Tamara

A seasonal absentee by-stander in all this - but a one-off offering from me. Its a pity that this post is getting taken apart in some quarters, because hypothetically it *could* promote some interesting discussion if the snow goggle biases were put away for a moment. This winter has seen a much greater disconnect of the atmosphere to the La Nina base state than anticipated. Intense seasonal bias confirmation processes customarily disguise this, but if one is prepared to stay detached from that, it has been something of a surprise to the neutral diagnostic elements enthusiast. What is meant by this disconnect? In simplest terms, much greater poleward rossby wave propagation has taken place than under a more 'connected' and stable w/QBO La Nina low angular momentum regime in early winter would usually provide. The autumn itself, heading into the first few weeks of winter, saw (overall) a much more typical EL Nino type presentation, with a downstream configuration that until the festive period did not, mostly apart from a brief period, feature the expected sub tropical Atlantic ridging and instead a configuration of warm air advection processes c/o an amplified Atlantic trough and downstream European ridging. This alignment ultimately provided the feedback catalyst to assist the wave breaking that has instigated instability of the polar field, albeit reversal of zonal winds are restricted closer to 10mb level. Interestingly, global relative angular momentum has been slowly falling since the festive period and continuing into the first days of January. The Global Wind Oscillation, a phase plot depiction of wind-flow inertia between the tropics and extra tropics has slowly slipped towards the La Nina attractor phases - reflecting greater harmony with the ocean base state. The effects of falling momentum are to switch greater inertia into the polar jet from upstream. The initial manifestation of this c/o the weakness across the polar field has been for the usual feed of this inertia to proceed eastwards closer between 50 and 60N to be somewhat roadblocked, and instead looped around the pole and create the blocking structures close by to the NNE and linked to the amplification of the more Nina-esque Atlantic ridge. However, the difficulty comes with how these blocking structures respond to continued displacement/split processes within the polar field at the same time as angular momentum continues to fall ( *in the shorter term absence of any westerly inertia supplied by tropical forcing) and much more closely match the underlying base state. This is where the post under reply holds the interest I was mentioning at the beginning and should be treated with better respect It is perfectly conceivable, at least for a time, for polar jet energy to make greater inroads eastwards at mid latitudes, as the polar field continues to re-organise and as the period of time approaches where zonal winds, at least temporarily, are increased within the lower stratosphere and troposphere boundary. Not to any great levels by any means, as the overall structures continue to look weak and unstable-but enough to allow a more westerly induced pattern to prevail, at least for a time, with a more traditional Nina-esque Atlantic ridge centred close to the west or south west and ebbing and flowing within the bandwidth of displaced vortexing to the NE. *The further wildcard is intraseasonal MJO forcing. In keeping with w/BQO Nina-esque regimes, and linked to known periodicity timelines for high frequency activity - it is worth watching out for signs of an eastward convective parcel progression attempting to negotiate the Pacific later in January or possibly into February. A further variable that could see a sudden and quite dramatic surge in atmospheric angular momentum, tipping the resultant synoptic patterns even more their head. Impossible obviously to know where and when these may set up hemispherically at this stage - but high impact synoptics are conceivable with very large temperature boundaries across the mid latitudes. That is all for another time. Perhaps. Closer to home, putting preference biases completely aside, whatever they maybe - from a neutral meteorological point of view its a very tricky time for numerical modelling ahead which encompasses the ensemble suites and representative upper air anomaly charts . The overriding diagnostic (GWO tropical>extra tropical momentum relationship vs a very disjointed polar field) susceptible to highly erratic global wind-flow patterns at the tropopause boundaries and creating precipitous jumps in synoptic patterns, at mid latitude,... in either direction. Falling momentum and weak high frequency MJO activity for the foreseeable future (* above caveat aside ) does weight probabilities towards a more typical sub tropical regime and polar jet flow arcing around this. At the same time and as can be seen, there are very hard to keep tabs of pockets of weak inertia popping op at relatively short notice closer to 60N which quite easily could stall westerly inertia and create subtle eddies within the jet which build very weak 'cold' ridges and derail the movement of troughs from west to east and restrict warm air advection At present, no outcome is to be discounted or dismissed out of hand, and any discussion should be unclouded by bias preferences and be open minded to the even greater than usual uncertainties. That is not sitting on the fence - its honest objectivity in the face of highly sensitive factors that require a watching brief ,rather than any impetuous call for the sake of wanting to appear confident in appealing to a specific popular outcomes.

So, I am posting in the SE regional thread, which has been my home for decades, and yet now have an official home 1200 miles away in the Santarem district of Portugal. Due to the COVID pandemic, the whole process from start to finish has been done from here in the UK c/o online applications, remote viewing of property, and finally a PoA purchase via a legal advisor who has overseen the whole process and has done a brilliant job as well Today I received my Portuguese/EU residency certificate - which means I keep the automatic rights of citizenship that would otherwise have ended at the close of next month and would have meant a much more difficult VISA process. There is some updating being carried out to the kitchen and the bathrooms and then its a case of taking the long drive down there (to avoid flying risk), before the end of the year, with a copy of the residency certificates for each of us and also an affidavit letter from the legal advisor to confirm the essential purpose of travel. Anyway, to keep it weather related - there are obvious differences as one might expect to the conditions down there and those here. Early morning temperatures today in double figures and mid afternoon temps still heading towards the low 20s in southern and parts of central Portugal (and Spain) Compared to temps close to freezing first thing this morning across much of SE England and E Anglia, and despite the relatively milder air coming in from the west, the maximum daylight temps staying in single figures for most. Finally, for this post, my Portuguese friend Susana who originally lived in this part of Portugal, but lived locally in Hastings for the past 7 yrs but is returning home with all of us, has sent a latest picture looking out from the house across the garden and the vegetable/fruit area. It will be lovely to wake up and see the citrus growing from the bedroom window. Also four little palm trees which were transported from my garden here in the UK in pots have just been planted out for me. The weather sure looks beautiful

