Tamara

The sub tropical warmth ahead of Storm Eunice evident in the mildness in the UK today and certainly helping further fulfill the 22C warmth down here in Portugal close to the dominant Azores High which has been robust since the start of the year, keeping the seasonal rains away(and increasing drought issues heading into Spring and Summer to follow) That baroclinic zone where Eunice is right now undergoing rapid cyclogenesis separates this sub tropical air from the conveyor of very contrasting arctic cold air exiting Canada. This symptomatic of the set pattern of deeply +AO across the arctic and the entrenched vortex to the NW and the intense sub tropical high pressure close to the Azores, so it was a matter of time perhaps before such a string of volatile storms occurred across the Atlantic and rather mirror the dominant pattern that lasted even longer through the entire 2013/14 winter. Storms Dudley ,Eunice and the further disturbance(s) to come over the weekend which already have yellow warnings out for southern most parts on Saturday being the epitome of sustained on-going 200mph jet streaks in such deeply +ve annular mode regimes. So lots of comparisons on this thread with storms of yesteryear but the 2013/14 yardstick looks to me a good one for the present stormy spell. If I recall correctly when I lived on the SE coast back in the UK, one of the strongest of those storms produced gusts in excess of 80mph (as evidenced in the late evening of that particular storm when a section of the neighbours roof was whisked off into the night sky and crashed into the street some 60 yards away. The street lamps were flickering on and off like disco lights!). I think that a high amber warning stuck throughout that storm (at least in my locality) so bearing in mind all the way along the south coasts it looks set for gusts up to at least 80mph, it will be interesting to see what happens this time an it is not a given there will any upgrading (there at least) from 'high amber'. The SW clearly justified with the red warning. It is though conceivable that the zone maybe most likely to see an updated red warning will be in the highly populated zone north of London from the east midlands into East Anglia - especially if any sting jet c/o dry air intrusion behind the cold front manifests itself. That puts METO in a difficult situation based on seeing how (and where) the Shapiro-Keyser explosive nature of this storm peaks to maturity. The modelling (to date) has suggested this to potentially occur over the eastern part of the UK and possibly into the North Sea as Eunice exits. But this is where t0 modelling of the passage of the storm is required and where the METO might be forced to wait, with the result potentially that a very late upgrading of the amber warning is merited. That leaves them a bit between the devil and the deep blue sea. So its satellite/sounding observations from this time onwards and seeing if the higher resolution micro modelling picks out any further of these indications of superimposed sting jet gusts on top of the general background intensity of the storm. From the peace and tranquility of another Portuguese day coming towards its beautiful sunset, stay safe at home, if possible, to all those of you in the path of this thing back in the UK

It wasn't wrong - if the usual eccentric deterministic and ensemble distractions that enrapture this thread (as reviewed in tandem with this product) are ironed out. The UK heights leading to diffused Icelandic/Greenland heights c/o decelerated -ve zonal wind anomalies within that region was fully advertised (correctly) by NWP in accordance with the diagnostic initial momentum transport from the tropics c/o the high frequency tropical wave advancing across west of the dateline and AAM tendency rising sharply accordingly Subsequent to that, and as also documented accordingly in this thread amidst the hysteria - there has been a boost to the Hadley cell at the same time as organisation of the polar field profile has reversed polar rossby wave transport poleward (again explanations given) and made a subtle but acutely disparate impact to the Northern Annular mode to what was potentially anticipated from earlier in December and the run up to the festive period.. Just because wintry impacts were far less than whipped up on this thread for a small island within the context of a very large northern hemisphere, doesn't mean that the equivalent macro-scale pattern numerical modelling was wrong.

