Tamara

Hello there - yes that represents the favourable outcome This is a very tricky and finely balanced period, with a myriad of possibilities suggested across the spectrum. Many of them are indeed at least reasonably favourable - but from a personal point of view, based on the distance involved and on what I can see within the non numerical modelling diagnostic elements that are putting a few nagging question marks in my own mind over previous assessments of how this summer might progress, I would ideally like to see any NWP solution from any computer model that has an element of a persisting Atlantic ridge within it, dropped completely in the coming days. Its perfectly possible this will indeed happen, but for purposes of being wholly objective and honest it has to be monitored. Its been interesting, though not especially at all welcome, that the EC operational has kept representing some kind of Atlantic ridge type of solution to the day 10 period, and hence persisting with some of the coolest solutions vis its ensembles and some of the other computer models,. Taken at face value, such persistence of an element of Atlantic ridging rather than downstream would suggest that any subsequent improvement in the time under focus heading to the mid month period was more questionable, certainly in terms of recovery of temperatures to levels many of us would like to see at this time of year. I think its worth reiterating the key points previously detailed in terms of progress: 1) The coming 7 to 10 days of this month are well documented, and so I think any daily re-packaging of the obvious negative aspects of it isn't sensible or helpful. Better I think to continue to remind of the causes instead and which put the further outlook into context of the possible outcomes. Those causes relate to an a-typical (La Nina type) forcing from supressed phase of convection in the Pacific resulting in uptick of trade winds responsible for the upstream amplified pattern (Atlantic ridge leading to our downstream longwave trough) The very strong tropical convection suppression in the Pacific causing the upstream amplification from us is circled here: The associated uptick in trade winds (blue shading in the wind-flow anomaly plot below) to the supressed convection phase are circled accordingly - the contrasting westerly winds (yellow/orange shading) associated with the current MJO wave are also part included within that circle to the east of the -ve zonal wind anomalies across the Pacific - representing the westerly winds heading across the Indian Ocean to the Maritime Continent to meet the wall of trade winds across the Pacific 2) Which brings us precisely to that further outlook and what the possibilities are for de-amplifying the upstream ridge, lifting out our resident trough, and presenting a downstream ridge profile in our vicinity instead The improvement is predicated on the suppressed phase of the tropical>extra tropical cycle completing and a return to a more active convection phase across the Pacific. The effect of this to replace increased trade winds across the Pacific with further westerly wind bursts, such as seen earlier in May. This sequence re-configurating the rossby wavelength and blowing a hole in the Atlantic ridge and implying a better wavelength for amplification of the sub tropical ridging (Azores high) further downstream instead The VP200 tropical convection anomaly forecast above ^^ above gives some kind of indication as to how quickly this might proceed. The week 3 chart correlates to the start of the time of hoped for improvement as the orange shaded suppression starts to ease away in the Western Pacific, suggesting angular momentum tendency starting to rise once more and the cue to de-amplify the pattern upstream and instead re-amplify downstream as momentum transport propagated from the tropics to the extra tropics starts to feed across to our side of the NH. What is vital in this process is that the tropical convection phase does indeed propagate back to the Pacific, without beating a retreat - and we see no sign that some longer term shift in the default position of the tropical wave from the Pacific to the Indian Ocean is underway. This would be a sign that a more fundamental longer term breakdown, rather than merely just a natural suppression phase within the "mini ENSO cycle (aka MJO) was in process. With that in mind, that is why whilst the EC ensemble suite continues to look promising for this more favourable change back to summery conditions , I would ideally like to see over the coming days the operational, and any associated cluster it represents, stop producing cooler spectrum solutions and start to follow the ensemble pack pulling the other way - rather that see any opposite trend to this. These cooler soutions indicative of reluctance to break down that Atlantic ridge and/or shift its mean position closer to us and replace it with the trough backed west into the Atlantic instead. At the distance we are discussing, and with the diagnostic approach pointing to two different possible outcomes - one favourable as discussed in considerable detail for some days now, and one less favourable - its important that all NWP (not just the ECM) sings from the favourable hymn sheet c/o the more favourable diagnostic solution gaining traction with persistence, at least for a time, of the very borderline El Nino standing wave in the Pacific. To illustrate the influence of the Nino standing wave since the winter, its position in the Pacific and its representative 500mb downstream synoptic pattern of Atlantic trough and ridging mostly placed downstream, I have borrowed these composites from twittersville - as I think they well capture what any shift in that low frequency signal would imply in terms of a reverse Atlantic ridge AND downstream trough Signs that this Pacific standing wave is succumbing to further pressure and that a shift in the low frequency signal is set to head back to the Indian Ocean would suggest something of a re-assessment of prospects for the UK looking further ahead. We are certainly not there quite yet anyway, and reason for the moment at least to keep looking at that hoped for improvement in the time period you suggest. Hope that helps

Thanks for your customary posting of the latest ensemble means and which is much appreciated by many I am sure - I know you provide these for the right intended reasons and without knowingly trying to create reaction from others or mislead anyone As you know, from my point of view the diagnostic approach to the outlook has stayed consistent with pros and cons as to how the pattern may evolve beyond the unsettled phase next week. It continues to be a case of having valid reasons to suggest improvement beyond this time - mindful as previously stated in equal detail what could alter such an improvement. The ensemble members can give a broad numerical modelling view, as a snapshot in time, of how each model perceives the signals influencing the synoptic pattern, but changes to these can occur over a period of days rather than necessarily from from one suite to another in a single day period. NWP can be prone to reversing trends as much as accelerating them based them on changing interpretations of signals - so the diagnostics prove highly useful in terms of trying to identify when such changes might happen and/or point to the likely outcome(s) within that given time. I also think that the continuing reference to operational outputs, even less of a reliable snapshot of time for obvious reasons, is proving especially unhelpful in distorting perception (and not helping stabilise mood swings) according to the particular modus operandi of the poster on each occasion As I see it, the GEFS has been a little progressive in terms of creating the impression of the pattern improving as soon as later next week and I think the potential advection of heat from the east has rather helped create this illusion as well in what has remained an unstable pattern surrounding the longwave trough. The real improvement chances have always been beyond this time - any heat equation injected into the unsettled pattern has always been an 'add-on' to create a little interest. But all the main model ensemble suites (ECM,GEFS and GEM) all point to this improvement beyond this to varying but quite reasonable degrees. The VP200 tropical convection anomalies clearly show up the suppression phase over the Pacific and active phase in the Indian Ocean that is responsible for the step up in trade winds which is creating the La Nina-esque type pattern downstream to the Atlantic and European sector in the coming period - the distinct amplified Atlantic ridge and longwave trough response over the UK. Quite the change there from the sustained and robust array of westerly wind bursts associated with the recent convectively coupled kelvin wave progression through the Pacific earlier this month The eastward progression of propagation of this activity, is on the slower side of the envelope. This is in contrast to the fast propagating CCKW related phase of tropical convection in recent weeks. Hence why it is unlikely that the trough solution will lift out properly until the supressed phase in the Pacific starts to fade out in advance of the convective zone proceeding through the Maritime region towards the West Pacific. This sequence changes the rossby wave response further downstream with the result that amplification responses also shift further downstream to assist pressure rises in the wake of the trough.at the same time as allowing the heights to the east to link up with this pressure rise and improve conditions. Present estimates do not suggest this evolution advancing sufficiently till towards the mid month period - so the extended ensemble suite reflection of improvement are not too far away from the diagnostic guide. The timing of associated rises in angular momentum tendency, as the tropical signal propagates, also clearly coincide with both the tropical>extra tropical diagnostic and the extended ensemble suite suggestion The caveat, as before, is that the tropical cycle retains some progression to allow that rise in momentum tendency - the diagnostic equally shows that any lack of verification of propagation implies a greater struggle to change the downstream pattern. Answers to that question however simply cannot be properly answered immediately, by anyone - and require a watching brief to check progress. On that basis, any gloomy prognostication bases on snapshot modelling is as counter intuitive and misleading as any, allegedly, over optimistic one. Mindful of these cautions and caveats, which in truth once provided as balance and information to help the thread, should not thereafter require to be endlessly inserted and repeated with every post as means to try to ease the truly tiresome battles of attrition on this thread that persist, there nevertheless continues to be, at the very least, reasonable evidence to back an improvement towards, or by mid June. Especially as stated in recent posts the background to global relative momentum is set more favourably than other years at this specific time of seasonal change and should help the process of the trough lifting out in tandem with the shift in rossby wavelength re-configuring the pattern.

Reference paragraph 1 : Clearer signs have emerged from NWP related to this suggestion - one that previously had first been flagged up last weekend related to possible tight margins of error related to this dividing line between the cooler Atlantic air and the increasingly hot and very humid air consolidating over the continental mainland. Hence the reference, however clumsy it might have been to interpret, to the void where low pressure 'seems' to gravitate no further eastwards. Remaining paragraphs: The final outcome of this scenario remains predicated on the technical factors outlined already in considerable detail - under the pros and cons influencing it. I think some are perhaps getting sucked too far prematurely into the 'against' argument rather than keep an open mind at least about the 'for' argument, or perhaps they have other reasons for the apparent pessimism for no obvious reason - other than using thesnapshot of an unsettled spell being extrapolated outwards for concocted periods of time by specially selecting NWP operational charts to try to prove the points. Operational charts which so often chop and change each and every 6 to 12 hours. However, whatever the motive, this is a perilous stance to take based on the fact that the doom-saying summers listed of 2007, 2011 and 2012 (and one or two others maybe such as 2010 which featured a -ve PDO and low angular momentum La Nina state which correlate to a -AO) have some considerable ostensible differences to this summer. One thing this summer does have in common with those years is the perilous arctic ice situation (and even more perilous than since any of those summers) . But we have also seen other summers in this period, such as from July 2013, some parts of summer 2014, the latter part of 2016 and most especially of all of course last summer where the pressure state of the Arctic has had no influence at all in contributing towards blocking patterns that are detrimental to some highly ideal spells of summer weather. Therefore it once again reinforces the message, and one that I fully subscribe to, that each season should be sensibly taken individually on its merits. How else can one be objective? One key difference to all those poor summers listed is that this one is being ushered in by a higher than average angular momentum regime feedback circulation. 2007 featured sharply crashing AAM based on a fast emerging strong La Nina (nowhere in sight this year) aligned with the very low sea ice and a strong eQBO to help reinforce the blocked patterns in the Atlantic and over the pole. Likewise both 2011 and 2012 featured low angular momentum Springs heading right into the start of summer - - with the former never recovering from this state during the summer at all until early September (which coincided with some of the hottest weather of the 'summer' during autumn 2011.... and the latter with a brief rally in angular momentum in late June 2012 into the early part of July which accorded with the spectacular plume seen close to the end of June and hence otherwise associated poor synoptics. No coincidence that both 2011 and 2012 both featured, like 2007, extensive periods of blocking in the 'wrong' places and a trough that refused to lift out properly due to the location of the tropical standing wave being in the 'wrong' part of the hemisphere and according with long periods of amplified Pacific and Atlantic ridges and downstream trough.. This summer begins featuring a weak El Nino, w/WQBO and overall we have been seeing a neutral to slightly +ve PDO. I have seen suggested by a very respected and knowledgeable forecaster in another place as something of a cross between 2002 and 2006. In terms of the UK, the former was not especially good though a few short bursts of warmth occurred, but the latter featured some wonderful weather throughout June and July and then a very impressive Indian summer from early September into October 2006 was also a w/QBO year like this year - a good antidote to any -AO patterns. There are other observations to make in terms of SST's and solar forcing here too in respect of both those years - and so again another argument for taking each season on its own merits. This summer also starts with a supressed phase of tropical convection in the Pacific, temporarily altering the PDO state a little, and already dealt with in huge detail - and which lends itself to some a-typical Nino forcing (aka more La Nina-esque) Hence the displaced and amplified Atlantic ridge and trough axis steered south on a very unhelpful longwave in our vicinity. But until, or if there are signs this is to become the default, signal (i.e through crashing angular momentum as part of a a longer term change of atmospheric circulation such as happened in the aforementioned poor years) it remains sensible to be suspicious of any NWP which appears to try to match some of the commentary on here by sticking the boxed in pattern for inordinate amounts of time. It remains to be seen how far any heat advection from the continent might advect this way - but whatever happens in that respect, the chances are higher of this trough lifting out much sooner than in any of the pessimist years. And even if the greatest heat doesn't arrive, there is no reason not to expect a general improvement in conditions more widely SE>NW in the more extended period. If its not good enough for the likes of someone like me to say this, then look at the METO updates, which essentially are saying the same thing as someone ordinary as me on a weather enthusiast page. Based on the fact that the polar field is now resembling a state adjusting from the fallout of the strength of the final late warming of the stratosphere, and assuming that angular momentum remains close enough to where it is at present, the pressure will grow according to summer wavelength changes to keep backing the pattern very slowly westwards the further June progresses and most particularly from midmonth towards July. Keep the pros and cons list handy - but try to remain objective about the possibilities of both, and not skew opinion according to other motives and reasons.