Posted August 29 (edited) What of the change of month and the previous suggestions of improved warmer weather once again ? Still on cue whatever numerical operational model vicissitudes appear to indicate - albeit as per the theme of the beginning of this post, never a straightforward process at this junction of the Northern Hemisphere. As was the case in late July and early this month, the high frequency MJO signal has growing confidence to embark on another eastward progression across the topics, much as discussed in detail in previous posts. The cue to promote attempts at further downstream ridging. Deterministic RMM modelling between the ECM and GFS is coming together with this trend, but this less reliable MJO modelling in terms of suggested amplitude path is very open to further forecast adjustment (i.e on the upside) At present NWP is very keen to retain an element of a more dominant Atlantic ridge and the tendency to allow fronts to slip south as pressure falls ahead of it sometime around next weekend. Some caution is required in the coming days as to whether or not any further retrogressive Atlantic ridge signal is overdone or not beyond this time - not so much as to how it evolves at face value - but more in terms of how the longer term pans out. The lessons from late July and the obfuscated lead up to when the intense heatwave suddenly started to appear in numerical modelling were that these same numerical models were, as anticipated back then, too blindsided by the low frequency upstream signal and made late adjustments to over progressive inroads on cold fronts and return of cool North Atlantic air, due to over amplifying the incoming Atlantic ridge to the west of the UK, and therefore digging the polar front too far south. The reality was that downstream ridging across Europe and Scandinavia proved more resistant than advertised and the subsequent heat dome persisted for much longer than initially suggested This time around, there is clearly not going to be anything close to any extent of advected heat source later this week. However, irrespective of this, based on the suggestion of an amplitude MJO wave progressing right into the Pacific through September, this ultimately supports the sort of de-amplification upstream (and subsequent downstream amplification in this neck of the woods) that happened in early August. On this basis there is support for the idea of any renewed ridging coming in from the west, adjusting itself eastwards to become positioned to the east and drawing a plume of very warm air from southern europe and up through France and to knock on the door of the UK. It is more a question of timing of this, rather than if it happens at all. I am, as ever, suspicious and questioning of NWP trends and do not take each and every output at face value. The ideas behind any further plume potential lie beyond next weekends proceedings and based on the movements of the renewed ridge advancing eastwards into the following week. This rather than trying to retract the high westwards and lower pressure again across the UK and other parts of NW Europe. It seems counter intuitive to expect further retrogressive flow after next weekend when the tropical signal supports increased momentum upstream in the longer term. So, and for what it is worth as I do not profess at all to be a "forecaster" as such and rather simply look at diagnostic probabilities -, it may take some time to benefit areas further north and west, but the evolution of ridging may ultimately manifest itself as stretched between the European mainland, the UK and with some slight and very subtle discontinuous retrogression to the Azores/Atlantic high pressure zone. But not too much discontinuous retrogression to allow pressure to fall too much ahead of it across NW Europe. Bias susceptibility to too much Pacific amplification possibly an element with this within some numerical products and it is on this basis to not be overly distracted by operational output but monitor the trends on numerical models c/o the ensemble means and clusters. This over a day or two at a time, rather than draw instant conclusions from each and every suite. A very conducive outcome remains possible for some calm, warm and sunny September weather and evenings still warm enough for sitting outside with the lanterns and tea lights glowing in the now darker evenings than were enjoyed during the previous summer type spell of weather Edited August 29 by Tamara ---------------------------------------------------------------------------------------------------------- The above lengthy extract from a detailed analysis post towards the end of August requires little apology for re-posting because the evolution of the pattern has duly exposed the suspicions surrounding NWP that were expressed on this, and other occasions, around that period of time. The many various bolded parts are especially prescient. So, beautiful summer-like weather is here to cherish for early autumn Those tea-lights flickering gentle shadows in the darker evenings, as suggested to come in the bolded post, being the one key difference as a reminder to the change of season since that early to mid August spell of classic summer weather The 90 day Global Wind Oscillation plot, a phase plot depiction of global wind-flow inertia, tells the full story of the summer pattern and the quite clear departure from the default pattern during August as the GWO reflected a higher angular momentum regime away the tight orbiting of from the low momentum La Nina phases 1 and 2 during much of June and July - returning once more to a low angular momentum regime in late August.. Namely, a switch during early August to anticyclonic wave-breaking downstream across NW Europe as the Pacific pattern switched to a less amplified signature and re-configured the downstream pattern - as the rossby wavelength switched downstream accordingly Now, heading into the middle of September, we see a further downstream anticyclonic phase occurring at the time that, once again, rising relative angular momentum rallies the GWO orbits out of the lowest momentum phases and traces an orbit path along the lines of what was in progress at the beginning of August - on the cusp of higher momentum GWO phase 4. Though, important to note, this is based on a two day consolidated lag to Sunday 13th. and doesn't include latest developments subject to forthcoming update. What of the outlook? As everyone knows, further glorious weather to come through to the weekend itself (though of course regional variations with the easterly winds). Beyond that is another period of suspicion as to how keen NWP is to re-introduce the return of the trough digging as far south as it suggests. Is numerical modelling, once again, far too fast in wanting to overdo the return of influence of the La Nina walker circulation? Further out still - If the high frequency tropical convection stays active, does not slip into the doldrums, and continues to shake up the rossby wavelengths, then it will remain possible deep into autumn to see a continued variable mixed meridional theme that sees more departures from the typical Nina-esque Atlantic ridge and downstream trough. Should the high frequency tropical signal weaken , then the walker cell will fully prevail and strengthen La Nina considerably and the pattern quite possibly becomes a little more predictable in terms of the pattern wavelength, I would think, for those who , unlike me, are forecasters . At least as far as it is possible to make suggestions as to how patterns may pan out in a world of long term and unpredictable climate shift. In this scenario, the Atlantic ridge is suggested to increasingly be in charge with less interruption - and lead to a fair amount of polar maritime influences as this season progresses, for north western parts of Europe at least. The wavelength then looking to adjust the position and amplification of this ridge thereafter - with a flatter regime flowing around the perimeter of the ridging and ensuing in tandem with the more organised polar vortex. Therefore much depends on the character of this La Nina - whether it follows a 2017/2018 more east based nature, or whether it is the more familiarly seen (basin wide) traditional type. For me, I will have long completed my posting on this thread by such a time anyway - until next Spring. All being well and fingers crossed, I am by this time 1250 miles further south to escape the British winter. From a completely different perspective to the vast majority in the UK, it will be be very interesting to see how a first winters weather pans out,1250 miles further south and with that, another highly fascinating learning curve to experience and enjoy

Yes indeed - very much so. Back towards making more sense, set against the direction of travel of forcing within the tropics>extra tropics The suggestion was made last week that the progress back to fine, warm spells on a more widespread basis would not be seamless in terms of how NWP handled it - with the tendency to try to shift rossby wave amplification back upstream and push troughs south east across NW Europe. Customary modelling difficulties evident with the high frequency tropical convection wave negotiating the passage of the terrain of the Maritime Continent as it propagates eastwards a factor in this respect - leading to susceptibility to defaulting back to the La Nina standing wave across Africa and the Indian Ocean. Quite a lot of SST warmth across the Maritime Continent and the Western Pacific, as the Eastern Pacific continues to cool further below average. Possible makings of a (continued weak) central/eastern based La Nina which assists the prospect of MJO related tropical convection convergence perhaps setting up a little further east than a traditional La Nina heading through this autumn. Therefore maybe greater downstream amplification patterns to mix up the default La Nina-esque mid Atlantic ridge than might otherwise be expected, and hence Indian Summer potential for October as well

The whole of this quoted extract is wholly relevant - but captured in essence within the bolded parts There remains some capping on the extent to which NWP is unanimously switching shoaling in mid stream, at this time - but EPS is most emphatically leading the way here. The persistent very strong upstream amplification as discussed in this previous post a few days back, harboured suspicions over the extrapolation of this signal. So the switches that are now starting to occur make complete sense as far as I am concerned within the context of momentum changes to come. I continue to anticipate that NWP is not going to progress seamlessly with any such switch. So one should not be surprised to see some degree of persisting with trying to supress the trough to the north southwards in some modelling. GFS perhaps most progressive and culpable here, though not single-handedly. However - as is always customarily sensible and advisable in my opinion: It makes no sense to take an inductive reasoning approach to NWP and extrapolate outwards to arrive at conclusions based on an individual suite at any given time - but rather to take a deductive reasoning approach based on continuity and a persistence factor within consecutive suites. Deductive reasoning compliments the diagnostic that is driving the evolution of change within the numerical modelling that starts choreographing the appropriate changes within the outlook. This involves drawing conclusions from trends that may take more than a day or two at a time - and which provides the desired continuity and compliments suggested outcomes and avoids impulsive and ultimately erroneous reactivity. In this way, a deductive approach develops gravitas that models are jiving correctly with the diagnostic and so inspires confidence in their performance. The coming days should flesh this theme out more coherently and further improve confidence of how well numerical models are in harmony with the diagnostic moving forward. In this way, the constant flux and change of atmospheric profile, as ever, provides fascination and opportunity to learn - irrespective of whether or not conditions on the ground are meeting ideal preferences at any given time

This is not true for everybody. What of the Septembers pattern for those who are still interested ? A lot of caution remains heading through this month with the upstream amplification pattern - based on the progress of another high frequency MJO signal across the tropics and an associated fresh jump in angular momentum tendency that may well bring about further curveballs with time. Whilst it looks like yet another La Nina-esque influenced autumn - and these have been numerous over the last decade overall, that doesn't mean that the atmospheric circulation does not wax and wane within such a default parameter. The first half and middle of August was an excellent example of a wane period The balancing act within the atmosphere, and how this impacts the jet stream, pivots around the substantial extent that AAM has fallen back once more, following the August rally and attendant downstream ridge and spectacular heatwave. NWP has been spooked by strong scrubbing of westerly wind inertia c/o of robust -ve mountain torques across the extra tropics. It is this that has been sending an equally robustly strong upstream amplification signal (very La Nina-esque). The Global Wind Oscillation, a phase plot depiction of global wind-flow inertia, represents this steep fall back in globally averaged angular momentum since the last 10 days of August, with a deep amplitude orbit return to La Nina phase 1 on its way to Phase 2. This underpins the NE Pacific amplified wavelength configured to equally amplified sub tropical ridging in the central and Western Atlantic and troughs peeling off close to Iceland around the perimeter of the Atlantic ridging and sending phases of chilly polar maritime air south eastwards in between humid sub tropical frontal sectors which are mainly being seen further south. The persisting issue, as discussed in previous summaries, is that the low frequency tropical signal that is driving this default pattern continues to blindside numerical modelling to this next further eastward propagation of the high frequency signal that has some hallmarks to what happened in late July and the first part of August.. Based on the steepness of the ongoing fall in momentum, an unstable wind-flow budget in the atmosphere is being created, which means any subsequent rebound and rally will likely trigger quite a further NWP shake-up as this month moves forward Ultimate Pacific engagement of the high frequency tropical signal implies another interruption to the African Easterly Wave pattern c/o the low frequency feedbacks in the region (such as happened in the early part of August.) This leads to a rapid de-amplifying of the upstream pattern and hence the jet stream, downstream, switched right away from a NW-SE angle and as a consequence cutting off airflows sourced from the far Northern Atlantic and instead engaging airmasses from increasingly warmer vectors It will be interesting to see how the numerical modelling continues handling this over the coming 10 days or so and whether another mid stream change of fish shoaling is around the corner.by the time we are getting through the second week of September and looking mid month period onwards. Much as happened looking ahead from the latter part of July into the first half of August. Whilst the coming period certainly sees the continuation of the strongly amplified upstream pattern, it remains wise to watch for the models eventually becoming a cropper if they try to persist with this pattern too far through the month. A further six weeks on from the August heatwave, as it will be by this stage of a very extended period, it certainly still doesn't preclude further reminders of summer to come. Plenty of time remains for that yet and those who continue to appreciate warm days and warm evenings, irrespective of less daylength, will consider it worthwhile sticking around a bit longer while such an opportunity remains