Yes - the perennial problem is the lack of promotion of the most probabilistic outcomes. If this was set up as an objective weather pattern discussion thread that sought to provide the sort of detailed, technical post you describe (to find the most likely outcome and not ANY preference bias whatever that might be) then the site would not get the traffic it requires. Next to impossible to achieve objectivity with the title of the thread having the word 'hunt' inserted in it. A word that invokes tribalism and x vs y divisiveness in itself (and somewhat Neanderthal in nature) so further augmenting the ridiculous factionalism. By way of balancing that - there is a separate thread devoted to detailed technical impartiality but no surprise that so few people go to the trouble of posting in it, despite its very creditable aim - as it is rendered futile and a waste of time and energy when the majority on this site are more interested in preference indulgence than observing how t0 arrives at the correct solution and not always the preference solution. By observing the correct solution, and analysing how that came about it provides a basis to learn more. There was an excellent technical discussion on this thread several years back, which was inhabited by a plethora of very informed posters - both amateur and professional. At the time it succeeded in getting quite a wide audience from within the standard model thread with people eager to learn from it. I for one learned so much from this and lead to the example I try to follow these days, (allowing for so many complexities to attempt to fathom out). However the drive for quantity over quality in the way I have tried to describe has resulted in the loss of quite a few of these regular contributors. That, and the parallel drive towards twitter and social media outlets (which bring problems of their own due to brevity of messaging/misrepresentation and clickbait popularity issues) So no surprise that my own view of the outlook within this thread, will probably have a 'bland' unpopular perception to it. Baroclinically induced Pacific forcing is leading to strong westerly inertia off equator - and has driven atmospheric angular momentum much above average (especially related to the base state). Equatorward fluxing ( rather than poleward) is creating anomalous Hadley cell strength ridging, rather than amplifying high(er) latitude ridging - dovetailed with an organised polar vortex associated with upper cooling. Wave trains continue to point to such anomalous ridging into mid latitudes while a shift of vorticity within the Canadian sector drives an increased thermal gradient exiting the US. The abundance of orange shaded AAM anomalies within the tropics (westerly inertia) identified clearly within the relevant GSDM plot as posted: This is where clarity in respect of poleward AAM anomalies is required to avoid apparent contradiction in terms. Momentum transport c/o rising AAM through the tropics into the extra tropics is very much governed by the vector of directional flux. Equatorward flux will restrict the amplification of sub tropical ridges that might otherwise transport poleward to become extensions of arctic height rises, (if the polar field is unstable enough to allow these). The current situation continues to see extensive wave breaking and significant Pacific jet extension c/o large East Asian Mountain torque - but directional flux and a well organised polar profile (vortices) restricts amplification of the associated rossby wave eddies to anomalous mid latitude Branstator ridges. I suggested that the outlook into the early part of 2022 would be interesting and fascinating either way, irrespective of preference, and that is proving very much the case (for me at least). The precise directional fluxing of these amplified westerly wind eddies in accordance with the ongoing tropospheric/stratospheric interaction proving the fine line and highly disparate outcomes that can result - depending on such relationship between them. Open mindedness comes with lack of the burden of bias intrusion and distraction, and has its rewards Muitos cumprimentos e respeitos

As was not apparent at time of posting. NWP duly advertises the Atlantic sector amplification - the degree of polar jet follow-on yet to be confirmed by the same numerical modelling. Hopefully the bulk of the cold air advection doesn't get trapped further south within the amplified Azores high before it flattens out. These 20C to 22C afternoons ere just bliss and cold nights next week with risk of frost can certainly do one for sure.