On the first comment, you didn't unfortunately include the relevant part of the post that showed it was a comparison between a model ensemble suite and a diagnostic (GWO plot) as a way of trying to helpfully and objectively assist readers in matching up where the pattern might evolve vs the movement of that diagnostic. The post yesterday fully explained the context of the "void" but the impression I get is that you knew this when you nevertheless felt compelled to want to create an illusion of error. More generally, in view of the continuing snap reactions to individual NWP operational outputs which maybe not unexpectedly and somewhat predictably have persisted since observation was made of them - it might be worth looking at how the extended ensemble clusters incorporated within on the EC ensembles and follow on immediately after that 0z mean chart are picking up on the possible GWO vs NWP evolution to reduce the influence of the trough, which is being extrapolated forward with inevitability by some, and return some influence of higher pressure building from the east and north east Clusters of that EC 0z ensemble suite reflects the steady trend of atmospheric angular momentum tendency, todays update now posted, staying very buoyed in association with that development As stated in the previous post, this is worth watching to see if it gains traction for the reasons given - but notwithstanding the cases for and against improvement probabilities provided in attempts of neutral objectivity - it at least provides some scope to stop writing off weeks of early summer before it gets here officially. It goes without saying that of course people can comment on all kinds of aspects of NWP - but surely its better backed up with more than just deliberately pessimistic hunches that have no foundation up to weeks ahead and seem purely designed to create reaction. This issue pervades this thread most often in winter, but clearly is present at this time of year as well and rather seeks to discredit any attempt at taking out time to try to be objective and helpful as various posters who are enthusiastic about their spare time here try to do. No-one is attempting, or believes themselves to have wisdom or is some kind of elite over anyone - such an attitude would be anathema. There are all kinds of ways to contribute. But ways which are respectful and manage to drop the need to be either sarcastic, discrediting and/or are reaction seeking.

If I might interject at this point. It might well be worth adding a bit more flesh to proceedings and come at this yet another way in terms of objective diagnostics vs model "hugging" hunches and speculative extrapolations of individual operational outputs For this purpose, lets take the updated ECM 0z ensemble at day 10 - which is a reasonable estimate of where we are likely to be based on current state of affairs. It also rather illustrates the points I was trying to make yesterday in terms of the pressure void to our east. Lets then compare it to how it might be reflected by the diagnostic Global Wind Oscillation plot, which is a phase depiction of wind-flow patterns in the tropics and extra tropics and shapes the jet stream patterns which carve out placement of pressure systems. Described here in deliberately simplified parlance. For this purpose we take the latest GWO plot, based on its typical consolidated lag time - at present time of typing to 25 May. This reflects a very low amplitude signal which has ebbed and flowed around a weak phase 0,4,5,6,7,8,0 type orbit for some quite considerable time - identifying the pale pink warm neutral/weak Nino signal standing wave circulation feedback in the Pacific. So once again in deliberately simplified terms - the predicted phase correlation for that EC ensemble plot is,very roughly speaking, around a Phase 8,0 position - according to month and seasonal wavelength. This phase progression is reflective on the GWO spectrum of some loss of angular momentum tendency having taken place upstream early next week which has amplified the Atlantic ridge to our west, and, downstream of that ,digs the longwave trough close to the UK. What is important here is how the GWO is likely to progress as we head through early June. Based on the supressed phase of tropical convection across the Pacific as the MJO has retuned to the Indian Ocean, then some increase of trade winds, as identified yesterday, is behind the upstream loss of momentum as -ve frictional torque mechanisms scrub some of the recent strong westerly inertia from the atmosphere. This trade wind uptick visible, shaded blue, in the Western and Central Pacific towards the dateline here But this is far from the whole story. With relative global atmospheric angular momentum comfortably above average (though still to account for some slight downward adjustment c/o the Pacific suppression phase)…. …..this will still likely leave the AAM budget of the atmosphere with a supported floor, ahead of the next uptick in the "mini ENSO cycle" as the low frequency tropical signal in the Pacific is allowed to re-emerge and add further westerly inertia to the atmosphere and re-boot angular momentum tendency. AAM forecasting still, steadfast on this Such support and uptick having the effect of re-orbiting the GWO back through Phase 4 towards the Nino attractor Phase 5....the signal to lift out the trough and pressure rise from the south (likely as suggested yesterday that the boundary of the heat will still be fairly close by and conceivably ready to advect back northwards and westwards). It remains something to keep watching in ensemble suites in the coming days, as this story is far from complete.... What could go wrong? The importance of re-energising of westerly momentum across the Pacific to neutralise the trade increase is a vital key here to support AAM and prevent a deeper slide. Any shortfall would add incentive for a more durable trough solution and retain a more amplified ridge profile in the Atlantic to lock the longwave trough in. What can quite conceivably go right (aka AAM forecast) Its also true though that if the atmosphere just stays as it is, then summer seasonal wavelengths on their own should work in our favour and increasingly means that just a persistence of the present GWO orbit regime inclines the trough to edge back westwards the closer we get towards the later part of June and into July. Any ideal scenario of repeated vigorous westerly wind bursts however, that pushes the GWO into higher phase Nino attractor orbit 5,6 and 7 starts to flag up the possibility of 2006 and 1983 type scenarios with prominent Western European heat ridges duelling with thundery cut-off lows to our west and south west....But that is for another time perhaps...

Aside from the daily commentary of updated model suites, from a regular few and which proves a useful, helpful and pleasant antidote to mood swings, unfortunately far too much typically absurd and unwarranted overreaction to the inherent erraticism of numerical modelling is omnipresent as usual on here from the usual suspects - and which serves no useful or rational purpose at all. This, much more of a nuisance than what NWP (apparently) most lately suggests itself So, by way of most recent example of this - we arrive at the point that sections of NWP, about a week back (including long term EC monthly!) was mapping out a good 10 to 15 days at least of cool and changeable weather with a trough anchored very close over and to our NE, phasing with further disturbances arriving across the Atlantic on a supressed jet stream. Instead we look at this to end the coming week and the weekend Its not, and was never likely in my opinion to be a long term sustainable ridge - but what it does do is represent a likely recurring pattern in the opening weeks of summer based on the detailed discussion of parameters discussed in the previous post, and many preceding it. As suggested in that post, this is not likely to be another 2018 type summer with weeks of blazing anticyclonic ridges and so expectations should be adjusted accordingly in my opinion. If not that, then at the very least learn not to flinch with each and every output, or better still, wait a period and see what emerges over a period of a day or two rather than every 6 to 12 hours! On the basis of a diagnostic, rather than purely wholly numerical modelling approach - for the opening period of summer (at least) it does look to me to be one punctuated by an Atlantic trough and downstream ridge pattern that breaks down and then re-programmes. Rinse and repeat. In that sense, the above snapshot of the expected weather pattern to end the week (and very pleasant conditions) conforms quite closely with those prior expectations. And its not a bad place at all to be at the very start of summer - irrespective of whatever else is currently suggested to follow. Looking ahead from this time, personally I am very sceptical of any NWP trying to break down the weekend pattern sustainably by, yet again, suggesting a repetition of a week or so back with a same extrapolation of breakdown into equally sustainably cool and changeable conditions in the extended period. Operational NWP shouldn't be taken at face value at any time, and that includes ensemble data at long range as well - which is equally quite prone to change direction like a shoal of fish in midstream, when they exert a bias signal and appear unable to "see" beyond the that signal to the next. After all, it gets forgotten on this thread by some - but its not the computer models that drive the signals....but its... So NWP in the last 10 days or so has been particularly poor, and is likely to continue to be unreliable which means that human reaction to every output should be adjusted accordingly. This also means that whilst upper air noaa anomalies do represent a best guide as an overview of NWP - they too still remain just a snapshot in time and will also change as computer models evolve with signals. So a good idea to adopt a cautionary word here also at not taking them too much at face value. Relative global atmospheric angular momentum remains comfortably above average and with a fair degree of westerly wind inertia in the atmosphere and more to be added as another MJO wave begins its course through the Indian Ocean to arrive in the Pacific later in June.. This to help programme further downstream amplification in this sector of the NH heading through the month. AAM forecasts continue as before to advertise this supported trend. Whilst the natural interim supressed phase of convection over the Pacific (shaded orange) temporarily creates some upstream amplification and a little extra downstream polar jet energy, -as cautioned above, the risk of overreaction to this shorter term signal is clear and questions any attempts by NWP to try to effectively "crash "AAM and as a synoptic response send sustained increases in polar jet energy over the top of a retracted and amplified Atlantic ridge such as the ECM operational is currently trying to do. As long as the tropical signal resides as default in the Pacific, and angular momentum therefore remains sufficiently buoyant, then an upstream ridge and UK trough remains counter intuitive on any holding basis. . Look at where the low frequency tropical signal resides close to and just west of the dateline in the Pacific It supports an El Nino type standing wave, even if not a "pure" basin type El Nino. In fact, the standing wave due west of the dateline is better than any east based signature which would supply too much of a southern stream (as unhelpful for warmth in our sector as too much polar jet energy) So its not the pure ENSO base state that is important at all here, it is the relationship that the atmosphere adopts to the base state - and this is one that drives a programme of rossby wave trains that should keep punching breaks into the downstream jet and filling them with the sort of ridges that are advertised this coming weekend. With that in mind, there looks to be a very slack downstream void setting up in quite a bit of the modelling beyond next weekend where the attempted phasing of the jet coming across the Atlantic (trying to bump up the strength of low pressure) hits the buffers. A very large percentage error exists here - the European mainland having already become anomalously hot by this time. A ridge, or at least relatively higher pressure is always most likely to reside here, on the basis of the above wave train analysis - so any modelling trying to fire up huge lows and ultra progressively sweep this continental airmass aside should be viewed as just one main source of suspicion. The UK is always likely to be on the cusp of the Atlantic vs European airmass difference, and so no surprise that a uniformly settled picture cannot be guaranteed in any way and is not likely (unlike 2018) - but also no surprise that some NWP will come up with solutions that produce over deepened lows where the phasing of the lows on the downstream jet, clash with hot and humid airmasses. But though it alas takes centre attention due to the vicissitude of NWP, this is simply the noise that masks the default, which will (for the time being at least) be to restore a trough/ridge pattern - rather than progressively try to usher in a low angular momentum jet stream scenario where pressure falls over both UK and Europe and pressure rises sustainably instead upstream over the central and western Atlantic. Please note the deliberately persistent italicised and/or bolded use of word sustainably in this post.

There are no apologies for self quoting a significant extract of the post the other day because its central focus becomes more primary to how NWP evolves in the remainder of this month and into the start of June Sustainability of next weeks trough solution already being examined by NWP - despite some continued extended products having recently appeared to insist it will prevail throughout the 15 day period and perhaps beyond The low frequency tropical signal shows little sign of abating amplitude across the globe. The next wave programming a further progress of convection anomalies eastward towards the western and central Pacific during the first week of June This fully endorses the consistent signal by the CFS seasonal model for quite some time(and still further consolidating) for angular momentum to stage its next rally in tandem with this event With westerly wind inertia already in the global circulation based on weak El Nino standing wave and previous robust westerly wind-bursts from the last wave, this one looks to be following on a rapid timescale envelope and should maintain an above average level of AAM through June.. A sign that the atmosphere doesn't want to let El Nino fade at least quite as quickly as the shallow ocean thermocline wants to suggest. NWP modelling should have little choice but to follow-up these signals and continue to build poleward ridges northwards through the mid latitudes and keeping the lid on attempts for the weather to turn unsettled for too long. Summer wavelengths also increasingly suggest that these ridges should embrace wider warmth parameters, despite the apparent reluctance of -ve downwelling anomalies left over from the intense dynamic final warming of the stratosphere to fade quickly - and hence the propensity for these ridges to become higher latitude than one might desire for optimal warmth. It is this factor aligned with the vulnerable sea ice that provide the wildcard flies in the ointment, and need to be watched carefully to check they don't persist in scrambling the attempts by the tropics to send the jet properly northwards. However, based on the current PDO profile and w/QBO not the same as has occurred in some of our poorest summers (and so assisted greater pernicious staying power of a -AO profile) then assuming the atmosphere stays in the same state through the opening weeks of the summer, the polar profile should succumb to a more neutral and even slightly +ve profile with time. Though the more strongly +AO profile of last summer continues to seem unlikely in my opinion.. Only beyond this opening period, (caveat: that polar profile allowing) does the ability of El Nino and associated tropical/extra tropical patterns begin to become under the greatest uncertainty - but as the last post suggested, no reason not to suppose that some very pleasant and increasingly warm spells of summer weather await us