So, deposit paid, and "escritura" scheduled to be signed (by a POA representative near Lisbon) during September to be the owner of new property in destination Portugal. Summer blazes on there, regardless of technical change of season. Back here, as ever not so straightforward - with airmasses from polar and sub tropical origins doing their usual act of vying for supremacy at this crossroads and higher latitude to where I ultimately will reside at 39N. Current momentum budgets underpin why this neck of the woods is, at present, on the "wrong side" of an Atlantic ridge, unlike southern europe, - and instead fully on the side of another departing trough and below average temperatures coming into the end of this week and leading to slow recovery (but still these highly disappointing temperatures for many of us) this Bank holiday weekend. Globally averaged angular momentum has returned below average c/o -ve mountain torques in the extra tropics - these torques and their associated easterly inertia to hemispheric wind-flow, have fully scrubbed out the angular momentum rally that inspired the seemingly distant memory of an anomalous downstream heat ridge and some blazing NW European summer weather earlier this month. This change equates to re-amplification of the upstream pattern within the Pacific sector as a result of the stabilising effects of tropical convective suppression (shaded orange), at the same time as the convectively active low frequency signal (shaded blue) re-asserts itself across Africa and the Indian ocean as illustrated by the velocity potential precipitation anomalies. N.B Note the forecast trend and eastward progression with time - more on that later. This re-sets the downstream wavelength of retracted Atlantic/Azores ridge and overriding NW European trough that was seen extensively in the mid summer period - encompassing almost the whole of July (bar that final day heatwave!) What of the change of month and the previous suggestions of improved warmer weather once again ? Still on cue whatever numerical operational model vicissitudes appear to indicate - albeit as per the theme of the beginning of this post, never a straightforward process at this junction of the Northern Hemisphere. As was the case in late July and early this month, the high frequency MJO signal has growing confidence to embark on another eastward progression across the topics, much as discussed in detail in previous posts. The cue to promote attempts at further downstream ridging. Deterministic RMM modelling between the ECM and GFS is coming together with this trend, but this less reliable MJO modelling in terms of suggested amplitude path is very open to further forecast adjustment (i.e on the upside) Following on from the above, this snapshot in time cluster analysis represents in synoptic terms the same interesting juncture. At present NWP is very keen to retain an element of a more dominant Atlantic ridge and the tendency to allow fronts to slip south as pressure falls ahead of it sometime around next weekend. Some caution is required in the coming days as to whether or not any further retrogressive Atlantic ridge signal is overdone or not beyond this time - not so much as to how it evolves at face value - but more in terms of how the longer term pans out. The lessons from late July and the obfuscated lead up to when the intense heatwave suddenly started to appear in numerical modelling were that these same numerical models were, as anticipated back then, too blindsided by the low frequency upstream signal and made late adjustments to over progressive inroads on cold fronts and return of cool North Atlantic air, due to over amplifying the incoming Atlantic ridge to the west of the UK, and therefore digging the polar front too far south. The reality was that downstream ridging across Europe and Scandinavia proved more resistant than advertised and the subsequent heat dome persisted for much longer than initially suggested This time around, there is clearly not going to be anything close to any extent of advected heat source later this week. However, irrespective of this, based on the suggestion of an amplitude MJO wave progressing right into the Pacific through September, this ultimately supports the sort of de-amplification upstream (and subsequent downstream amplification in this neck of the woods) that happened in early August. On this basis there is support for the idea of any renewed ridging coming in from the west, adjusting itself eastwards to become positioned to the east and drawing a plume of very warm air from southern europe and up through France and to knock on the door of the UK. It is more a question of timing of this, rather than if it happens at all. I am, as ever, suspicious and questioning of NWP trends and do not take each and every output at face value. Latest updated 12z GFS ensembles indicate a shift away from what the operational continues to insist and with the Atlantic ridging element overdone on the latter as suggested might be the case within this post. The ideas behind any further plume potential lie beyond next weekends proceedings and based on the movements of the renewed ridge advancing eastwards into the following week. This rather than trying to retract the high westwards and lower pressure again across the UK and other parts of NW Europe. It seems counter intuitive to expect further retrogressive flow after next weekend when the tropical signal supports increased momentum upstream in the longer term. On that basis the 12z GFS ensemble mean is closer to that of the ECM mean this morning and makes more sense. So, and for what it is worth as I do not profess at all to be a "forecaster" as such and rather simply look at diagnostic probabilities -, it may take some time to benefit areas further north and west, but the evolution of ridging may ultimately manifest itself as stretched between the European mainland, the UK and with some slight and very subtle discontinuous retrogression to the Azores/Atlantic high pressure zone. But not too much discontinuous retrogression to allow pressure to fall too much ahead of it across NW Europe. Bias susceptibility to too much Pacific amplification possibly an element with this within some numerical products and it is on this basis to not be overly distracted by operational output but monitor the trends on numerical models c/o the ensemble means and clusters. This over a day or two at a time, rather than draw instant conclusions from each and every suite. A very conducive outcome remains possible for some calm, warm and sunny September weather and evenings still warm enough for sitting outside with the lanterns and tea lights glowing in the now darker evenings than were enjoyed during the previous summer type spell of weather

...And so the "worst" of the pattern switch is duly very much focussed within this last week of August and with an underwhelmingly cool introduction to the Bank Holiday. However, the renewed stranglehold of the low frequency tropical signal across Africa and the Indian Ocean, which has re-set set this unwelcome pattern wavelength, faces a further interruption as September progresses. NWP is still a bit tentative with this, but then as stated in last weeks post this is to be expected based on how strongly they congregate around such a default signal - until the flag is really waved that something else is about to shake-up the wavelength. As the MJO cycle progresses eastwards once more through the first part of September then this is the starting pistol for another AAM rally and re-booting of downstream ridging. We cannot guarantee record breaking late seasonal heat, but every reason for a warming trend to renew after an initially disappointingly cool start due to the incoming amplified Atlantic ridge trapping lower uppers as the next trough passes east of the UK at the end of the week The switch in the upstream pattern should, with time, start to send Atlantic lows further to the NW and allow warmer air to mix out the coolness. Updating tropical convection forecasts will start to define the extent of the next rise in angular momentum as associated rossby wave-breaking propagates into the extra tropics - and on that basis the extent of a further downstream ridge will be defined accordingly. But the theme of the final paragraph of the quoted post still very much applies

The reality is that NWP is wholly congregated on, but susceptible to be blindsided and over preoccupied by, the re-ignition of the low frequency tropical convective standing wave across Africa and the Indian Ocean. Cue the drop back in angular momentum in progress as this signal re-emerges c/o steeply -ve trending global torques across mountain ranges scrubbing out the recent surge of westerly inertia (the +AAM anomalies that created our downstream heat ridge) within the atmospheric circulation - - and hence the programmed re-amplified Pacific and flatter downstream pattern in the outlook as the GWO reflects loss of momentum through phase space with an orbit back through negative tendency Phase eight Recent posts have attempted to deal extensively, ahead of time, the suggested problems to come surrounding this next phase of weather, and that period of focus is during the remaining third of August. A period where long term extrapolatory assumptions over the longevity of return to this "default pattern" are prone to be outed with time As rightly stated by a few people, the coming week still looks relatively warm for quite a few, despite rain around and the increasing cyclonic influences just to the west. That condenses the "worst" of the pattern switch into the last week or so of August. This is where the further evolution becomes especially problematic and subject to questioning. How much credence is there in all these "early autumn" prophecies ? The answer is they are equally prone to be outed as being unduly overstated. I am not referring to Equatorial Atlantic tropical cyclone/hurricane developments , which are a manifestation of the active African Easterly Waves that develop due to the low frequency tropical standing wave default pattern in that part of the tropics - but much as stressed in previous posts of my own leading up to and during the heatwave - the eastward propagating high frequency tropical convection MJO cycle looks set to continue to be more active heading into September than during the earlier summer when it hit the doldrums and allowed that amplified Pacific wavelength and flat downstream pattern to dictate for several weeks in this part of the Northern Hemisphere. The signs are that the high frequency signal may once again interrupt the upstream signal and another downstream ridge sequence emerge - eventually cutting off the cooler changeable flat North Atlantic influences. The cluster analysis from @Man With Beard of late has reflected this ultimate evolution, and this trend continues today, mirroring tropical>extra tropical forcing trends in the longer range The comments by @johnholmesare correct as ever related to the anomaly charts. Just to add that these also evolve according to numerical modelling interpretation of the signals and so current charts are equally snapshots in time of the period in question and are likely still on the cusp of the extended period they represent. However, coming days should provide greater traction to trends, because as suggested at the start of the post, NWP is somewhat blindsided and preoccupied by the Indian Ocean low frequency tropical signal and likely will perform a mid stream shoal switch - much as they did in late July, once they cotton on to the next eastward progression of the high frequency "mini ENSO cycle". In short, plenty of scope remaining yet for the few who haven't pressed their bias preference switch from the furnace to the freezer, for further summer-like weather to come . September could become a pleasant and welcome extension of summer in terms of weather type, irrespective of change of season - at least for those of us who would greatly look forward to this weather type

Astonishing rebound in momentum transport that is behind the current spectacular synoptics and associated high impact weather. Both the heat, and also the powder keg potential for thunderstorms. The GWO has smashed its way through the El Nino attractor phases through into a weak amplitude Phase 5, based on the two day two consolidated lag. A reversal of the standing wave pattern between June and July and a resounding blueprint for the anomalous European and Scandinavian ridging presently in evidence. With that switch in mind, NWP has been suggesting, up to now, a wholesale collapse and reversal back of the present regime during next week. Times like this, ahead of a suggested vigorous pattern change, (or more accurately switch back to a previous pattern) require extra caution because while the atmosphere ""remembers" its previous behaviours, there is no linear one size fits all response from one season to another. Therefore expecting the same responses from the forcing within the tropics and extra tropics, season on season, is over simplistic and erroneous. Especially the size of the correction these models appear to have been predicting. Personally, with all the above waffle in mind, I am watching the disruption of the upper trough next week and how that phases with the residual heat lows from the stagnant heat dome mass that will have sat there for an extraordinary amount of time by the end of the coming weekend. No surprise at all that the Met Office have issued so many thunderstorm warnings day on day and no surprise at all if these calculations of how the Atlantic troughing phases with this stagnant slack low pressure mass runs into some difficulties. There have already been some incremental corrections westward with this process and there is time up to 7/8 days ahead for more of these All in all, a lot to be decided and not take for granted, from my own point of view at least, whatever the present modelling may presently suggest, from the early - but more especially mid to latter part of next week. Edit 6pm This was written earlier this afternoon until home commitments took over. Irrespective of the details (which are irrelevant in the circumstances at this distance in terms of what each model goes on show thereafter), the trend indeed continues further on latest 12z suite, so far, to take the main trough ever west of the UK in the middle of next week. This is one such example of not taking each and every NWP operational and indeed ensemble data at face value but treat them as a snapshot in time and watch instead how they trend over a day or preferably two at a time Should probably pin this as a rider to every post...signals lead models, models do not lead signals.

Its hard to not write extensively about this because I am very much inclined to agree with you.. So that is what I will have to do. Advice would be to bypass this post, those who participate and/or read this thread and who are not interested in detailed perspective analysis that comes from a diagnostic and proactive point of view, rather than reactive to NWP operational.. Assessing wind-flow inertia, and how it influences angular momentum tendency, across the hemisphere is very useful guide as to jet stream pattern behaviour when used objectively and with no preconceived agendas. However, unfortunately, as technical diagnostic tools like these (most especially the forecasting products), become more familiarly spoken about by larger numbers of people they tend to become dumbed down and treated as over simplified and misconstrued magic bullets as a means to arrive at x+y= outcomes. Such outcomes often bias skewed to what a given individual would like to see, rather than the helpful guide to the range of probabilities they offer in proper practice The twitter-sphere doesn't help in the respect. Far too many over simplified absolutes are conveyed in limited text communication which leads to incomplete, inaccurate and misleading information being accepted as a given, the further the message travels outwards. This in turn leads to discrediting and ignorance of the true purpose and value of these essential meteorological aids - and the science that drives them is lost as the expense of being abused as a novelty fixit. Additionally, the feedbacks to these wind-flow patterns are subject to variation due to seasonal wavelengths and the effect they have on adjusting the longwave patterns as changes occur between the tropics and the pole. So trying to extrapolate the same feedback over straddling seasons further and further out in time will inevitably lead to error - due to the effects that seasonal wavelength changes have on patterns. So, taking all this back the here and now - its worth repeating, once again, a lot of caution is required trying to make assumptions that the extended period is going to be some precise carbon copy of the earlier summer and that somehow this makes an early autumn (in terms of weather type rather than merely the anticipated change of season) an inevitability. If predicting the weather was a simple as that, most of us average jo's would have cleaned up financially be now and would be putting our feet up somewhere that meets all our dreams. Wherever, and whatever that may be. As stated yesterday, and all the models now agree with this, amplification of the Atlantic ridge is highly intuitive to the eventual return of influence of the African and Indian Ocean standing wave as angular momentum slips downwards. It is also likely that the heat will relax its grip over the weekend, but this is a slow process of attrition and all the hyperbole of descent into autumn needs to be put into context of just how hot, both day and night, the current pattern is and will stay like for more to days to come yet. Any westerly influence, should it ultimately arrive, would feel a contrast to such anomalously high levels of heat that will continue to dominate through this week. Timing of all these changes is also very problematic. Such is the nature of numerical models that will pounce on an emerging signal and tend to go into overdrive and overdo the change it suggests Analysis of the here and now is always a good place to start to get some proper perspective on where we are and where we might head, rather than jumping on extended forecasts that are not based on any knowledge of consolidated, present day, data Angular momentum has actually surpassed expectations with its rally since late July. A deficit of more than -2SD (standard deviations) below average has been overturned in the last 10 days and globallly averaged angular momentum is even a fraction above average. This, based on strong propagation of +AAM anomalies from the tropics to the extra tropics. Mountain torques in the extra tropics have scrubbed out considerable easterly inertia Take a look at how the passage of the MJO high frequency signal is overriding the standing wave across Africa and the I/O . The velocity potential convective anomalies, depicted blue, have flipped the pattern heading towards and into the Pacific - and with the low frequency standing wave under supressed convection (shaded orange) The long persistence of easterly trade inertia across the Equatorial Pacific has been cut off. Westerly winds are apparent close to and east of the dateline The Global Wind Oscillation, a plot depiction of net wind flow inertia after additions (or subtractions) via global torques has rocketed in orbit, subject its two day consolidated date lag, into amplitude Phase 4, and is even testing the El Nino attractor Phase 5 threshold. The significance of this is to show how the low frequency standing wave (low momentum default) has at least temporarily (almost) arrived at the point of reversal. Hence the spectacular showcasing of strong downstream heat ridges leading to our NW european cauldron of pressure cooking. That isn't hyperbole, it is factually what has happened. So what is to come? The caution, and usual suspicion with NWP as far as I am concerned anyway, is how fast and to what extent, they are handling the breakdown of this pattern after next weekend. The ECM continues to be a day or two faster than GFS, And there is a risk all NWO could be too aggressive with this. In the extended period, and based on how the momentum budgets have been stretched upwards like elastic from the summer low benchmark, the numerical modelling will be susceptible to being spooked by an equally sharp snap-back of a wind-flow inertia rubber band. However, while the atmosphere often "remembers" its previous default feedbacks based on known intra-seasonal tropical and extra tropical phenomena - it is not always as simple as "what goes up, must come down" on some like for like quid pro quo basis. Based on the persistence of the low frequency signal into the autumn, this does imply persistence of that Atlantic ridging, and the inherent inclination for numerical models to want to angle the jet stream around the northern and eastern perimeter of this ridging and dig a trough far southwards. More on this shortly. Question marks persist over the enigmatic behaviour of the QBO. There is now a very high chance of failure of transition, despite some (future seasonal!) wish-casting in other arenas. Note that July has now reversed the easterly downwelling phase 2020 : Jan -2.51 Feb -3.20 Mar -4.36 Apr -5.03 May -4.86 June -2.78 July +0.34. QBO Calculated at NOAA PSL 30mb zonal wind at the equator, zonal average For info Climate Indices: Monthly Atmospheric and Ocean Time Series: NOAA Physical Sciences Laboratory PSL.NOAA.GOV US Department of Commerce, NOAA, Physical Sciences Laboratory With this, there are now implications for the high frequency MJO signal to remain more active heading into the autumn proper and this would have the tendency to persist in anticyclone wave breaking based on continuation of a +NAO profile and a stable profile across the polar field. On that basis, there risks overreaction to any poleward amplification of the Atlantic ridging, and therefore cooler polar maritime incursions to be over exaggerated in the, aptly named, unreliable timeframes. in short, advisable not to let the exceptional nature of this hot spell, which looks set to last up to the weekend with only a slow cooling process over the weekend itself, over skew perceptions of what may follow it. That is why present day analysis of these AAM tools is much more grounding in terms of assessing future potential impacts, rather than proclaiming what are sometimes over speculative definitive outcomes based on numerical model forecast suggestions -without first having any knowledge and awareness of the existing momentum budgets and then looking at the computer models afterwards. Signals lead models, models do not lead signals