Aside from the standard posts scattered around designed purely to enkindle reaction and usually predilected and deliberately exaggerated through baseless claims and skewed interpretations of sometimes spurious sources of information, the late Spring pattern has evolved very much akin to expectations of previous analysis. That is - the substantial downwelling of -ve zonal wind anomalies at higher latitudes vs attempts for seasonal ascendancy of the tropical/ferrel cell to assign the jet stream northwards assisted by a weak El Nino forcing pattern in the Pacific. The former strongly -AO regime making the latter much more difficult to achieve than was seen under the +AO regime of late February that ironically managed to achieve temperatures not too dissimilar to what many reasonably hope for at this time of year. The recent convectively coupled kelvin wave (CCKW) has produced a considerable train of strong westerly wind bursts across the Pacific It can also be said that whilst the -AO regime has made any heatwave elusive, this Pacific pattern has still provided some fair weather and prevented an even cooler May CET from evolving than if we had been in a low angular momentum regime under substantial higher latitude blocking As the associated MJO wave concludes and passes into the Western Hemisphere, on into the Indian Ocean at the start of the next cycle and attempts at upwelling cooler water occur in the Pacific as trade winds increase once more, its to be expected to see angular momentum tendency drop as the previous span of westerlies are scrubbed out from the atmospheric circulation The ECM wind-field anomaly identifies those upcoming trade winds in association with the lull phase of the tropical cycle as it passes through the I/O and suppression returning at the same time to the Pacific as identified by weeks 1 and 2 VP200 (velocity potential) anomalies This upstream change and scrubbing out of recent westerlies signifies re-amplification in the Pacific transferring to a downstream Atlantic ridge response exacerbating the Greenland heights signal at the same time as a wedge of vorticity is allowed to sink SW'wards over Scandinavia and the northern most Atlantic towards the UK and presents a trough solution evolving immediately after the Bank Holiday. What is problematic is that this signal is to be assessed at where globally averaged angular momentum sits relative to the orbit of the Global Wind Oscillation. These indicators show westerly wind inertia throughout the global atmospheric circulation to be higher than average - with the standing wave still echoing a weak El Nino signature in the Pacific and the GWO well away from the low angular momentum octants. This puts some question as to the sustainability of this medium term Atlantic ridge and downstream trough signal - especially as the tropical convective MJO signal continues to be active and churning across the globe - which means that further westerly wind bursts can be generated. Despite its jaundiced reputation in relation to NWP interpretation of wider signals, the CFS seasonal modelling has some value in its indicators. It has remained steadfast for the last couple of weeks on angular momentum retaining a floor relative to the Pacific signal for June at the least - with the next upturn of angular momentum tendency signalled to follow the upcoming trade wind increase, in tandem with eastward propagation of the next MJO wave On this basis at least, and as this forecast is not without support from other modelling, and notwithstanding the CFS tendency to over amplify tropical signals - and still its worth being very cautious of trying to extrapolate the 5 to 10 day trend for cool and unsettled weather to take hold c/o Atlantic ridging and downstream trough for large swathes of the month to follow. Hence ensemble mean suites needs to be gauged carefully against this over the coming days and certainly not worth taking too much notice at all of intra day operational swings that are, frankly, best avoided at any time anyway. Looking forward ahead even further than this naturally becomes very highly uncertain and even more problematic - its to be seen whether the present ocean trend to want to cool the Pacific ENSO zone succeeds in overriding the ability of the atmosphere to sustain WWB's and therefore unable to prevent a more sustainable switch to a more la Nina-esque Atlantic ridge and downstream trough scenario. Avoiding this for the greater part of the summer looks less likely than seemed possible earlier in the Spring, but it is not, yet, inevitable by any means that the first half of this summer at least won't seen some pleasant seasonal weather.

Hello Mike - this cross section of the layers of the atmosphere clearly illustrates the downwelling process of that final warming - quite the contrast to the +AO dominated close to winter and first half of Spring. I think that puts a perspective on this Spring and its mixed character. Aside from the anomalously very cool start to May which is concordant with the switch to -AO c/o the dynamic nature of the final warming, the CET means for March and April are fully contemporaneous with that earlier generally +AO profile - both months comfortably above average. So the capricious surface conditions of the season have rather masked the fact that this Spring as a whole (so far) has not been quite as poor as some might try to make out. Also the highly anomalous warmth of the second half of Spring 2018 is a distracting and distorting comparison perspective - even in this warming world and increasing occurrences of noteworthy temperature anomalies breaking frequent records, consecutive late Spring and Summer seasons of such homogenous warmth still represent an upper benchmark. This is the UK after all and not a sub tropical climate as much as it is not a cold continental one in winter. Expectations and preference ideals are too often asymmetric with geographical latitudinal realities Looking ahead, and striving to build on previous analysis - some further thoughts to add extra substance. There are signs a more coherent +ve PDO pattern might try to set up with come modest relative cooling in the West Pacific - and by comparison, warmth consolidating in the eastern Pacific. This accords with El Nino standing wave signature being assisted by the eastward propagation of convectively coupled kelvin wave (CCKW) MJO envelope. As such, it accords with MJO Phase 8 imprint and downstream rossby wave train amplification of the wavelength Furthermore, this contradicts the identity of Springs heading into summer like 2007,2010, 2011 and 2012 where a -ve PDO was part of either an imminent La Nina (2007 and 2010) or a long established La Nina (2011) or an even more established Nina struggling to transition to ENSO neutral (2012) If we take the globally averaged angular momentum budget, the characteristic low angular momentum state (as manifested by -ve PDO tendency) is clearly choreographed during the Springs of both 2011 and 2012 Quite different to 2019 A w/QBO prevailed as is the case this year in 2011 - however the Pacific feedback was very different and clearly much more La Nina-like than this year - and the characteristic mid Atlantic ridge and downstream UK trough increasingly played a hand as summer was arriving and seasonal wavelengths changed. Furthermore, the QBO phase does not accord with the very easterly state in Spring/Summer 2012 where some other background parameters were closer to this year than 2011. The strong e/QBO overrode any attempts made by the atmosphere to try to become less La Nina-esque during early summer 2012 and sustained blocking at the "wrong" latitude instead.. Calculated at NOAA/ESRL PSD 30mb zonal wind at the equator, zonal average For info https://www.esrl.noaa.gov/psd/data/climateindices/list/ 2012 -16.07 -15.25 -16.74 -17.62 -22.04 -25.89 -27.82 -27.93 -26.60 -24.51 -18.95 -10.02 2013 -6.07 -1.23 2.85 8.39 12.64 13.38 14.27 14.66 13.12 11.69 12.45 12.55 2014 13.13 12.68 11.72 7.15 -2.81 -13.98 -19.29 -21.64 -23.24 -23.86 -23.65 -25.38 2015 -26.70 -28.62 -28.15 -24.38 -12.33 2.18 7.45 10.97 12.07 13.38 12.79 11.39 2016 9.34 6.77 3.16 0.64 2.37 3.86 6.25 10.07 10.48 12.83 14.16 15.09 2017 14.92 14.78 14.35 13.88 8.01 -3.18 -10.48 -14.42 -15.28 -16.79 -17.20 -18.12 2018 -19.02 -19.37 -19.77 -21.41 -24.23 -28.45 -29.10 -20.41 -9.91 -2.79 3.36 8.05 2019 9.02 9.25 11.82 13.36 - (April update) Strengthening westerly phase evident. A +ve (westerly) QBO, in tandem with an El Nino forcing on the atmosphere is a more "hostile environment" for any final warming feedback to prevail than occurred in 2012 and failed as a result of too much La Nina -ve PDO forcing in 2011. Summer 2006 is that excellent w/QBO and El Nino combination example, previously referenced, where HLB in May gave away to anomalous warm mid latitude blocking early in June. In respect of the CCKW as a noteworthy event, it will be interesting to see seasonal model updates in terms of ENSO SST predictions as benchmark for how Nino might prevail. -May is often a key month here.. Should El Nino rally once more, with the standing wave engaged in the Pacific and the atmosphere reflect a coupled feedback loop with the extra tropical GWO starting to repeat orbits returned back to the Nino attractor phases, then portents are good for summer. Different to last year in respect of the likelihood of no persistent ridge for weeks on end and the Atlantic trough and downstream ridge breaking down and re-setting and allowing thundery breakdowns, and then cooler changeable interludes leading to re-set of the default warm pattern. That is not a forecast - but its quite typically traditional weak summer El Nino. There is little danger of the southern stream getting too strong as it did in the start of the "super Nino" of summer of 2015 - its more a case of avoiding increased trade winds breaking down the Nino-like trough/ridge pattern and creating a more persistent downstream trough instead. Especially because this years final warming has had a "dynamic" element to it, with considerable poleward heat flux drawn into the final warming and further amplifying the downwelling -ve zonal winds to the troposphere/stratosphere boundary. This might serve to give it that little extra resistance to the natural effects of seasonal wavelength changes and advancement of the ferrel cell in the tropics as assisted by the favourable effects of wQBO usually creating an elevated tropopause due to westerly shear in the lower stratosphere. But there are signs as previously discussed the odds are stacking against such resistance and whilst, as we know, the seasonal models like UKMET et al are not bullet proof - such general analysis as attempted here does provide some logical resonance and credence to such predictions of a warm summer pattern Time as always will tell - but much more will continue, as ever, to be gained from comparing progress of ensemble NWP suites in reading all these signals than wasting time trying to draw too many observations and conclusions from the smorgasbord of unicorns offered by intra day multi faceted operational output.at long distances All the computer models have their inbuilt biases, but the GFS continues to often display a -ve tendency (low angular momentum) response to its interpretation of tropical signal imprint on the synoptic pattern - particularly as the MJO completes its cycle and returns to the Western Hemisphere onwards to the Indian Ocean. Such timing as due later this month and where differences of modelling are already apparent with the ECM .So some further erratic output and disagreement could crop up as the coming week progresses in terms of the extended period. Good reasons therefore to particularly not overreact to operational data and also be cautious with ensemble suites and compare their progress over 2/3 days rather than 1-1..

In view of the provocative game-playing (that I had gladly escaped during the winter) and the absurd quick fire reactionary responses attributed to NWP operational data, attendant with not enough attention instead to ensemble data (which in itself is simply only a snapshot of time in evolving patterns) its probably a good time for some more analysis. Signals after all lead models, models do not lead signals The seasonal demise of the polar vortex, amplified by tropospheric heat flux forcing as described in an earlier post, continues to dominate the NH pattern and inhibiting any late Spring build up of real warmth over the nearby continental landmass due to persistent feed of cold air advection from a displaced and rapidly dissipating area of leftover vortex broken away from the the eastern arctic into Scandinavia The persistent height rises to the north - a manifestation of the symmetry between the tropics and the pole and differentials between them which enhance tropical convection activity (MJO) and the resultant wave train in turn triggers equatorward flux in the Southern Hemisphere (i.e subsidence away from the Antarctic) and a +ve pressure signal (+AAO). At the same time in the Northern Hemisphere, the engagement of the eastward propagating tropical wave at the dateline teleconnects the opposite way to the SH with a -AO/-NAO - the latter attendant with upstream jet extension across Asia and the Pacific and greater flow into the southern stream. The Southern Oscillation (SOI) in response to the eastward propagating MJO signal has trended sharply negative in the past few days - an indication of -ve Outgoing Longwave Radiation anomalies ( OLR) and associated deep clouds and thunderstorm development close to the dateline - also as illustrated by the intense and sustained westerly wind burst continuing across the Pacific to the dateline https://www.longpaddock.qld.gov.au/soi/ As well anticipated in recent posts in advance of this WWB, angular momentum tendency, subject to 2 day lag, is significantly ramped upwards. The +ve AAM anomaly depicted around 30N in tally with the proxy data in the Pacific as depicted in previous plots The Global Wind Oscillation, a plot depiction of wind-flow additions and subtractions as supplied by frictional and mountain torques and other related mechanisms, has responded with a leap into high amplitude Phase 4 as these large amounts of westerly additions are supplied to the atmosphere. This is the kind of repeated GWO pattern which, with repeated westerly wind additions c/o tropical activity, would return the GWO to El Nino attractor phases 5 to 7, and would accord with seasonal wavelength changes and start to build further anomalous mid latitude ridges close to and just to our east during June (and possibly July) at the same time as height rises to our north start to fade out.. More on this uncertainty further below. So for this, and all the other repeated detailed reasons given in previous posts, it continues to be premature and pointless in equal measures to extrapolate assumptions of the resultant -AO led pattern into the early stages of summer (and in some cases evidently even beyond!). A lot could, and quite conceivably will happen over the coming few weeks at such a time of seasonal pivotal change. In the medium term, its worth watching the developing low heights towards Iberia and Biscay - these could play a role in providing a stark thermal temperature boundary between the cold air advection to the north and a much warmer humid circulation becoming ensconced around the low heights to the south and south west. How far north such a boundary might set up is uncertain at the moment, but some NWP is playing with this scenario in the extended period and its conceivable that another round of retrogression of the heights to the north could lead to a similar evolution to this week, but this time with less of the deeper upper cold air around to tap into further to the north and east and any warm air advection adjusted further east into Western Europe With that type of subtly adjusted wavelength evolution in mind, if (as discussed above) tropical convection patterns and angular momentum tendency can remain conducive, and this still isn't a given and needs to be watched for a while yet, then seasonal wavelength changes and gradual ascendency of the Hadley/ferrel cell heading into new month and new season change could flip a stubbornly (relatively) cooler pattern into something more sustainably warmer. Such as happened in some of the aforementioned summers in previous years that featured May's that seemed reluctant to embrace a substantive warm-up All that said, and returning to the opening theme above, some proper perspective is needed here - next week still looks to feature a much more pleasant spell of weather after this weeks anomalously below average temperatures, with some welcome sunshine that will lead to warm feeling days out of the breeze, especially in the north west and away from eastern coastal districts where low cloud could be a nuisance. In that respect not an unusual pattern for mid May and nothing untoward whatsoever to lead to writing off swathes of a season that is yet to begin and remains too uncertain to call (either way).