You are welcome Subtle changes often happen within a given ocean>atmosphere paradigm which, as we are currently seeing, can have quite dramatic changes to the macro scale pattern for a time - and on this occasion has certainly changed fortunes for this part of NW Europe. It is simply the superimposition of high frequency MJO forcing over the "default" (long term) standing wave across Africa and the Indian ocean. This, rather any shift within the long term standing wave feedback Later in August the tropical signal will be re-coupled, that is, meaning that the underlying strength of the walker cell circulation will return to determine the wider pattern. But this does not inextricably mean back to square one before the hot weather. The outlook from an NWP perspective looks, as anticipated, full of potential minefields. The ECM is at present most keen to try to return the pattern towards the early and mid summer pattern by the end of the coming week - whilst other modelling is illustrating what I suggested the other day, in that the present blocking pattern may reload to some degree. Such a rinse and (partial) repeat would still allow the most intense heat to filter out but would leave, and as the UKMO modelling very well illustrates, a decent level of warmth in place to keep the high summer mood going. It makes sense that an Atlantic ridge re-builds to the west - this in keeping with angular momentum adjusting back down to a degree once more as the MJO forcing fades from influence. However the ECM seems to be pushing the envelope with this at present, and on this occasion could be the model to overdo the heatwave breakdown and its suggested unsettled pattern thereafter. Yet another one of those occasions where anything past day 4/5 is open to question at the moment. But a good chance that Atlantic ridging will overspread the problematic cut off feature and keep a settled, and still quite warm (though less hot) theme going into and beyond the mid month period

@Tamara I wonder if you might answer a couple of questions I have about this. - You note the inevitability of the low frequency tropical signal returning (which you have previously described as "default") and promoting upstream amplification etc. - You also later refer to the default pattern and seasonal wavelength changes in August. I wonder if you could flesh this out a little more. Firstly I'm interested in what you mean by the default pattern. Do you mean on a global basis, or are you specifically referring to default patterns for a UK summer, because there seems to be an implication regarding the effects of this change on UK conditions? I understand that there are more and less typical broader patterns, so an Azores high is typical while a Scandinavian high is less so, but even if you imagine a default Azores high in place, there are many ways for the UK to retain a warm/continental flow, for instance, just as there are ways to be cool and wet with warm southerly or easterly draws. So what is the implication when you say the default pattern will return? Do you just mean, default broader patterns will return, accompanied by relatively moderate conditions and all the usual uncertainty, or are you implying specific conditions in the UK? I suppose I'm asking here because if default broader conditions are to be significant (as you suggest they are) in this context they must imply specific effects for local weather. And yet looking through recent summers we have had prolonged spells of both warm, settled weather (probably majority overall) as well as wetter, cooler weather, such that it is not clear to me what our default summer weather is. So it's not clear to me either what default conditions locally are, or that globally default conditions have predictable effects on weather locally... Secondly, can you add any further detail about seasonal wavelength changes? Is this to do with the jet firing up? Does this mean August should actually be expected to be a less summery month overall than June/July? My understanding was that our recent run of our Augusts was anomalous set against long term averages. A few questions in there I know. I hope you can pick that apart... ----------------------------------------------------------------------------------------------------------------------- An interruption to the heatwave nowcasting being monitored on this thread, but still an important one in my opinion nonetheless. This valuable post by @wellington bootrisked being lost as the previous model thread was closed yesterday morning very soon after it was posted. So I have re-pasted it because it covers the essential points that have been made throughout this summer, so far, and which will continue to be primary influence for the rest of summer and into the following season as well. When I have spoken of the "default" pattern, so often as it has been referred to, that means the primary driver in the tropical> extra tropical circulation that has created the negative inertia wind patterns that translate to an upstream amplification across the Pacific - and which has led to the synoptic downstream configuration of an Atlantic ridge and trough across NW Europe and Scandinavia. So the default pattern hasn't been referring to traditional UK summer patterns - but specific signals that have been primary influence over the summer (at least up until very recently) across the UK and other parts of NW Europe and Scandinavia The low frequency tropical convection signal (the default) controlling the pattern has been rooted, since the beginning of June, across Africa and the Indian Ocean. The persistence of this signal is denoted by the velocity potential convection anomalies (shaded blue). Note how the twin low frequency signal faded in the last part of May across the Pacific and left the African and I/O standing wave to dominate. This positioning of the dominant singular low frequency signal creates uptick in low level easterly trade winds (the African Easterly Waves or AEW) that steer the tropical cyclone developments westwards in the Equatorial Atlantic along the southern perimeter of amplified Bermuda and Azores ridges to the west of the UK. On the northern perimeter of these ridges, has been compensatory increased polar jet energy - hence the low pressure sequences that have kept passing close to the NW of the UK with persistent relatively cool air flowing across the UK from the Northern Atlantic. Recent posts, at least since the second half of July, anticipated a break in the influence of the low frequency signal as the high frequency intra seasonal MJO has overridden this c/o an eastward progression across the tropics which has scrubbed out quite a significant amount of this easterly inertia which has re-configured the upstream pattern (de-amplification). This added momentum transport between the tropics and extra tropics, c/o positive frictional and mountain torque mechanisms, creating rossby wave-breaking downstream, carving our our downstream anticyclone, reversing the pattern configuration of the summer to date and promoting heat advection from southern europe, . The extent of the uptick in momentum (and the reflected extent of the pattern change for the better downstream) is evident from the Global Synoptic Dynamical modelling (GSDM). This is a diagnostic (non numerical) model that calculates wind-flow inertia across the globe. In other words in simple terms it is a useful guide to deciphering jet stream patterns. With so much of the excess easterly inertia scrubbed from the atmospheric circulation at this time, AAM has recovered with a sharp uptick to average levels for the first time since the end of Spring. It is no coincidence that the fall in AAM began just as summer appeared and at the same time as the low frequency tropical signal switched away from the Pacific. It is equally no coincidence that AAM has recovered just as that low frequency (default) signal has been interrupted for the first time since the start of summer. The longevity of the current heatwave centres around the wavelength (timespan) of the influence of the MJO phase cycle that created the distinct upturn in angular momentum tendency. When this intra seasonal signal fades, this will be the cue for the upstream pattern too re-amplify and what I refer to as the underlying default pattern to return. NWP is playing with this evolution in the extended period with hints of a return of an Atlantic ridge with the strngest anomaly to the west of the UK, rather than downstream to the east.. However, considerable intense heat build-up across huge expanses of the mid latitudes, very much including all of Europe means that this process is nowhere near as clear cut in terms of any progressive cool-down as during the earlier summer. Additionally late summer seasonal wavelength changes make it likely that pressure fall across Scandinavia will not be as easily achieved as earlier in the summer. Therefore the pattern could well evolve into an an anomalous Atlantic ridge bridging with enough strength to keep deflecting the polar front far enough north to retain a warm airmass in place for some time. Whilst these will be almost certainly less intense then currently, any dramatic modelling over the coming 10 days that tries to show a sharply retracted ridge, falling pressure to any great extent over and to the NE of the UK - and substantial inroads of very much cooler air very far southwards should be, at present at least, treated as suspicious and an overreaction to falling momentum upstream. That said, angular momentum will definitely fall back when the high frequency signal completes its eastward progression across the tropics, and the trade wind influence returns across the Equatorial Pacific and Equatorial Atlantic, but it is far from certain that the downstream pattern will revert to precisely as it was before the heatwave. The coming several days, at least, are set for some quite remarkable summer weather - irrespective of whether 38C is breached or not in the immediate future

I admit especially to tuning out of individual operational outputs, but more especially the GFS which provides too much of a distraction with its intra day vicissitudes and inconsistencies when trying to look at trends day on day. To put it another way, I would conserve energy and not spend much time at all poring over intra day details - especially with so much erratic continuity each and every 6 to 12 hours. Most especially several days away. From my own viewpoint things are shaping up still for an excellent spell of summer weather. The earlier attempts by GFS operationals and a minority of its ensembles to wedge a trough between the Scandinavian high pressure and the Azores ridge to the west is redolent of the default regime we have moved away from (for a time anyway) and is typical GFS bias in these type of situations to want to overplay Atlantic ridging. This has the effect of lowering pressure too much downstream c/o excessive retrogressive pull which over weakens the "bridging" between the two anticyclones. In some respects, and this was alluded to in a post of mine last week, we are are likely to see the opposite trend of the bulk of the summer so far, for a little while, whereby NWP (more especially GFS) might be over progressive with negative momentum bias and will be coerced into re-adjusting upwards increase of pressure on strength of downstream ridging rather than the opposite trend of the summer to date to overdo Azores ridging eastwards and then incrementally retract it afterwards.. There is bound to be something of a rinse/repeat element to the upcoming very warm/hot spell but the transitioning ridge from the Azores to take over from an existing downstream ridge (to then become the renewed downstream ridge) is a classic evolution of some of the more extended very warm spells in summers at this latitude. With that in mind, I think the weekend sequence is rather a distraction to the fact that prospects look very warm, in general, for a rather longer time instead of extremely hot for a shorter period of time. There is a reasonable chance that the relatively cooler North Atlantic air encroaching into the northern half of the UK at the weekend will "warm out" heading into next week as the renewed anticyclone absorbs the heat advection from the continent. The first half of August is increasingly looking very promising. Uncertainty grows thereafter in terms of possible return to default - as the superimposition of the eastward progression of the high frequency MJO forcing on the pattern completes its orbit. This MJO wave is presently cutting off the upstream Pacific amplification process that has been the spoiler for NW European downstream pattern this summer. Once it fades and its influence is removed, then the low frequency walker cell across Africa and the Indian Ocean resumes charge of proceedings. When that happens, easterly inertia will be added back to the atmospheric circulation from the tropics into the extra tropics as torque tendency switches negative which in turn will promote falling angular momentum - and cue re-amplification upstream. This signal, in basic synoptic terms, for this part of the Northern Hemisphere would likely be indications of the main upper heights shifting decisively westwards and a trough migrating around the perimeter with pressure falling more generally across NW Europe and Scandinavia. However, as stated yesterday, that is subject to further clarity at this distance and can wait for another time. Its time to enjoy summer proper for many

Excuse the self-quoting and the extent of the re-post but over two weeks further on in time, it is surely interesting to revisit this as a means to demonstrate how the utilisation of the global wind-flow and angular momentum budgets can prove highly useful in gaining clues when NWP gets carried away by negative momentum inertia and is blind-sided to the sudden switches in global inertia that lie hidden around a future corner. Hence, akin to the dropping of a red hot potato in the hand, the volte face of the retracted Atlantic ridge and downstream trough that blighted most of July. The largest westerly inertia push since the Spring is underway. Steadily rising frictional torque tendency within the tropics is finally engaging mountain torque tendency within the extra tropics Based on the low level of global momentum up to this time, this injection of wind-flow c/o rising torques within both the tropics and extra tropics has created a very strong rally angular momentum tendency set against such a below average level vs parity. This has been the cue for the models to ditch the persistent Pacific amplification evident since the start of the summer and re-configure the downstream pattern. The end of week plume this past Thursday and especially Friday has been just the opening salvo - much as anticipated in recent posts. The Global Wind Oscillation, a plot phase depiction of global wind-flow inertia, reflects the sharp uptick in momentum and underpins the pattern re-configuration with the downstream ridge tendency that is returning to NWP c/o an orbit dash out of the La Nina attractor phases and into Phase 4 for the first time since the start of summer This feedback builds towards the programming of the substantial heat ridging development through this week The wavelength/timeline of the downstream switch augurs better for at least the first half of August fortunes. The test will be how quickly the default low frequency signal across Africa and the Indian Ocean regains authority and starts to re-amplify upstream once more. However, that is for another time. Very good support for a phase of very warm/hot weather, and with the nuisance upper trough displaced sufficiently away to provide good injections of hot continental air. As stated the other day, secondary low pressure systems will provide interplay with the downstream ridge and this ups the chances for thundery activity including elevated importations c/o some destabilisation over the Spanish plateau. However, for the period under discussion at least, harder for cool North Atlantic air to make the ease of inroads so far east as seen so far this summer. The extent and intensity of the heat advected from southern europe is of course a subject of great interest from a meteorological point of view. From an enjoyment summer perspective many will be grateful for some stunning seasonal weather and it won't matter if temperature records are broken. Very warm evenings for sitting outside with dinner and some glasses of wine will be greatly savoured. Hopefully this will become a familiar form of relaxation when, eventually all being well, I arrive here at a new home an hour from Lisbon. Just got to get the builders in to provide a pool! But first of all fingers crossed that the deposit gets safely paid this coming week:

...Like shoals of fish they are starting to point the other way in midstream... Recent NWP, up to the start of this week, had seemed suspicious the way it had reverted to extrapolating the default summer pattern out in time throughout the first half of August. Quite a subtle, but distinct shift has duly occurred over the last 2 days. Clear indications now that the return to cooler unsettled conditions after the upcoming plume is not in for a long haul, as by the second half of next week the axis of the upper trough looks set to re-orientate further away to the NW of the country and the Azores ridge allowed to create a downstream extension to extend and sit over and to the east of a greater part of the country. There is still likely to be some secondary trough inroads swinging around the main upper trough, but crucially the approach of these on a subtly different track - and advecting incursions of tropical continental airmasses ahead of them on a more regular basis and with the rather unseasonal North Atlantic airmass incursions less dominant. This implies an increased thundery nature to a greater number of the rain bands, with very warm/ hot sunshine in between, and winds much more often from a southerly vector than seen for much of the summer so far. There, I have said it.,..

There is an indisputable correlation between rising global atmospheric angular momentum and associated poleward advection of rossby waves created via torque mechanisms propagating such eddies in the jet stream between the tropics to the extra tropics. - this subsequent wave-breaking leading to the production of downstream anticyclonic ridges. The synoptics that arise are led by seasonal wavelength changes throughout the year, but rising AAM indicates westerly inertia being added within the tropics c/o eastward propagating tropical convection waves and which leads to ridge development in the extra tropics as the westerlies within the polar jet are prone to amplify and buckle downstream as the upstream pattern at the same time sees a de-amplification and jet extension across the Pacific. Understanding these processes and awareness of them gives a good insight into how NWP models the jet stream and this has to be seen as a useful tool in deciphering synoptic pattern evolution. It only becomes perceived as a "cure all" if such diagnostic processes are attempted to be fitted to idealised weather pattern biases without consideration of all other probabilistic outcomes, and which may be more likely at that given time. That said, and as outlined already by process - rising AAM is a very important ally to the creation of very warm Western European Branstrator ridges in summer. It is encouraging that the well respected Euro atmospheric modelling is on board with other model suggestions of recovery of angular momentum heading into August. This increases the chances of further ridge development to a higher level than seen during July and tendency for the jet stream, over time, to adjust somewhat northwards. With that in mind, I would be taking all NWP and ensemble/cluster data with a lot of caution beyond the limits of the plume advection later next week and weekend. The suggestion of the walker cell low frequency signal across Africa and the Indian Ocean continuing to control the pattern quite as rigidly as up to now as set against the uptick in momentum budgets and associated increase in wind-inertia which will have a knock on effect on upstream pattern may well be overplayed on this occasion - rather than the underplaying of that signal that has occurred since the plume event of the last week of June. Such underplaying as demonstrated by the Azores ridge flattering to deceive beyond about day 7 to ridge fully and expansively eastwards - only to correct backwards as closer time periods arrive. So it is not unreasonable to start to see some corrections the other way around with time - that is, notwithstanding some initial erratic progress probable early in the month which might lead to the illusion on a thread of fickle emotions of a thread like this, of August as a whole simply being more of the same. Potential model error likely occurring due to such over estimation of default Nina influence increase the further that August progresses - leading to the Nina-esque "wane" period as discussed in previous posts. This arising, as the intra seasonal high frequency MJO signal progresses eastwards across the tropics and (temporarily) "destructively" interferes with the underlying low frequency signal that has been dictating the pattern since the beginning of summer. One might expect the underlying La Nina-esque signal to return heading into the autumn, but the chances are increasing for the third phase of summer to improve with time and the upcoming plume scenario should not( in my non-forecaster appointed opinion) be seen as a last taste of summer proper

The quoted section and especially the bolded part are duly the focus for the coming period. It will take the convective MJO related envelope to propagate to the Maritime Continent before rising angular momentum tendency can start to alter the downstream pattern more sustainably. So underpinning expectations of a further changeable and cooler period to get through before any changes become possible into the new month. Extended NWP evolution is hampered by the fact that RMM MJO analysis modelling is totally scrambled by being superimposed over the low frequency standing wave across Africa and the Western Indian Ocean. So until a clearer trend emerges of the high frequency tropical convection signal propagating eastwards into August, away from the Indian ocean, then the numerical models will keep extrapolating forward the default pattern of the Atlantic ridge and downstream trough and keep delaying any suggested improvement out in time. However, taking NWP and its associated ensembles and clusters at face value requires even more caution that usual because they may act like shoals of fish pointing one way upstream and then suddenly change direction mid stream when the underlying signal emerges from all the "noise" within the tropics. Signals lead models, not the other way around The bottom line is it worth taking erratic extended NWP and modelling such as the EC weeklies with a large spoonful of salt until the signal from the tropics becomes clearer - fine margins in this situation could result in significant changes. Even if such a change would not be likely to produce a long lasting pattern change. However, aside from the very pleasant weather outside my window at the moment in my own locale and where some of the better weather has been - for a summer such as this from a more widespread perspective, many would take a true taste of seasonal conditions however long it might last. This is good analysis by you and it echoes the diagnostic sentiment as described above The final bolded text sentence is the only bit I would not agree with. Based on where the low frequency signal is aligned across Africa and the Indian Ocean and how angular momentum is likely to re-align from any upward trend during the first part of August (depending on the eastward extent of the high frequency MJO signal) there is no prospect of any orbit towards the El Nino attractor phases .(i.e from GWO phase 4 into Phase 5). The last attempt to lift angular momentum in the last week of June as we know was half hearted and only resulted in the GWO transiting through Phase 3 - before heading back to the Nina GWO phases and hence the heatwave/plume of that week subsided to the suppressed pattern that has largely dominated July to date The GWO is wholly restricted within the tropical>extra tropical negative wind-flow inertia created by the position of the low frequency tropical convection standing wave across Africa and the Indian Ocean - and the associated suppression across the Pacific. Hence the default phase is orbits focussed converging mainly around La Nina attractor GWO Phase 2 The maximum outcome this time around, based on long term GSDM wind-flow budget inertia within the atmospheric circulation is a more progressive push through Phase 3 into a low amplitude Phase 4 at the very most. That said, to achieve Phase 4 is arguably at least as difficult to achieve as late June and also based on shifts into high summer wavelengths - such has been the slide in global atmospheric angular momentum this summer to data. Adopting the same principle as with numerical modelling, care also needs to be taken with taking any composite for GWO phases at face value. These composites need adjusting to cater for all relevant drivers influencing the macro scale pattern at any given time and each situation taken on its merits and individual interpretation taken of the composite accordingly. This type of bounce in angular momentum tendency, related to the base state circulation, would likely be enough to promote a downstream ridge as the progress of any incoming trough would slow down sufficiently to promote a plume scenario - but would almost certainly be at behest of when the low frequency signal in the tropics returns to default. If there is shortfall in the intra-seasonal MJO propagation beyond the Indian ocean, that would push the trough through and then revert back to a retracted Atlantic ridge and downstream trough At present, there is certainly still time for adjustments to improve the chances of the former plume type scenario - but this is very dependant on the propagation progress of the tropical wave within the "mini ENSO cycle" - aka the MJO. If this is insufficient, weak and soon aborted, that means the default supressed pattern returns swiftly. On the other hand, if the models underestimate the (temporary) effects of the MJO masking the dominant walker cell circulation, the numerical models may tone down the upstream amplification and instead switch to a more expansive downstream ridge scenario and provide some relief, for a time anyway, from the tedium of the default pattern