This convectively coupled kelvin wave (CCKW) I have continued to reference over the last week or two has been of such strength heading through the Indian Ocean that it had much to do with the development of the devastating tropical cyclone Fari which sadly crippled the Indian state of Odisha in recent days. It has now passed eastwards through the Maritime continent through to the Western Pacific and in terms of the feedback processes from upwelling warming across the ENSO zones (see below tweet for illustration purposes|) has possible significant future implications to downstream patterns in terms of the ushering in of summer wavelengths - and when the dynamic response to the final warming of the stratosphere eventually starts to fade from influencing tropospheric responses. The bright red - intense white colours in the Hovmollers plot pick up the strength of the WWB heading through the W Pacific. On the VP200 anomaly chart depicting the CCKW tropical convection, notice the eastward progression of the CCKW in the first half of May to engage the dateline in the Pacific - and how suppression (yellow, orange. red colours) of convection appears in the wake of the tropical wave across the Indian Ocean and then Maritime continent respectively These westerly winds in the Equatorial Pacific propagating eastwards, tend to migrate poleward through the extra tropics with time, and programme swathes of downstream amplification. Where this amplification sets up is dependant on the longevity of the more transitional seasonal downwelled -ve easterly zonal winds across polar latitudes. The question is - how sustained are they likely to be? Any collapse in angular momentum caused by increase in easterly winds across the Pacific has the effect of transferring the amplification wavelength upstream to the Pacific and create a downstream ridge>trough>ridge trough response. The final ridge trough response being that in the Atlantic and European sector respectively. On that basis it could be seen that any Atlantic or Greenland blocking default created by a -ve ENSO atmospheric circulation feedback is a natural extension of any pre-seasonal higher latitude blocking such as we presently have. If the ocean/atmospheric standing wave is held back across the Western Hemisphere because of the effects that easterly trade winds have on supressing tropical convection in the Pacific - the consequential stable atmospheric environment is conducive to High pressure in the Pacific and a downstream blocking response also in the North Atlantic.. That is why the setting of a +ve ENSO wavelength in the Equatorial Pacific is useful for warm air advection patterns in our part of the Northern Hemisphere in summer because it encourages MJO related and associated separate induced cyclonic activity in that part of the tropics - which in turn helps promote a downstream Pacific trough/ridge pattern, and replicated Atlantic trough and downstream European ridge In this way, the CCKW is a very important event in reducing the risks of a more la Nina like atmospheric response generating sustained easterly trades winds heading into summer and also therefore less chance of any Atlantic blocking configuration to follow on from final warming HLB and then stick for any length of time. Ultimately, the current MJO wave will return, naturally as part of the "mini ENSO cycle" to the Western Hemisphere - note the suppression arrive in the Pacific as the signal returns from Africa back to the Indian Ocean in the last third of the month Based on the fact that the latest wave is already likely to adjust the Pacific SST pattern favourably, as detailed above, and help tip the balance more towards a summer ridge pattern downstream, then the return of the tropical wave pattern to the Indian Ocean later this month might well see a higher floor to the fall-back in angular momentum likely later this month - and that usually always occurs at the end of each MJO cycle. Currently globally averaged atmospheric angular momentum (GLAAM) is about +1SD above average which is a quite good place to be in ahead of the upcoming passage of WWB's associated with the CCKW. CFS and other model products are picking up on a possible further CCKW event heading into June. Its early to have high confidence in this, but its one to keep an eye on. . This watching brief then also ties in with what happens to the polar field pressure pattern in terms of the remnants of the final warming. Some current NWP extended products are hinting at wanting to try and retrogress the poleward amplifying sub tropical ridge that evolves through days 7 to 10 again and sustains the block to the north in the more extended period. This then potentially inviting another swathe of troughs to have the potential attrition to undercut from the west and promote further cool unsettled weather How credible is this? Assuming that longer term prospect chances have been improved by upstream tropical developments sustaining a supported angular momentum regime heading into summer, then the lifespan of dynamic fall-out from the final warming has limited sustainability heading into summer period, with a combination of seasonal wavelength changes, +ve momentum forcing from an El Nino type atmospheric feedback and assistance from the +QBO phase to also underpin this. So whilst a further round of attempted Greenland blocking cannot be ruled out in the medium term, then this might give way to a warmer mid latitude ridging pattern as Spring heads to early Summer. Though the sustained anticyclonic conditions of last summer seem less likely this year, and a potential thundery element exists which many would welcome on the basis of a reloading warm pattern and to add some interest to conditions. May 2007 was a generally +ve AO regime which gave way to higher latitude blocking through June and the rest is history. It was also a year where angular momentum collapsed heading into summer at the same time as an easterly -veQBO assisted downwelling of -ve zonal winds into the troposphere to align with weak sea ice and produce a very poor summer. The previous year, as documented in a post last week, had the opposite configuration with a higher latitude blocking in May followed by mid latitude heat ridges as soon as the calendar turned into June. This year the chances of more sustained relapse have not gone completely yet, and it will require a close watch on both the ongoing blocking patterns and angular momentum/tropical wave trends during the remainder of May heading into June - but latest events should have at least reduced the risks vs what was beginning to seem increasingly possible in recent weeks and create a better chance that this summer evolves closer to the better variety than the likes of 2007, 2011 and 2012. So that improvement from the south comes as suggested of late by the weekend to follow what looks a truly dismal week - albeit due to the persistence of the higher latitude blocking signal it looks now to set up initially further west than ideal to cut off the cooler air staying close by. However, the depth of cold air compared to this weekend is not there and liable to mixing out further with time. Its also not the end of the world if it morphs into attempting to retrogress with further fall of pressure undercutting it. It doesn't mean summer is destined to be over before it has begun. Such an evolution is far from certain anyway at this stage and some fine weather, albeit not a heatwave, still looks likely heading into the following week. But should this month continue to struggle to follow through on any recovery promise, the philosophy of ice in November and sludge and muck to follow comes to mind with its opposite counterpart in May. It remains the case that what follows afterwards matters much more in terms of hopefully disposing of the Greenland and higher latitude blocking..

Eagle eyes obviously present... Based on yesterdays assessment those day 10 means reflect good cross model continuity for re-set of Atlantic trough and downstream ridge and warmer air to approach from the south. Taking account of the caveats that could still alter this evolution for all the detailed reasoning also given yesterday and recently, it still pays to do some QC on NWP assessment in terms of latest suite consensus (at the very least) before reacting However, the next few weeks continue to advertise a significant westerly wind burst propagating east through the West Pacific to the dateline and providing a timely shot to keep an atmospheric Nino feedback going into early summer. We are not quite there yet, but Its worth keep repeating that this is increasing the chances of warm plume scenarios rather than cool and changeable North Atlantic flow around a mid Atlantic ridge and downstream trough response

Extracts posted last week on closed thread: The other part of the equation is the seasonal transition within the stratosphere from its winter default vortex position to its summer state. This is bound to increase -ve momentum (easterly) zonal wind activity across the pole and manifest blocking tendencies at high latitude which adds further to NWP vicissitude...….. ...….Mitigation of HLB sustaining too long also probably aided by a weak El Nino forcing which acts to help the ITCZ further north in late Spring heading into early summer sooner than under low angular momentum La Nina conditions...... Forthcoming prospects? ….. assuming active tropical >extra tropical forcing sustaining angular momentum tendency through the greater part of May c/o weak El Nino circulation and aligned with w/QBO, increasingly inhibiting Atlantic and Greenland blocking heading towards the important seasonal wavelength setting as Spring turns towards Summer, then whilst such a pattern will inevitably sustain backwards and then forwards once more- its not unreasonable to hypothesise further re-setting of the Atlantic trough and downstream W/European ridge and re-cycling of increasingly warm patterns from an ever warming continent as the days keep growing longer. ---------------------------------------------------------------------- The most recent post following these extracts spoke of fine margins in respect of much of the wider discussions ongoing in these summaries. Categoric underestimation in that analysis, no doubt, of the vast amounts of vertical heat transfer generated into the pole by the recent Scandinavian height anomaly - - which has further augmented the final warming and thus created the "dynamic" response to the final seasonal warming of the stratosphere that has made the difference between a more conducive Branstator (mid latitude) ridging arrangement to late Spring warmth and the highly anomalous Greenland block verifying instead. The result being the polar vortex splitting and leaving a core cold element to our NE which is set to spoil the B/H weekend temperatures and increasingly also looking set to usher in a southerly tracking procession of lows from midweek. So one might assume such a reverse polarity outcome has also scrambled the polarity of the whole analysis, and left credible extrapolations of cool cyclonic conditions for weeks on end in its legacy instead? No, not really. The fundamentals remain that the seasonal stratosphere/troposphere transition might well be simply masking the direction of travel to an ultimate destination that sees transport of the end of season cold vortex back across the pole c/o the reverse polarity, helping to converge a mid term cyclonic mess in the process right over our heads, but with the Greenland height anomaly also fading and backing west towards Newfoundland and Canada with the result that something of a west based -NAO allows pressure to rise from the south and starts to draw up warmer air in the process. There are some hints of this in extended means, albeit the exact timing of this is open to question circa a few days bandwidth. Such an evolution would go full circle to re-setting the original analysis suggesting of an Atlantic trough and downstream ridge and return of something much more seasonal. As depicted by the sharply defined edged red anomaly in the Hovmollers plot below, this pattern evolution also draws on the significant westerly wind burst about to be set in motion in the West Pacific as the strong tropical CCKW related MJO wave propagates further east (and ultimately engages the standing wave at the Pacific dateline) - -and hopefully keeps a borderline Nino signal going into the start of summer and which assists in choreographing a rossby wave pattern suitable for downstream mid latitude ridge building (as the wildcard -ve easterly anomalies at higher latitudes fade out to summer transition of the stratosphere/troposphere state) As previously outlined, the importance of this is vast - because seasonal wavelength changes from Spring to Summer would lock in this type of default for periods during June, and possibly July in much the same ways as, by way of a few examples, during 1994, 1996 (to some degree) and 2006. Most of those years also had some Nino upstream forcing attached to them - and 2006 perhaps closest in this respect with an establishing and growing +QBO phase which helped supress sustained higher latitude blocks and facilitated low heights over the pole to follow the blocking of seasonal transition. The most recent post discussed some of the elements that could make things go wrong, and which also would encourage Atlantic blocking mechanisms to become the default instead - and which would persist or follow on from the upcoming unseasonal supressed jet stream conditions. So of course there is nothing inevitable about such repetition in 2019 However all three of the aforementioned years (1994, 1996 and 2006) saw highly underwhelming May's with a supressed jet stream and plethora of Atlantic/Greenland blocking prior to very good June and July's - but summer variants followed this in each case centred around extended dry very warm/hot settled spells and some spectacular thundery breakdowns and re-loads of the Atlantic trough and European ridge pattern. 1994 was especially notable for the thundery variant during both June and July.

The shorter term is the easier bit to update on. Alas, the seasonal warming across the Arctic looks to make the pivotal difference to the amplification wavelength and latitude of the ridging and put the UK just enough on its "wrong" side with a much cooler start to the B/H weekend than seemed possible.. Such fine lines here.... Hopefully a recovery of temperatures as the polar flow gets cut off and feeling much more seasonal to round off the long weekend Thereafter much hinges on how sustained the westerly wind bursts across the Pacific are c/o the tropical cycle propagating eastwards during the first 10 days of May and discussed in thorough detail in the full post from which the above quotes are extracted from.. That post dealt with the upside to the possibilities. This one equally deliberately looks at what can, and might, go wrong. Wrong at least for those of us hoping for warmth. Seasonal models are most unreliable with atmosphere and ocean trends during the Spring but start to gain greater reliability once more heading towards May. These forecasts are generally becoming less supportive of El Nino than they have been, with indications it will not last far into summer. What is important here is not so much the base state per se, but the travel of direction of the relationship the atmosphere adopts to it and takes the lead. A downward spiral of Pacific SST's implies westerly wind bursts scrubbed out at the expense of strengthened trade winds and consequentially - an associated falling angular momentum trend with the amplification wavelength dictated well upstream in the Pacific rather than our closer downstream European sector The MJO looks due to return to the Western Hemisphere later in May at the end of its cycle, and much will depend on how successfully the tropical wave will imprint the standing wave in the Pacific as it passes through the region and provide maximum upside to angular momentum before it starts to fall back again as it completes. Any shortfall in this, lends itself to a collapse of AAM tendency at the most pivotal time as seasonal wavelengths attune to summer changeover. Cautionary caveat attached to any CFS forecast and associated model biases, but based on the programmed evolution of the forthcoming tropical cycle, this prediction has good enough credibility and is fully consistent with the daily updating trends. In any shortfall scenario (should it verify) the baseline for global momentum would fall below that of the low point just arrived at in April which in itself followed the descent from the late winter "peak" of this weak El Nino. Such a situation lends itself to a quite different scenario outlined in the above post with the Atlantic trough and Western Europe/Scandinavian ridge scenario reversing to an Atlantic ridge and downstream trough. Therefore cool, changeable and showery rather than any warm/very warm plume scenarios. Clearly its far too soon to know exactly how this is going to play out - so no need for any undue suspicions just yet. Its too close to call and warm scenario A could yet still prevail. However, May as often is the case, looks set to be a crossroads month for the high season that follows it. So its worth in my opinion looking at possible things to look out for in the modelling in the coming few weeks, and the scenarios and different pathways that might start evolving as we get nearer and nearer to the start of summer proper.