This ties in with the suggestions made yesterday about intra-seasonally driven "wane" periods within the default tropical > extra tropical circulation The strong trade wind surge during the third week of this month is what is driving NWP at the moment and they are going to struggle to "see" past it. Most especially the GFS which invariably overreacts to robust signals and then goes to town extrapolating it outwards in time. A poor period of weather is quite likely during this time, but its worth waiting and watching for what happens thereafter. The powerful intra-seasonal MJO related kelvin wave driving this strong upstream signal will be followed by strong convective suppression in the area where the low frequency walker cell has been strongest and dominant so far this summer. This is tied into those longer range atmospheric angular momentum forecasts and the rebound associated with the lull in easterly inertia that will occur as a result of the convective suppression across Africa and the Equatorial Atlantic - and westerly wind propagation occurring (increasing wind shear across the main development region or MDR of the Equatorial Atlantic) as the convectively coupled kelvin wave (CCKW) heads eastwards across the tropics. The velocity potential anomalies depict this sequence with the blue shading (active convection) during the coming week and into the following week being replaced by the orange (suppressed shading) as the convective envelope heads into the Maritime continent region of the tropics by the end of July Little point in looking for clues for this in deterministic and 10 day ensemble NWP this early for quite a while yet because they will be churning out a suppressed downstream flat jet-stream for this period, but while the actual strength of the EC weekly bounce in AAM ,as reflected by the distinct downstream anticyclonic wave-break heading into August on the synoptic level may be open to debate - the evolution it shows is very credible based on the passage of this intra-seasonal phenomena which could well shake up the NH pattern from the default tedium - at least for a time. So caution is advised to assumptions about the Pacific heading into August by over extrapolating the suppression there which occurs due to the convective signal across Africa and the Indian ocean. On the other hand instead, an extended watching brief on that default low frequency "wane" (suppression) arriving as August arrives and how much it can create a change of pattern for a little while.

Pacific cyclones are a good thing for Western European summers - it demonstrates that the ENSO region is convectively active, trade winds reverse c/o the westerly wind bursts which accompany the thunderstorm and cyclone development and can help the downstream pattern reverse from what we are seeing this summer with a corresponding low pressure anomaly centred to the west in the Atlantic and a perpetuating downstream ridge which ebbs and flows over and to the east (in the same way in reverse that the default retracted Azores ridge is to the west this summer due to the opposite much less helpful configuration) Hence, much more continental influence than we are seeing this summer and also thundery potential as the default Atlantic trough disrupts against the downstream ridge from time to time. Rinse and repeat in a very good way. In the present however, with so much influence from the North Atlantic arcing around the perimeter of the retracted Azores/Atlantic ridge it is no surprise to see the cool and changeable pattern that has dominated and with the upper trough being constantly reinforced to the N and NE and preventing any continental influences. Also no surprise that, last week of June aside, thundery potential from plume scenarios has also dwindled to nothing. The reality is that it was possible to see back in May that a traditional La Nina-esque type circulation might well develop and associated Atlantic ridge and downstream trough - what couldn't be predicted was precisely where tropical convergence would arise and on that basis it was quite possible that a more blended pattern may have resulted. Indeed for a time this was actually the case as the late June mini heatwave demonstrated. However,. the high frequency MJO tropical cycle has become very quiet since late Spring, and following the active cycles during Spring itself which generated the westerly inertia and sustained atmospheric angular momentum above average. More about the high frequency signal later into the post. Frictional torque tendency has been buoyant as described in previous posts. However following what has transpired to be a brief westerly wind burst east of the dateline, and which if sustained would have helped the possible signs of improvement that were apparent up to recently, much of the inertia is now concentrated south of the equator in the southern hemisphere and trade winds across the Pacific look set to see further quite strong phases through the second half of July This will certainly serve to keep angular momentum below average and the Global Wind Oscillation spot welded in the low angular momentum La Nina attractor phases. Notice the contrasting westerly wind anomalies at 30N prior to the start of summer driving the downstream anticyclonic wave-breaking and the switch to easterly inertia at 20N to 30N in recent weeks - switching to upstream amplification at the inflection point of the negative torque mechanism - scrubbing all the westerly inertia from the system .....and this in turn will perpetuate the upstream amplification and the downstream response to that of an Atlantic ridge and upper trough that keeps trending into Scandinavia. NWP at present is showing some signs of e/QBO type stratospheric interference which up till now has kept at least the sub tropical ridge in play and prevent a sustained supressed jet stream sinking further south. On previous occasions the modelling, for reasons given back then, has rowed back on this development, so a watch is required as to how this plays out this time. What could help alleviate or provide some respite from this disappointing pattern? Signs of some activity from the high frequency MJO tropical signal propagating eastwards from the Indian ocean and at least temporarily acting "destructively" on the default walker circulation which is rooting the low frequency tropical signal across Africa and the Indian Ocean and which in turn is boosting all the easterly (trade wind) inertia across tropics and into the Pacific. It is this atmospheric chain that sets up the downstream responses that are so evident. Also heading ultimately into the latter part of summer it could be that just a small recovery of momentum c/o of some MJO related activity allied to ex tropical activity curving around into the Atlantic may create some warm air advection to shake up the pattern - at least for a while. Even in default long term term regimes such as these, it is natural for there to be a "wane" phase. Low angular momentum summers quite often see this the further one heads into August and often September as well based on intra-seasonal factors that can drive these temporary changes before the ridge/trough regime returns into autumn. Though some will consider this too late, the reality though is there is a good 10 weeks or more where summer like temperatures are quite easily achievable with better synoptics. I would recommend looking into the diagnostic element as described and at the whys and wherefores - understanding why patterns entrench themselves like this is a constructive antidote to the rather pointless and self harming processes of agitating and moaning about the weather outside the window and which should be in the appropriate thread anyway, if one has nothing better to do with their day

Pode me elucidar por favour? This isn't picking on any given individual before I am duly charged and confined within a nunnery? But such a sweeping assertion is very questionable . It is one model suite - as a snapshot in time. Till the next model suite and the next snapshot in time - and so on and so forth. Why should this particular suite have any more credence than any other? There has been apparent agreement for all sorts of outcomes in recent weeks and yet countless varying outcomes - such is the customary nature of numerical models that evolve according to the signals and so it is not possible to make definitive assertions about any apparent computer model consensus over any other. If there is indeed any at any given time. As emphasised so often in recent times, NWP is going to keep struggling based on the persistence of the low frequency tropical signal in the Western Hemisphere amplifying the upstream pattern and tending to accentuate the jet stream downstream vs on the other hand wave breaking within the tropics at times propagating rossby waves into the extra tropics and building transitory series of downstream ridges within the mid latitudes. Hence the ebb and flow of the Azores ridge. What is now happening, and what is different to what has been seen in the previous 15 days, is that sub tropical ridging is becoming more prominent downstream due to greater anticyclonic wave-breaking and with relatively less erosion of the ridging from the north and less pronounced retraction of the ridging than previously. As also stated just yesterday, there is probability still for innate tendency for this ridging to pull back westwards from time to time and much of NWP vicissitude will revolve around this equation - but for such interruptions to be appreciably shorter and less extensive than seen since late June. On that basis it is not intuitive to pronounce any particular NWP as more consensual than any other as there are also bifurcating processes at work downstream to combat the attritional effects of the jet riding around the northern perimeter of the Azores/Atlantic ridge This will continue to be an evolving process and not the essence of one given model suite to rigidly extrapolate its findings out in time.