It is indeed proving to be the case that the models have been over progressive with the eastward intrusion of the trough - too far east (and for too long) General NWP culpability evident - but just as illustration here is the ECM ensemble change even over 24 hrs representing the early part of next week @Singularityhas nicely described the tropical>extra tropical forcing in the mix which is behind some of the NWP erraticism and which also encompasses my own suggestions in the wider post (of which the selected extract above comes from) re CCKW/MJO progression that might cause the models to find something of a reverse gear on the processes of extending the trough too far east into and across the UK. The downstream ridging proving more resilient than the modelling suggesting. The other part of the equation is the seasonal transition within the stratosphere from its winter default vortex position to its summer state. This is bound to increase -ve momentum (easterly) zonal wind activity across the pole and manifest blocking tendencies at high latitude which adds further to NWP vicissitude. This cross section of the atmosphere layers clearly shows the stratospheric/tropospheric evolutionary story since the SSW of January and the equatorial flux of AAM anomalies which resulted in sub tropical and mid latitude blocking (including the spectacular late Feb "heatwave") rather than the expected high latitude cold block coming courtesy of poleward momentum of +AAM anomalies - usually seen under conditions of active MJO tropical convection propagating towards, and then through the Pacific. To follow this immediate post SSW period, the return of zonal westerlies and re-organisation of the polar vortex apparent as per the blue anomaly representation through the layers of the atmosphere during March. In turn, now leading to the relatively late final warming of the stratosphere which is also complicating the models handling of tropospheric placement of these blocks . This interaction of seasonal downwelling from the arctic with influence of maladjusted angular momentum related wind-flow patterns at mid latitude, is proving to reduce the amount of Pacific amplification that has been expected by NWP - and in turn the amount of downstream polar jet flow extending the fragmenting and decaying end of season vortex and resulting trough out of Canada and US across the Atlantic. Furthermore, and to complicate matters further, the trend of equatorial zonal winds related to the Quasi Biennial Oscillation (QBO) are slowly increasing westerly in the new +ve phase. This suggests at least at face value that effects of any "dynamic" tropospheric fall-out from final warming (easterly zonal wind anomalies) of the stratosphere into the troposphere should be mitigated, relatively supressed and shorter lived - despite the late break-up (certainly later than last year c/o Feb SSW 2018). QBO Calculated at NOAA/ESRL PSD 30mb zonal wind at the equator, zonal average For info https://www.esrl.noaa.gov/psd/data/climateindices/list/ 2018: -19.02 -19.37 -19.77 -21.41 -24.23 -28.45 -29.10 -20.41 -9.91 -2.79 3.36 + 8.05 2019: +9.02 +9.25 +11.82 (Jan, Feb and March figures) Mitigation of HLB sustaining too long also probably aided by a weak El Nino forcing which acts to help the ITCZ further north in late Spring heading into early summer sooner than under low angular momentum La Nina conditions. If we take a look at where globally averaged angular momentum is at what is looking like the bottom of the "lull" in the tropical>extra tropical wind-flow cycle, then there is scope for quite a pre-summer rally to boost prospects for warm spells in this run-up period. Still a little above average at this time, having lost the momentum peak of late winter Forthcoming prospects? As tropical forcing progresses eastwards through the tropics and westerly wind inertia builds in response, this increases +ve frictional torque tendency at the inception point where these westerlies add momentum to the atmosphere - starting the process of angular momentum tendency rising once more. Notice the red anomalies (westerly winds) progressing towards the Western Pacific as the easterly inertia (blue shading) ahead of the tropical wave is scrubbed out with time Another way of looking at the wind anomaly progression across the tropics is through the VP200 convectional anomaly chart - the convectional suppression (yellow and orange shadings) presently over the Pacific gradually being eroded by the advancing -ve convection anomalies heading from the Eastern Indian Ocean through the Maritimes and towards the West Pacific (equal to MJO Phases 3,4,5-6 on deterministic model MJO plots) These eastward progressing westerlies added to the atmosphere circulation from the tropics register a response from the Global Wind Oscillation (a phase depiction of wind-flow pattern additions and subtractions) and result in orbit from falling angular momentum Phase 8 to rising angular momentum Phase 4. This likely is a prelude to the GWO fully engaging the El Nino attractor phases 5,6 and 7 once more in the further outlook as the Nino standing wave in the Pacific is re-engaged during May and hence why in my opinion NWP is starting to advertise a ridge response once more rather than sustaining the trough On this basis, assuming active tropical >extra tropical forcing sustaining angular momentum tendency through the greater part of May c/o weak El Nino circulation and aligned with w/QBO, increasingly inhibiting Atlantic and Greenland blocking heading towards the important seasonal wavelength setting as Spring turns towards Summer, then whilst such a pattern will inevitably sustain backwards and then forwards once more- its not unreasonable to hypothesise further re-setting of the Atlantic trough and downstream W/European ridge and re-cycling of increasingly warm patterns from an ever warming continent as the days keep growing longer. In the shorter medium days 5 to 10 period, its a case of seeing how the models resolve the ridge placement now that the ejected trough is backed west into the Atlantic. This will determine how much cooler air advection comes south before the suggested higher heights close to Greenland fade in the longer term and hopefully set up a mid latitude ridge as day 10 ensemble means are trying to converge on. To copy paste the last line of the quoted post however : "A case though of keep watching to check this evolution stays constant and seeing how NWP responds"

Extract taken from post of last week to assess progress, at the end of a beautiful Easter weekend and when some of us are looking for clues when the next warm and settled spell may appear Had been looking for some further evidence to keep backing up the quoted suggestion and to prevent any slippery slope into a late Spring cool trough gaining further traction into a more extended period. Here it is: This velocity potential (VP) 200 anomaly cross-section represents an eastward moving convectively coupled kelvin wave (CCKW) modelled to cross the Pacific during the first half of May. This matters because it provides a further boost to westerly wind bursts across the Pacific (to follow the current ongoing increased easterly trade winds close to the dateline) ensuring that upwelling of sub surface warmth can prevent any continued decline in this weak El Nino - and re-set the rossby wave pattern downstream once again back to being pre-disposed to building ridges instead of programming a persisting trough In association with the CCKW, deterministic MJO modelling has started to pick up on increased eastward progressing amplitude activity which should support a recovery in angular momentum tendency to resurrect from the the current downturn as part of a next uptick in ridging to replace the trough Unsettled for a time, yes - however its conceivable that the models might over programme the duration of the trough, but in any case, there are at least signs that the suggested improvement has some reasonable support to ensure that further fine weather has a chance of returning following the upcoming unsettled, cooler and windier spell A case though of keep watching to check this evolution stays constant and seeing how NWP responds

Hi James - a thoughtful summary and deserves a response I think in the context of seasonal wavelength changes in addition to the transition period of the stratosphere to its "summer" position there is bound to be flips in patterns when wind-flow patterns undergo sequences of retractions and then forward momentum once more as we have been seeing this Spring - and is quite common to see in many Springs, notwithstanding the unprecedented changes in climate that inevitably will amplify and/or distort responses. But the physical responses of changes in wind-flow patterns and the torque mechanisms that drive these changes remain constant nonetheless and continue to play an important role in attempting to decipher pattern changes. In that respect its another reminder that its not the ENSO base state per se that matters in terms of direct correlation, but the relationship that the atmosphere adopts to it in terms of weather pattern adjustments (in tandem with all the factors between the seasonal changes between tropical and polar stratospheres and also of course how the seasons impact sea ice at such pivotal times of switch. What of those wind-flows? We can see from the Hovmollers plot that since the first few days of the month we have seen a drop in a long period of westerly wind bursts across the Western Pacific towards the dateline, as part of a response to tropical convection patterns having finally weakened through March following a very long active period since November. With an imminent trade wind burst in the Eastern Pacific as can be seen shaded blue on the plot this represents a peaking of something of a "lull" phase in this Nino phase - which is not unusual in any ENSO cycle. The effect of greater easterly trade wind inertia or reduction of Nino westerly's c/o eastward progressing tropical convection is to create a deceleration of wind-flows upstream in the Pacific and a consequential fall in angular momentum tendency Initially this serves to weaken the Jetstream heading downstream across the Atlantic, and hence has helped to promote and preserve the downstream ridge across Scandinavia, but as at the same time as the lobes of polar vortex start fragmenting in response to approach of final warming of the stratosphere, then breakaway troughs appear such as programmed next week to disrupt from Greenland to close to the west of the UK. Meantime the downstream ridge is preserved to the east and north and east as the negatively tilted trough process gets underway as modelled. The response of the Global Wind Oscillation, which is a plot depiction of these changes of wind-flow, is to head towards Phase 8, as reflection of falling angular momentum tendency of at least temporary loss of surfeit Nino westerly wind inertia in the global atmosphere. This gradual orbit from the Nino phase 7 represents the first part of the jet retraction process of recent days with the blocked Scandinavian High. The second part of the process is the trough splitting the ridging pattern as the amplifying Bermuda High hoovers up our ridging to the west and allows pressure to drop in its place. There are indications from the modelling that this retrogressive process c/o decelerating tropospheric and stratospheric zonal winds might complete with a further backing west of the trough and pressure rising from the SE to follow the more unsettled phase starting from the end of the Easter weekend. This needs to be watched to see how it might unfold for the extended period in ensemble products in the next few days

I stepped aside of the winter season on this site for a variety of separate, but also rather inter related reasons - and the perennially occurring type of nit-picking discussion exchange on this page is just one of them. Nothing changes here - even if the UK maritime driven weather at a crossroads to a large continental landmass traditionally changes back and forth as always The reason I am exceptionally reappearing and responding to the latest sequence of misrepresentation and erroneous judgement from the posts in accord with the sentiments of alleged arrogance expressed above is because they blight not only the most busy winter period, but are demonstrated, smaller scale, a lack of respectful judgement and correct perception all year round. The original culprit most generally attributed to extreme over focus and expectation built around one preference bias solution according to the season present - that is bound to increase risk of error rate if all other probabilistic solutions, that my be present, are discounted. Such error rate leads inevitably to predictable disappointment, and equally predictable finger pointing and baseless unhelpful and judgemental blame game over the failure as part of the perennial ritual. Sadly the methodology (and methodologist) suffers the fate rather than the erroneous sky high bias expectation that feeds the negative reasctions Several things: 1) The diagnostic approach, commonly referred to as "background signals" is *supposed to be* a guide to help assess a pattern framework on a macro scale basis using intra seasonal timescales and feedbacks from recurrence factors existing within the tropics and extra tropics. These then invariably become dumbed down in acronym land as silver bullet snow-making mechanisms (in winter), hiding and invalidating the real efforts in attempting to learn and then reference their proper use 2) The reason that posts might start with statements such as "models are not picking up the signals" is not to brook arrogance and supremacy over computer software, or express the gift of a clairvoyant that other posters simply cannot possess - but as a means of matching a wind-flow (jet stream) diagnostic evident at any given time, to the "drawings" of numerical modelling and stating an interpretation of where NWP may evolve differently in future according to expected changes of wind-flow propagation from the tropics to extra tropics. It is therefore disrespectful and cynical to imply true motives of anyone styling their posts this way are disingenuous otherwise. - irrespective of the final outcome. 3) The diagnostic is extra complicated in the winter through consideration of the seasonal polar vortex - but the tropospheric wind-flow diagnostic remains an essentially valuable guide as to determining how the different levels of the atmosphere may behave, and whether they interact or assume a disconnected or overriding state. None more so than this season for example. 4) Clearly a very complex boundary area and formidable to attempt to dissect, and humans will continue to make mistakes and hopefully learn from those mistakes (if they are allowed the grace to) - but the advent of putting stratospheric research collected over some number of years into active practice on both an amateur and professional level showed it has been possible to make mainstream breakthrough in this aspect of science. Most importantly a degree of measured acceptance and respect.. That has proved inspiring to many, and many who have sought to attempt the challenges on sites such as this and others. Regardless of starting level of knowledge. Therefore equally, a tropospheric diagnostic that can help assess how conducive a hemisphere pattern *may be* to amplify through propagation of planetary waves (which may in turn impact the stratosphere in winter through the assistance of extra tropical mountain torque processes) is an invaluable complimentary guide next to stratospheric practice, to try to start disseminating its own processes. The "bottom upwards" approach within the atmosphere has also therefore started to become inspiring to many. in the same way as stratospheric research pushed boundaries. 5) However, the value of these diagnostics has to be to assess all probabilistic outcomes that may be evident, and so any post starting with "the models are not picking up the signals" is best attributed to the extent of all probabilities and not just the favoured bias solution. 6) It is certainly possible in the pursuit of humility to caveat posts with the disclaimer that more than just simply the bias solution is present (and I for one used to attempt to do this) but this clashes with the dichotomy apparent of where cold science meets weather enthusiast. Both have an obvious valid place. But on an internet forum there is never going to be a one size fits all answer to this and parochial attitudes relating to "background signals" will always exist on the one hand when the bias solution goes AWOL - and on the other hand intellectual snobbery will exist of those deemed allegedly trying to be too clever for their own good. 7) With all this mind, I don't see a duty of anyone to have to come back by order of proxy to explain where they went wrong, or why their allegedly "lofty" stance has ended in failure. I am sure most would wish to do this of their own volition, in the cause of positive explanation and assist to others as to what may have happened if and when the time was pertinent to do so . And last but not least as part of a process of learning from mistakes made themselves. But there is no contractual obligation to do so. I am sure all this does is serve to create wariness of ensuring they do not come back at all In conclusion I would suggest those who, commendably, persist still on these pages with the diagnostic approach, at any (or all) levels of the atmosphere are therefore between a rock and a hard place and will continue to not satisfy anyone as long as the true purpose of their posting is discredited and picked apart accordingly - through not pleasing the different ends of the reader spectrum. Minus a salary (in some cases) as just very ordinary "weather students" who are simply wanting to test their own curiosity.

The coming circa 10 days are fully documented in the +ve momentum sequences and timelines as described in detail. In my own words, and not your incorrect interpretation, representation of them this translated to 'highly amplified' and not 'weak amplification'. The models are duly following an amplified pattern within the 10 day time period. If you must dissect mine or anyone else post please do not put words into mouths. Thank you. Is an amplification temporary taken in context of the period under the radar within my own post evolved? Its still late autumn for another two weeks. before winter proper begins There is much one *could* reply wrt Ed Berry's presentation and your own interpretation of what you consider short-comings, but this is not the thread for it and I suspect you would not be interested anyway as your focus skews approaches you do not personally subscribe to as restricted to the attaining and sustaining of cold weather patterns. However, the GSDM is created as an all year round diagnostic tool to supplement numerical modelling, not as a mechanism to fit signals/teleconnections into the desire for cold weather patterns 3 or 4 months in a year. I personally attempt to utilise these processes during the summer and adopt the same perspective as of today of fitting them to likely patterns and not preferred ones. Hence a pros and cons approach. Plus I do not pretend to be any self appointed clairvoyant forecaster anyway - simply try to progress understanding and learning. And offer a differing perspective that hopes to avoid too much bias loading. Orbit of the GWO through the Nino attractor phases does support retrograding to Iceland, Being a long post and only so much time and space to complete it, I omitted to state it and certainly accept that could have been made clearer- However, with that said and in support of the thrust I was emphasising - the focus was ultimately questioning what happens when the amplitude tropical forcing fades and angular momentum falls back. It seems you have chosen to omit that part to try to further discredit my post, A fall back in angular momentum would imply some recovery in polar jet flow and a possible return to a more mid latitude pattern. Clearly the Scandinavian ridge has mileage to support a higher latitude pattern in the medium term, but again in that post I was, and still am, seeing this in perspective of it being late autumn still and not winter itself This is the UK and not the tundra - putting aside the likes of 62/63 even the coldest winters are not uniformly cold and allow for relaxation of the pattern, Unless you are suggesting this is the start of a protracted uninterrupted 3 month freeze? Maybe you are suggesting just that, judging from your prophecy further down. Yes I had already fully anticipated you would scour it before your post appeared. Some things can be wholly predicted ahead of time. If I was making a 'formal' LRF (which I wasn't) and detailing every single forcing variable,(which wasn't the purpose of the post) I wouldn't have posted it in this thread. And momentum processes within the troposphere can be both augmented and detracted by unstable polar temperature and ice profiles. Again not the thread for this debate. Your own priority is to lead a charge for cold patterns and find reasons to support them both now and in the future. Different priorities, and also your own somewhat skewed (and incorrect) understanding and perspective of the GSDM, does not make the diagnostics wrong even if as a willing interpreter of the GSDM I am not bullet proof like anyone else when it comes to making forward suggestions based on them or any other method. Whether those are based on numerical models, or other approaches. Maybe you might include yourself in that bracket as a mortal who can (and does) get things wrong as well?.... .....Or perhaps not. You are to be envied and possess a certainty of the future that surpasses anyone (professional or amateur and regardless of contributory approach) and so there is nothing left to discuss And as stated previously, I'm heading back to my planet anyway So just let it snow.. See some of you maybe next year

This quoted statement strikes quite a chord - looking in as I am from the outside of the thick panelled window of this particular utopian world The METO are professionals and interpret numerical model data, as well as utilising the considerable investment they have in tropical modelling - and make their own conclusions. As such they are completely divorced from any hysteria attached to weather preferences. One can of course dispute their own findings and conclusions based on other collective/ individual professional or amateur analysis and many rightly do this. Assuming that is, that such alternative offerings remain credible in themselves and designated such that the signals are derived from applied meteorological science, and so fit a realistic range of probabilistic solutions, and not merely as given lip service to try to fit an idealised one Sensibly, the Met are quite right to state things as they see them - and not instead on a regular basis feel obliged to please (and equally reassure!) their public of any perceived preferences they may have through making extemporaneous interjections in the interests of satisfying bias neurosis. Therefore as a consequence making a skewed mockery of probabilistic solutions (and making them redundant as a credible service provider at the same time!) On that basis, the idea of them trying to inject a little calm into the proceedings takes on a highly absurd quality if one stops to consider that the "hunt for cold" thread world is a very different planet - objectivity and non pre-occupation with ice age synoptics being deemed boringly alien and kill-joy because it spoils the entertainment and threatens a tribally protected species. So, with that in mind, some one-off attempted analysis before vanishing back to my own planet of Venus. Process of +ve momentum transport c/o CCKW driven tropical convection has been progressing through I/O and has spiked frictional torque tendency +ve with associated westerly wind burst anomalies representative of this around 30N (orange shading) The frictional torque response reflects the inflexion point of where additional westerly winds are added to the atmospheric circulation through progression of advancing organised MJO thunderstorm development - and convergence of macro scale wind-flows creates a turning force on the Jetstream and changes the direction of travel of relative angular momentum. So this increased +ve momentum transport and associated +ve frictional torque response registers immediately as a fairly significant spike in atmospheric angular momentum tendency - - A mechanism that leads towards programming the highly amplified pattern as advertised by numerical models within the atmospheric circulation, as according to the spatial poleward propagation of those +AAM anomalies over time. It is the lagged timescales according to this progression which provides the lead for further +ve mountain torque tendency in the extra tropics and sustains the amplified pattern through the majority of this momentum accounted for over the Himalayan ranges, setting up an extended Pacific jet and supporting phasing of energy into sub tropical flow (of which the models are presently balancing the equation over the medium term) The 'however' that is coming is that the adage is "what goes up must come down". Catalysts as they are for major pattern changes, there is no indication (to me at least) that the present tropical convection cycle matches those that have been a prequel to some of the classic early to mid winters. Meaning by this that the tropical signal aborting and losing amplitude sooner as seems currently likely than on those occasions implies a faster reverse scrubbing out of more sustained +ve momentum to follow that may, in turn, prevent the upscaling of the switch to an ever colder and colder pattern as some of the extended numerical model fantasy, filling the cracks of these pages, panders to. So a complicated sequence in terms of the lagged +ve momentum propagation which impacts the tropospheric/stratospheric pathway in the second half of November and reverse sequence to follow reflecting loss of the tropical signal which suggests greater polar jet flow in relative terms to the generally weak vortex and some relaxation, at least for a time, of -ve AO. This makes making any prediction moving forward sensibly erred towards caution this side of the glass window, and irrespective of any personal preference. Unfortunately the link to the consolidated Global Wind Oscillation plot has been down for some time to confirm precise position, But taking into account the relationship existing between an atmosphere co-operating well with an establishing Nino standing wave in the Pacific, as reflected by GLAAM retaining generally around a +1SD to parity... ….is suggestive and supportive of the GWO progressive into the El Nino attractor phase 5 (illustrative of +ve extra tropical momentum poleward propagation as discussed above) to be followed by an orbit through the Nino phases and depending on how far AAM falls thereafter, possibly returning to Phase 8/0. These phases within the tropical>extra tropical "mini ENSO cycle" being in simplest terms indicative of a 'waning' sequence during the major long term evolving +ve ENSO phase This forcing sequence roughly translates to the possibility of Scandinavian ridge and euro trough morphing into mid latitude Atlantic /UK ridge as the Pacific pattern re-amplifies somewhat (GWO Phase 8/0) following the relaxing of the +ve momentum phasing, switching some energy back into polar jet flow There is also a notion of argument to subscribe to, that the +ve QBO transition has already passed its optimum tropospheric/stratospheric disruption potential as it continues to downwell further, and moving ahead the onus becomes increasingly further on active amplitude tropical forcing resuming to re-stock +ve poleward +AAM transport processes and keep the tropospheric/stratospheric boundary unstable in the face of ever colder mid and upper stratospheric layers- and so manage to sustain blocking mechanisms at higher latitude. This occurred during early/mid winter 2009/10 under an emerging El Nino, bottom rung of solar forcing and being within -ve QBO phase and away from advanced westerly transition - but, notwithstanding polar stratospheric ozone distribution and concentration is (presently)elevated indicating the Brewer Dobson circulation between tropical and polar stratosphere is at least (currently) buoyant, the variables are not aligned the same this year. Any shortfall in all these respects renders the chances of a closer struggle between the upper and lower atmosphere, and a 'less warm' tropopause boundary which alters the complexion of the establishing Nino pattern evolving more towards a mid latitude European blocked one rather than one sustained long term between vectors at higher latitude to the NE and NW. Following what for some off us, away from the sole priorities of this thread, is a very pleasant and highly welcomed mid November week of weather to help save unnecessary heating bills, its certainly a cold outlook with perhaps some equally cold rain to match it and possible marginal wet wintry mixes for a few But whether it morphs into the freeze frenzy that is the utopian paradise the other side of the glass from where I am sitting, remains (at least in my humble opinion) yet to be seen.