Um bom dia para todos The switch of the tropical signal to Africa and the Western Indian ocean as summer arrived has marked the extent of the fall in global atmospheric angular momentum since the beginning of June. Registering at -2SD (standard deviations) below average, global momentum has been depicting a typical traditional La Nina type atmospheric circulation as manifested by anomalous Pacific and Atlantic ridging and downstream trough arrangement. Up to this time, this has fully underpinned the elusive nature of the Azores High in flattering to deceive with its eastward ridging to deflect Atlantic low pressure areas from moving west to east too close to the north of the UK and the complete absence of any continental influences since the very warm spell in the last week of June. However, upward trending frictional torque tendency has continued further, which as explained previously, translates to some added forcing to tropical>extra tropical propagation processes leading to a lagged subtle alteration of the downstream pattern c/o greater rossby wave induced anticyclonic wave-breaking from the sub tropics . This process is not invalidated by the constant distractions of cent capricious numerical models, and still looks set to provide some diffusion to the unfavourable progression of low pressure areas. Mostly, NWP is now advertising this much more consistently - albeit some erratic progress towards it has still continued from one suite to another. With that in mind, the latest post by @johnholmes aptly observes within his customary very experienced summary, how numerical models struggle in these situations, and why taking them at face value with their flip flopping around over the finely balances wind-flow inertia upstream is better advised to monitor over a day or two rather then drawing instant conclusions with each and every operational output. To manage expectations - based on the unfavourable La Nina aspect to the global circulation, rossby wave propagation in this sector of the NH is not going to be as effective as it was so consistently during the Spring to create long lasting anticyclonic spells. There will still be some innate tendency for NWP to want to retrogress the pattern from time to time. But the second half of July continues to hold the prospect of more settled and warmer conditions with the constant interruptions of cool air from the northern and eastern Atlantic c/o of low pressure systems passing too close to the NW, becoming fewer and further between. The essence of the upstream pattern has been due to North Pacific ridging configuring the flatter supressed jet downstream pattern and retracted sub tropical ridge. The e/QBO signature has not been ascendant, which with a La Nina type forcing would have augmented a much more definitive -ve AO signal, and also with the absence of the Azores ridge trying to adjust eastwards from time to time, a persistent southerly tracking low pressure c/o an authentic -ve NAO would develop instead. As occurred during summer 2007. Despite the jet stream following a path at a lower latitude than once might hope for, a bone fide -ve AO/-ve NAO has simply not been the case through this first half of the summer Clearly this illustrates that low solar activity has no one size fits all correlation to a suppressed jet stream, but depends on whether the polar or ferrel cell is in ascendancy. As many posts have illustrated so many times - there has been sporadic forcing from the equator evident so far this summer which has bolstered the ferrel cell, and while it has still not prevented the less than inspiring conditions of the last fortnight, still shows that actually it could have been worse still than this. A fast downwelling e/QBO would have had profound effects on stratospheric downwelling of negative zonal winds and coupled with low angular momentum and low solar forcing - creating a locked in cool and unsettled pattern. Whilst some for whatever reason, are evidently dreaming of this even in high summer, despite the disappointing end to June and first part of July, the pattern is not locked in this way, with some warmer drier spells evidently appearing in the modelling. This subject has been previously discussed and outlined in detail at least a couple of times in recent weeks as to the plethora of factors that are interactable and determine the macro scale pattern. There is simply no one size fits all x=y correlation. Much of this though is related to weather preference bias (especially when out of season winter aspirations keep being referenced). Obrigada e cumprimentos

Subtle, but interesting things happening. Various posts so far this summer have referred to off equator forcing from the tropics providing some interruption to the default summer pattern, and have been watching frictional torque tendency for a while which has reflected this with an almost imperceptibly slow trend upwards. There is now a slight surplus of westerly inertia (increased momentum) across the tropics. This inertia is circled on the plot between 20N and 30N I know that @Singularityhas been looking at this as well What does this mean, why does it matter, and what relevance does it have for this neck of the woods at mid latitudes? Frictional torque tendency increases, tend to migrate wave eddies poleward with time into the extra tropics and increase angular momentum tendency. As the rossby wave packets propagate northwards and eastwards from the tropics, positive torques are induced via mountain ranges in the extra tropics and this turning force adds amplification to the jet stream further downstream. This is in contrast to the negative tendency regime where frictional torque tendency is negative towards 30N and which create a negative inflection point in the Equatorial Pacific and adds momentum to the jet stream downstream in the extra tropics. As we know the latter has been a dominant feature, mostly, of the summer so far as the last ten days illustrate clearly with a flat pattern and retracted Azores ridge. This regime has a further few days more to play out before things start to change by the end of the week. The process of propagation to the extra tropics is somewhat lagged and takes up to 14 days approx based on intra-seasonal cyclical timescales. But the slow upward trend has been in progress already for some while and signs are appearing that this is about to start re-configuring the NH pattern in the coming days. On the GSDM plots (from which the plotted FT plot is taken) and which are consolidated lags of two days and relate to the present regime - the changes occurring within the tropics do not register yet in the extra tropics, and torque tendency and angular momentum tendency remain supressed. Note the sharp drop in tendency towards the end of June which timed precisely with the mini heatwave concluding However, based on the propagation lag of the slow trickle upwards of tropical inertia of late, this has some implications for what happens in the extra tropics heading towards the middle period of July and the start of that process is being advertised within NWP suites with ridge building occurring in our locale and possibly towards Scandinavia with time. This adds credibility to some seasonal modelling such as the CFS which has been flagging this up for a while previously. Recent EC weekly modelling was pointing to continuation of the default low frequency tropical signal throughout most of July and from my own point of view I have been cautious up to now as whether the off equator forcing would be sufficient to diffuse the effects of low frequency walker cell signal somewhat and for how long such dissonance might last. On that basis it is interesting to note the improvement on the latest EC weekly suite to last Thursdays update - which seemed to be suspiciously over egging the background low frequency signal and now better reflects events as they are now unfolding, taking away some of the short and medium term uncertainty. Present trends are encouraging for a much more settled and warmer spell of weather heading through the mid month period. How long might this last? That remains difficult to know, based on the background Nina-ish low frequency signal, but it could well be there is also assistance, as discussed yesterday, from the high frequency MJO signal shifting some forcing into the Eastern Indian Ocean later in July and adding some further westerly additions to the tropics to support the upcoming break in negative tendency east of the dateline in the Equatorial Pacific. In that scenario is it is conceivable that fine warm/very warm weather may be further supported and continuing to provide a very nice contrast to the end of June and first 10 day period of July. At some stage, based on the step change of the background atmospheric shift to a more Nina-ish default, it is likely that a downstream ridge would come under pressure and more of a trough regime would return downstream as the Pacific amplifies upstream once more, but clearly little point in speculating too far ahead based on nearer term developments which are encouraging for summer proper to return for a while

There was talk recently of the usefulness of continuity analysis and this is one of those very good opportunities to put it into practice to measure progress. NWP is back on the trail of suggesting an eastward adjustment of the pattern, the interpretation of that being that the troublesome low frequency tropical signal that roots the unfavourable downstream pattern of Atlantic ridge and overspreading trough is being, at least temporarily, masked by some influence from the cyclical high frequency MJO signal. . Still room for this to muddy the water a little, but a reasonable chance now of at least a break in the tedium that has been evident since the end of June Overall, the message is still the same one as before: Better to not expect long settled spells, but reasonable to continue to expect interruptions to tedium, and within the much more restrictive (to NW Europe fine weather spells) macro hemisphere scale wind-flow regime that has taken over since the Spring The QBO database has been updated and shows that June continued the stuttering evolution of the easterly downwelling phase. Worth repeating yet again, that whilst this summer is not proving a classic, the eccentric nature of the QBO has so far saved NW European weather from a worse fate such as seen in 2007. 2020 Jan -2.51 Feb -3.20 Mar -4.36 Apr -5.03 May -4.86 June -2.78 (to date) QBO Calculated at NOAA PSL 30mb zonal wind at the equator, zonal average For info https://psl.noaa.gov/data/climateindices/list/ units=ms-1 https://psl.noaa.gov/data/correlation/qbo.data Compare that to 2007 2007 Jan +2.61 Feb +2.43 Mar + 1.24 Apr -5.18 May -14.06 June -21.33 > (July -24.92 Aug-27.41 Sept-28.13 Oct -29.05 Nov-27.61 Dec-19.48) Very robust and rapid downwelling of -ve zonal easterly winds stratospherically generating the anomalous strong heights across the arctic that summer, at the same time as augmenting the strong La Nina influence on the tropospheric pattern The nature of this relationship will be important for the remainder of the summer, and also have implications for the season(s) to follow in terms of how the weather patterns play out - bearing in mind the longer term continued likelihood of a La Nina-esque circulation adopted to the atmosphere, and relating to how the downstream wavelength is configured accordingly