Yes I stated the other day that August had featured a cancelling out process of the tropics vs the extra tropics, but one that still appeared to be cuing up very nicely for the re-start of the next atmospheric circulation to resume the progress up to late July. Which up till then had beautifully read the script and meant that not just was the weather itself lovely to savour, but also there was satisfaction in seeing the difficult processes and explanations play out and provide personal happiness and enjoyment of attempting to oversee developments as well. Humility is indeed a vital quality, and one of the first we should all should try to eschew - but on the other hand no-one earns a salary to take time out to post on this thread and on that basis it should be a personally enjoyable and rewarding experience at the same time as one shouldn't be beholden to expectations of total clairvoyency in terms of suggestions made ahead as to how the patterns may evolve. On that basis maybe not too many apologies should be issued for not reading the weather patterns correct 100% of the time should be made - the likes of the METO as a professional institution would not provide a service at all if they had to issue apologies and explanations for every time Mother Nature didn't follow the best measured expectations. Additionally, the complexities of the atmosphere defeating human attempts to study them, does not justify those minorities who always seem to appear at these times to suggest the 'teleconnections don't work'. They almost certainly do work - its just the hardest task to try and understand them, relay and explain them - and those that don't try but criticise first and foremost instead - should practice their own humility first Such sentiments of humility within these contexts become even more important as the audience no doubt starts increasing and changes ahead of yet another autumn>winter chase for cold patterns. Its my experience that whilst most on this thread, and large parts of the site as a whole, consider the busy season the most exciting and absorbing, it is also the time that unfortunately humility is usually least evident. With all the above in mind, this is the last post on this thread of the sequence of summaries since May covering this wonderful summer season just ended ( a small few at least will no doubt be relieved and delighted to see the back of the never ending length posts) and it has been a wonderful summer despite the relative disappointments of August and the now apparent stuttering of developments for any further sustained warm and settled conditions in this month also. The starting point of discussion far back in May was whether the step change to a contrasting much warmer second half of the Spring from the first half would successfully negotiate the hurdles of previous years where warm late Springs have only led on to a downward path to cool and more unsettled conditions. The pros and cons revolved around stubborn persistence of La Nina type circulation lag set against the suggestion of gradual changes to more Nino-ish patterns in later summer>autumn and hence trying to 'risk assess' mid Atlantic ridge propensity for preventing warm air advection building sustainably (and more especially from the south) or, whether a greater eastward extension of ridging would prevent this happening (again) and instead , over time, encourage development towards trough activity to our west in the Atlantic and further assist plume like activity from a southerly vector to take over from the Azores High. As it turned out the ocean signal indeed continued to build towards an El Nino standing wave, whilst the atmosphere in terms of the extra tropics rather lagged the vestiges of the La Nina regime with low angular momentum and easterly trade wind patterns evident during June. The Atlantic pattern in the first half of summer was in many ways was not too dissimilar to a traditional La Nina summer pattern with the core heights centred around an intense sub tropical Azores ridge,and one which occasionally waxes but mostly retracts westwards due to the effect of upstream amplification c/o easterly trade wind domination in the Pacific However, this time around, the the unusually north and eastwards displacement of these core Azores/Atlantic ridging heights means that the downstream trough that is usually reserved for the UK and NW Europe was erased quite some distance towards NW parts of Russia So instead of customary low pressure upper cold pool - to the much greater extent and by way of total contrast, home grown heat build in situ from these ridges with the NW of the UK first to benefit from this. As the summer progressed, and as the Pacific pattern started to couple the atmosphere towards the steadily warming ocean signal, the pattern did indeed start to signal a gradual transfer of core heights NE - culminating in (what turned out to be) the one and only real plume scenario of the whole summer in late July with the much anticipated and discussed Scandinavian heights and Atlantic trough. More of these plume scenarios into late summer and early autumn were envisaged and commented on several times.This late July Pacific forcing passage has however unexpectedly proved to a brief culmination of steady development since Spring What was subsequently envisaged some four weeks back was a res-set of the late July pattern through the middle part of August to repeat the sequence during the last third/last week of August. As we know, and as has been reasoned and explained, this sequence was delayed and August turned out to be more changeable and cooler for longer than seemed likely. So to September, and the modelling expectations of upturn once more in angular momentum remain present, but seem to keep remaining in real time at the end of arms reach. This not just applying to the CFS and its well know biases (those starting to re-appear here for winter attendance should beware its tropospheric>stratospheric modelling) but the ECM is also proving culpable to its own biases much as the GFS with its own -ve tropical biases. *So famous last words of hers when she said that seasonal modelling of AAM had been proving reliable* Ha-ha! Another example of freely given humility.. Extrapolations of re-engagement of the Pacific standing wave have stayed around the equivalent of chasing optimum synoptics at day 10 with NWP. Instead we see a return to Western Hemisphere engagement of VP200 tropical convection anomaly at the expense of the Pacific. This renders suggestion of recently rather void that the opposite seemed likely to prevail in the Pacific instead. At the same time no surprise the African wave signifies greater hurricane activity transferred from the Pacific back to the tropical Atlantic. An echo closer to September 2017, and against wider earlier expectations for September 2018 The greater -ve tendency in the atmospheric circulation sees transfer into -ve Asian mountain torque tendency in the extra tropics This is the signal to amplify the Pacific profile through deceleration of the Jetstream there, and downstream retracts the sub tropical ridging in the Atlantic. This ends the recent attempts for Scandinavian heights to prevail along with the suggested longer settled spell some days back and also means that the attempted re-set of the ridge from the SW is more supressed from the weekend allowing the jet further south and greater trough intervention It had been envisaged the GWO might hold ground in weak signal and then resume progress to higher AAM orbit sooner. However, eventual capitulation in the extra tropics (as discussed above) has been sufficient to re-engage the Global Wind Oscillation and end the long period of resistance in weak and incoherent orbit with a move into low angular momentum Phase 1. This definitely precludes any immediate re-positioning close to the opposite end of the spectrum in El Nino Phase 5 (as briefly achieved in late July at 'peak' of the Nino standing wave development). Its now about revising expectations in relation to how the eventual natural upswing in the tropical>extra tropical pattern can recover angular momentum back to a lower amplitude Phase 4 This reduced orbit expectation wavelength is indicative of the faltering of development of the progress to the expectations of weak El Nino through this autumn and then into winter. Its not a preference of mine at all, but not a surprise either in the 10 day period to see the models move away from warmer and more settled solutions as the re-engagement of higher angular momentum tendency keeps being put back in time. Instead the mid Atlantic ridge is being programmed in most modelling at the moment to engage a greater balance of polar maritime air, keep the jet stream straddling close to the UK and prevent it lifting further north and allowing sub tropical warmth to approach from the south and build back an Indian Summer. This said, the GEM has been suggesting that some cut off may occur in the jet stream to our west and provide a break on advancement of the migrating Atlantic ridge and a loop of the jet to our S and SW to engage something of a plume scenario through next week. This is not an uncommon scenario based on past similar situations, and would be lovely to see emerge - though at present it remains a low chance probability. Something will give with this situation at some stage, as natural tropical.. extra tropical cycles dictate but essentially the low frequency signal in the Pacific has gone more AWOL since August at precisely the time that it was set not just to allow August to repeat the feats of 1976 and 1995 - but quite conceivably surpass them with a marvellous September as well. 'What ifs' and 'could have been' aside, its still been a fantastic summer and thanks should go to the regular contributors on this thread who have taken time to provide model sum ups and also enjoy the positivity of the weather suggested by them, at least until relatively recently taking the season as a whole. A long way it seems from now until next summer, but time never stands still

The initial estimates, have proved to be a little on the extended end of the envelope ( and hence the fate of the last week or so of Augusts prospects less inspirational than hoped). But overall the pattern has progressed reasonably well in terms of expectations as expressed initially some 3 weeks back. The one part of the jigsaw remaining, and which has been proving slower still to appear ( up to now) than envisaged is an upper trough showing signs of diffusing energy underneath any of the blocking. This element is actually now materialising in NWP, however the suggested difference to the early analysis is that it might manifest itself amidst more of a -AO regime rather than the long prevailing +AO regime. Taken at face value, this implies quite a possible difference in surface conditions to those envisaged in the captured quote because a less +ve NAO set against a still prevailing +AO infers a mid latitude Sceuro block with upper trough in Atlantic disrupting to the SW to provide late season influx of humid air northwards (as per early August assessment) On the other hand, and purely at face value (which is an important thought to retain) a -ve AO profile suggests the type of smorgasboard on offer from elements of latest NWP - with cooler upper air seeping around the blocking re-positioned at higher latitudes and a hard to pinpoint array of shallow troughs appearing in the upper cold pools of these troughs, at the same time as the direction of approach from troughs approaching from the Atlantic suggests possible fusion with these shallow features to erode the higher pressure regime further and further northwards. But, as implied, this is getting way too far ahead in detail at a range that is still open to question in terms of why the broadscale changes are being suggested - and of course how likely and to what extent might they might verify. On that basis, probably best to do some analysis of events since early-mid August. to try to make sense of it all moving forward This period has been frustrated by some contradictory signals between the tropics and the extra tropics. The former attempting to resurrect something of a Nina flow with increase in easterly trade winds amplifying the pattern upstream (Atlantic/more supressed ridge and greater trough influence) and the extra tropics which have retained much of the westerly wind inertia of the tropical signal in late July (with the Scandinavian ridge and Atlantic trough that concluded the final days of that month) The tropical>extra tropical cycle is looking to set to resurrect this sequence again through September (as stated at the beginning, slightly later than first thought a few weeks back) and its no surprise to see the Scandinavian heights programmed accordingly. Total global angular momentum is currently broadly neutral... ..., despite the on-going lull in the low frequency tropical>extra tropical cycle and has been largely dictated by micro scale tropical activity to a small degree in the Atlantic but more especially the Pacific as one might expect with an El Nino standing wave erratically implementing itself. This balance of activity has helped support westerly wind inertia in the extra tropics whilst the easterly trade wind inertia in the tropics c/o -ve South Pacific Oscillation influence (SPO) and also weak phase of the low frequency signal since the start of August has been represented by -ve frictional torque tendency which is now starting to trend more +ve (ahead of likely changes upwards in angular momentum tendency through September as discussed later) The propagation of those -ve easterly winds in the tropics and subsequent degree of amplification in the Pacific has been negated of late in the extra tropics through the continued presence of the developing Nino standing wave and high levels of accumulated cyclonic energy (ACE) in the Pacific. Hence any -ve Asian MT tendency, which serves to help decelerate the jet in the Pacific in response to the lead set by -ve frictional torque tendency from the tropics, has shown a much more muted response to the previous (much greater) trade wind response that propagated to the extra tropics in early summer and prevented total global angular momentum from repeating the falls to the low point indicated of 8 to 12 weeks previously... ...and which at this time of year would be manifested as a very different outcome not just to the conditions of early summer, but to what we presently see nowcast synoptically according to the growing shift in seasonal wavelengths since then Kind courtesy of @Snowy Hibbo if we look at an ECM plot of forecasted angular momentum tendency,the advertised rise through September as mentioned earlier on, is in evidence. Backing up the CFS seasonal expectations (of which has proved a little progressive with the Nino standing wave) Note that both of these plots are overall tendency plot forecasts which have to be set against where total global AAM currently sits close to parity. The upward trend in tendency imminently commencing from the current 'low point' (and as indicated by the rising frictional torque tendency plot as above) helps bring about the re-set return of extended heights to the NE and the suggested approaching trough from the west to replay the sort of sequence seen late July. So this leads to the question of the suggested difference to late July in terms of the switch of heights from mid latitude orientation to progress to higher latitudes. A large part of the complex global blocking picture is the effects of a northwards recurvature of super typhoon Jebi through the North Pacific (with ominous threat to Japan), as part of a tropical>extra tropical interaction creating a knock-on anticyclonic wave breaking pattern which shakes up the NWP picture in term of trying to fit the precise positioning of these blocks set against the broadscale factors already outlined. There has been uncertainty as to the speed of downwelling westerly phase of QBO looking ahead through autumn (and then into implications for winter). This appears, to be now rapidly advancing through 30mb as is often the case when phase switches are becoming increasingly imminent Much the same principle as stratospheric vortex switch regimes, the advancement of +ve westerly winds starts flushing out the existing -ve easterly phase winds to the surface and these manifest themselves as they exit into the tropospheric surface level as +ve height anomalies. So on the basis of so many factors in play, then a lot of caution is needed in terms of NWP and how all the cards in the air fall to the deck. The background trend in angular momentum tendency to be rising through September well supports the overall continued evolution to an Atlantic trough and one which looks increasingly likely to undercut where the blocking mechanisms ultimately position themselves. The higher AAM potentially climbs through autumn then the more sub tropical jet energy seems likely. In terms of detail this is obviously almost impossible to gauge, but suffice to say and as per some previous analysis during August, the more unsettled conditions perhaps further south and south west and more settled the further north one heads. But with the suggested changes in latitude orientation of the high, this makes interpretation less reliable than was envisaged with a more mid latitude block. Nonetheless, with residual summer heat to the south, the track of any meandering troughs could have big local impacts in terms of boundaries of temperature and rainfall anomalies from one place to another. But I suspect based on the presence of troughs appearing more and more to the west and south west that any cool air filtering around these large ridges further north and north east is going to have a hard time this early advecting far south and west enough to the greater part of the UK and the potential remains for very warm unstable and humid air, (especially with SST's still close to peak from the summer) to be some influence furthest south.

The best answer, in my opinion, lies within the bolded section of the first post extract made earlier this week. So no garden path exists so long as every operational and ensemble NWP set isn't taken at face value Latest angular momentum update c/o the Global Wind Oscillation continues to show the support from within the extra tropics to add westerly winds to the atmospheric circulation which essentially are helping to neutralise attempts to make AAM fall more substantively through easterly trades currently added within the tropics. This means a slumbering stalled orbit is in evidence reflective of latest weak low frequency tropical forcing signal rather than any orbit fall-back. This limits the degree of persistence of an Atlantic ridge as previously suggested and explained. . This willingness for the extra tropics to support angular momentum shows that the atmosphere is going along with the on-going slow and erratic movement towards a weak El Nino through the coming autumn. The presence of a further convectively coupled oceanic kelvin wave developed in the Pacific is, as Mr Ventrice describes, of a greater amplitude event than the one that helped propagate warmer waters across the Pacific during Spring and bring La Nina to a close in early summer - and should further help develop the Nino trend in the coming few months. This suggests further support for angular momentum in the longer term and also the prospect of a sharp upward surge in AAM tendency as soon the tropics spring to life again across the warmest waters of the central Pacific. With this in mind, many of these post summaries have anticipated an atmospheric response to ocean CCKW signal by way of an amplitude MJO wave, and this continues to be the case. Seasonal modelling maybe inclined to be progressive with this signa in the last couple of weeks, and I will happily admit my own weather pattern enthusiast wish to see this happen is hard to contain and not resist over anticipating. However, it still objectively looks to be a correct signal and should have impact repercussions for the global pattern this coming autumn based on timeline recurrence - and also quite possibly also for the development of the seasonal stratospheric vortex looking even further ahead on the basis of a perceived strong Brewer-Dobson circulation between the tropical and polar stratosphere this autumn/winter. In the meantime, latest ensemble means (ECM,GEFS and GEM) indicate (on the above proviso that one reads between the lines a bit when viewing them) there is nothing untoward at all suggested in my opinion. Further fine and warm weather still seems very probable into the new month most especially further south and east - and despite the expected changeable look to the weather heading through next week its notable how the Atlantic ridging of the more immediate term has been put into the more transitory perspective suggested in summaries of the past week to 10 days. Mostly a trough/ridge arrangement advertised and no sustainable ridge/trough - so with time a return to generally above average conditions for the time of year and still the opportunity for troughs to dig southwards to our west and inject greater humid warmth northwards and for mid level convective thundery features to occur - and that are not uncommon at this late season. These events are never especially well modelled and tend to come into full focus at relatively short timescales - but as long as the jet axis is generally SW-NE, rather than W-E or NW-SE - there is always a good chance for some extra amplification to inject some of the warmest air advection northwards September looks set for the time-being to continue to follow the long term dominant +NAO profile - but should that Pacific amplitude low frequency signal come into focus sooner than later - then this suggests the sub tropical Jetstream waking up a bit which would further upgrade late season plume and thundery prospects initially as the NAO becomes less +ve and encourages lower pressure to develop to the SW.

Overall there is little I would change from any of those excerpts from the previous post. First the matter of the programmed Atlantic ridge as explained and discussed in previous post and now well within the modelling radar. It remains the case that the cool air Atlantic ridge sequence is a snapshot in time set against what has been and what is still likely to resume thereafter. It has to be said though, those of us wishing for a summer bank holiday to reflect the stunning overall quality of the season - that the weather looks set to certainly not read that particular script and the timing is quite unerring indeed. It is looking increasingly likely that the upcoming Bank Holiday, at least for many southern and south eastern parts who have been fortunate to see the best of conditions, will be the most underwhelming for a few years, Last year featured a fast track to early autumn conditions both prior to and immediately after the Bank Holiday, however the whole holiday weekend period itself was very warm and sunny for many at least further south, echoing that of 2016 and also 2015 which were both fitting for the end of season. Some enjoyable mid level plume conditions and evening lightning displays also added an extra element in 2015, and 2016 went on to feature these conditions well into September itself Another way of measuring related analysis to the programmed Atlantic ridge sequence, is the short term tropical convection VP plots that identify the loss of signal and meandering track to the Maritimes and emerging there by the closing days of the month. This is sufficient, in the shorter term, to be amplifying the upstream Pacific pattern and also pull the ridging pattern westward downstream and invite cool air advection from the NW. Much as described in the last post. However, the emergence of the tropical signal from the Maritimes (and then subsequent track back eastward into the Pacific) is what is a key to reduced trade winds serving to de-amplify the upstream pattern once more and help return to warmer and hopefully also more settled conditions downstream in our sector with time This links on to the updated zonal wind analysis below used as focus for those who wish to welcome in autumnal type weather and which reflects the interim short term loss of tropical forcing in addition to transitional CCKW across the tropical Atlantic (which was covered in last post). I think the particular point has been made before that taking these wind anomalies plots in isolation of the broader picture associated with it tells one part of the story but not the complete picture As hinted at above, it also means that any autumnal flavour to the weather is quite likely a temporary feature with further summer-like weather delaying the eventual seasonal changes that happened faster last year. This doesn't 100% guarantee any heatwave, though still plenty of time for one to develop in the new month, and still plenty good enough to continue to enjoy outdoor life for a while yet despite the B/H spoiling things for quite a few. The bigger picture, beyond the short term analysis, remains one of trying to assess the present lull phase in the tropical cycle and associated increase in trades against a larger atmospheric/ocean trend that has been heading in the opposite direction to any La Nina type forcing through the latter part of this summer. Albeit, with erratic progress which is now proving perhaps a little slower and more frustrating than seemed to be the case (hence the window for end of official summer plume that seemed possible in the last week of August has been put back a bit as just discussed) Its worth reading the attached link which discusses the role of the South Pacific Oscillation (SPO) in regulating the strength and influence of easterly trade winds (to which the immediate post under reply reflects with the short term zonal wind anomaly forecasts) The link was provided originally kindly courtesy of @Blessed Weather https://www.climate.gov/news-features/blogs/enso/look-south-enso-forecasters A +ve phase SPO suggests a weaker South Pacific subtropical ridge and weaker trade winds reducing the cold-water upwelling in the eastern Equatorial Pacific and allows for easier propagation of the warm waters from the Central Pacific to the Eastern Pacific. This is consistent with speeding up the development of an El Nino circulation and associated high angular momentum regime. However the converse holds true with a -ve phase SPO and this means that propagation of the warmer waters from the CP to the EP are slowed and retarded. This is reflected in on-going ENSO zone profiles. It doesn't automatically mean that an El Nino won't develop at all - it just makes it likely to be slower to develop in this instance and also much higher margins to be on the weak end of the spectrum and more centrally based. This summer the SPO has been in slightly -ve phase and this has been a factor in the erratic progress of the development of the Nino standing wave as just mentioned. The relevance of a -ve SPO to this thread and the synoptic upstream and downstream pattern is that the relationship that the atmospheric circulation acquires to the ocean base state is one that features a more amplified Pacific pattern with stronger sub tropical ridging in the Pacific when the tropical>extra tropical set-up allows it. Stronger sub tropical ridging in the Pacific in turn regulates the development and orientation of the Azores/Atlantic sub tropical ridging in our sector. When the trade wind effect is greater, -ve frictional torque around 30N (and associated falling atmospheric angular momentum tendency) creates the mechanism for amplification to occur upstream to retract the Azores ridge westwards and means that cooler incursions and trough influence from the NW is greater. As soon as the trade wind effect reduces, then the developing Nino standing wave Pacific pattern has chance to regain more influence as soon as tropical convection patterns create westerly wind bursts to de-amplify the Pacific pattern as AAM bottoms out and then rises back up - and in turn return the downstream ridge eastwards and reduce the influence of polar maritime airflow with the jet axis SW-NE rather than W-E or even NW-SE. Such a more "Nina-like" sequence with the jet pattern switching to a more looped southern track occurs later this week before the pattern attempts to return to the flatter more cyclonic Pacific pattern that allows greater downstream amplification of the Azores high in the UK/European sector rather than to our west. This is yet another further element to the GSDM diagnostic process. Knowing how the upstream pattern is affected by factors such as these is a help towards determining how jet flow and trough and blocking features may also behave further downstream. So how much is the developing Nino standing wave likely to be "destructively" affected by any interim trade wind anomaly? With a new CCKW presently passing though the Pacific, easterly inertia is likely to be mitigated in the short term and then developmental to resumed progress in the medium longer term Additionally to this, the presence of a further CCKW in the Pacific also adds weight to the expectation of that low frequency amplitude MJO wave to develop across the Pacific as part of an atmospheric response to this. Signs of this starting to happen are discussed above. Furthermore, the CCKW passing through the Pacific also supports development of "non low frequency" activity in the form of cyclones. The tropical modelling (up the page) is starting to reflect the following on upstream pattern change more clearly on a shortening time period and this helps start to increase confidence that NWP will carry the signal forward more consistently and with better continuity. Both types of tropical activity, low frequency and non low frequency, are conducive to supressing trade effects and replacing with westerly winds that add an Asian jet extension across the Pacific, de-amplifying the upstream pattern - and, downstream from this, reversing the ridge/trough pattern in the Atlantic and returning the jet northwards once more - that is, following the loop south over the short medium term. NWP has kept the signal stubbornly beyond day 10 in recent days, but cluster and ensemble data should start to read better developments and though next week may still take a few days to settle down beyond the cool and changeable holiday weekend - things still look reasonably set for more fine and warm summer-like weather to return at the turn of the month and heading into September Latest 0z ECM clusters and GEFS ensembles continue to reflect these much improving prospects by the end of the month. With all the above in mind, there is much to agree with this third highlighted post. I would question the first and second lines though. From my own point of view there is no need to be silenced by any NWP currently (or indeed at any other time for that matter) and as based on attempted forward looking assessment. The end of week and B/H has been well anticipated in recent days and continues to not require any further reaction beyond what it is. An autumnal interlude as a snapshot in time that is best put into perspective of what preceded it and also the longer range likely perspective of which your final sentence quite reasonably assumes Personal optimum preference would certainly prefer ridging setting up just to the E/NE to invite a quicker route to plume scenarios and southerly winds. Up till recently this seemed quite possible. Though cautionary reality now suggests initial position of ridging my be a little further west ridging into the UK and across mainland NW Europe as dictated by the Azores ridge. Which in itself is not an unwelcome prospect by any means

The ECM difference you illustrate as measured by the Global Wind Oscillation in reality being in "transitional" Phase 8 as of yesterday 14 August.... .....rather than heading through low angular momentum La Nina Phases 1 into 2, (as suggested at face value by those operational outputs a week out) which models a sharper Atlantic ridge and hence greater cool air advection from the NW than has been the reality The reasoning as so often discussed in these posts being the degree of upstream amplification in the Pacific which serves to retrogress the high that is downstream from this in the Atlantic - and with the polar jet pulling troughs around the top of the high c/o of a more NW/SE axis. If the model overcooks the upstream amplification, then it overcooks the polar flow and looped jet axis Looking at the Pacific we see an increase in trade winds (covered in last post) following the passage of westerly winds c/o tropical activity which is now in end/re-start cycle - simply a natural pause in the tropical cycle and coinciding with CCKW towards the Western Hemisphere. Its this that modelling is jumping on and gunning for as pattern evolution to Atlantic ridge. This actually mirrors the sequence during the first half of July - the difference being that the trade wind increase is set to be weaker this time (albeit seasonal wavelength changes since then mean that the polar Jetstream is rather naturally adjusted further south according to this latest global wind-flow position than earlier summer). Notwithstanding that, the significance of the trade wind increase being weaker compared to back then is that in related terms, global atmospheric angular momentum is significantly higher than it was in early-mid July - - so extrapolating an establishing mid Atlantic ridge ahead on this basis needs some care and caution and none of the usual MOD discussion premature declarations in terms of an early autumn (beyond a possible transitory cooler interlude) This isn't 'teleconnections' saying "No" to an Atlantic ridge either - its an attempted diagnosis as to how likely a wind-flow pattern is to assist it developing and lasting for any meaningful time. Compare current AAM establishing closer to parity to the more -ve longer term establishing pattern of August/September last year c/o the persistent easterly trades and La Nina standing wave. N.B Its seasonal wavelength changes that are the essential importance here which heighten the Atlantic ridge synoptic response to -ve AAM conditions from mid summer increasingly into autumn This on the other hand looks very much like a transitory phase and certainly not a shift regime phase such as was seen during late July and August 2017 where a persistent signal for a mid Atlantic ridge in late summer and early autumn was well supported. The summaries since May have been covering this aspect and fully anticipating it and stressed the differential as to mid summer crossroads *should* a La Nina type signal persist (which it hasn't) and force the first half of summer Azores ridge eastwards extension to retrogress and force a homogeneously distinctly cooler, showery and unsettled second half of summer countrywide - such as seen in 2017. This clearly has not happened and instead we have seen a more traditional UK summer pattern emerge, still largely warm or very warm at times further SE and most changeable and average conditions further NW. The jet axis favouring tropical maritime, mostly, rather than mostly dominating polar maritime influences Compare a one year snapshot of tropical activity imprint of summer 2017 to summer 2018 A stark difference - and emphatic evidence of the role that tropical>extra tropical momentum transport has played in sustaining programmes of amplified late spring/summer ridges, whilst the standing wave regime of last year and associated lack of activity allowed a much more repressed Azores ridge to take over semi-permanently the closer high summer approached and Atlantic/polar maritime regimes increasingly prevalent Its also clear from this year how the mid to late July low frequency signal has transferred to the Pacific rather than defaulting back to the I/O and the relevance in that sense as to where we are in the current tropical cycle to early-mid July this year is very similar. Its this benchmark that the remainder of the summer and heading through September is best assessed in my opinion. This means that, overall, we should expect an Atlantic trough (not ridge) to prevail) and a case of looking for signs of the next Pacific low frequency engagement to re-emphasise a sharper trough to the west and south west and downstream European/Scandinavian ridge CFS vs 2 seasonal modelling identifies the next Nino standing wave engagement with associated surge in angular momentum tendency and tropical momentum transport beginning around the last week of the month and especially into September. Such a surge sharply reversing any interim easterly trade wind burst, and with it, any intuitive reason for mid Atlantic amplification to prevail too long Anticipating ahead comments about CFS capabilities here - these daily updated angular momentum forecasts have been consistently flagging this next upturn - and proving highly reliable and useful guides since becoming available c/o Victor Gensini maproom. Dr Gensini is co-working on developing further this maproom faciility to include further and larger data-sets with Ed Berry (former NOAA scientist and co author of the GSDM) Based on NWP behaviour since mid summer, then confidence in picking up extended signals in good time and then carrying them forward consistently is pretty low to say the least. Much as the mid July period, the models were as suggested at the time blind-sided and over pre-occupied by the tropical phase lull and were programming a mid Atlantic ridge and downstream trough to take over from the sustained heat of early to mid summer. This clearly didn't happen and instead over the ensuing days the path to a plume c/o Atlantic trough and Scandinavian ridge and a reload of heat into early August came into focus instead. In conclusion, in my opinion anyway, best to perceive any Atlantic ridge programme as a temporary blind-side to the bigger developing picture and no permanent feature- should it evolve to any extent in the medium term. Focus looks very much on an extensive ridge adjusting east with time (if it doesn't evolve more quickly from within the circa 10 day modelling) ahead of a defined Atlantic trough. As so many recent updates have suggested, this keeps prospects of further plume scenarios very much alive if nothing for a pinpoint micro scale island such as this can be guaranteed by anybody But as late winter 2018 starkly showed, it makes little difference to the weather if a season changes, it will do what it sees fit regardless