Hi there The intraseasonal tropical cycle has supplied the wave driving to overlay the walker cell for sure, but looking at the SST arrangement in the Pacific there remains plenty of fuel for the standing wave to not move far yet (keep imprinting and firing convection) from Indonesia into the West Pacific (consistent with a La Nina type feedback). The stalled intraseasonal MJO cannot sustain indefinitely and without any immediate evidence that the present westerly inertia c/o the MJO is altering the spatial arrangement of SST's (and upwelling) in the Pacific in the near term , then as soon as the effects of the intra-seasonal dampening are removed then angular momentum, (currently operating at close to 2SD (standard deviations) above average (and a considerably precipitous El Nino'ish disconnect to the base state) will likely undertake a sharp correction. It could indeed be that La Nina is peaking and that transition will start occurring by the Spring, but on present evidence, the feedbacks remain for a while yet with the walker cell well west of the dateline and the intraseasonal MJO is/has been massively blind-siding the reality while it has been stalling at apparent high amplitude. Timing of all this (ie when the intraseasonal signal loses its grip is difficult based on the very extended stall). But, in synoptic terms, the next minefield for the numerical modelling will be when this happens and in the absence of any steering within the stratosphere to the troposphere (very much the other way around) then it seems logical to think that a traditional +NAO/+AO is a probabilistic outcome. Evidence to the contrary can and may emerge to change this, but until or if that happens then this is the watching brief in my opinion.

The MJO is the tropical convection forcing equation in the global atmospheric circulation and adds westerly wind (forward inertia) within the tropics as it heads eastwards. However, as has been repeated several times over the past couple of weeks, it is just one part of the global wind=flow budget. Propagation of wind-flow eddies c/o of tropical convection westerly wind bursts can rise into the extra tropics and head poleward from there to influence the stability (instability) within the polar stratosphere - but the discussion needs to get away from any interminable x+y=magic bullet to deep freeze conjuring act because momentum transport can just as easily result in anomalous mid latitude Branstator ridging as is now clearly advertised. Notwithstanding that AAM did not produce sustainable blocking at higher latitudes on this occasion (but certainly did create anomalous ridging at 50N to 60N in the pre-festive period) a correction is (ultimately) inevitable in the extended period based on the fact that the tropical component of the GSDM budget has been propping up the rally in AAM and which is precipitously incongruous to the base state, For all of the chatter and excitement about MJO Phase 7 in the Pacific - its just a matter of time before the current high frequency cycle gives way and a sharp correction leads to the return of the polar jet. As a numerical modelling signal of that, the vacuum created in the jetstream upstream as -ve inertia suddenly returns to the Pacific, may force a short lived sharp amplification in the Atlantic sector (leading to further hysteria in this thread) but the over-run of the polar jet highly likely to flatten any cold air advection and introduce a much flatter westerly flow. In the meantime, a quite glorious week awaits here in SW Europe with the muggy damp warmth of the festive period evolving into dry deep blue sunny sky warmth to end the year and lead into 2022

I do understand the feelings of UK members about the lapsed cold spell, but I am excited about the upcoming sub tropical ridge resident across my new homeland next week which looks set to produce afternoon values nudging towards and maybe even above the 21C mark and which seems surreal for the heart of winter..Its one thing having a winter break and some winter Mediterranean sun, but when its your new home it makes you stop and try to remember what month of the year it is because you are not getting on a plane back anytime.... Quite surreal for a first winter living out of the UK. Feliz Natal x

@Singularityanswered this question very well a few pages back *and very much a similar message I repeat many times). The MJO is one component of the global wind-flow budget (a very important one) - but the extra - tropical circulation also has to be considered to get a fuller picture of the overall tropospheric circulation and how in turn this affects the relationship within the stratosphere As far as I am concerned the tropical MJO and extratropical GWO (Global Wind Oscillation) have produced a' harmonious' wind-flow signal based on amplified rossby wave propagation from the tropics to extra tropics and created a profile for deceleration of zonal winds between 50N and 60N. Hence the presence of the wedged heights circa Iceland to Greenland, following on from the mid latitude block close to the UK. However, the impression among the disappointed within this forum that the diagnostics 'doesn't work'is not just down to the need to fit the diagnostic to one favoured outcome and which obscures that the synoptic response has actually been a close match in terms of height anomalies and placements (notwithstanding the failure for strong enough and sustained enough cold air advection to deliver the weather that UK members want to see). Its also to do with the following: 1) Insufficient cold air close by to the UK to tap straight into and create a stronger dense profile of cold air - hence harder to get enough negative tilt on low pressure areas to disrupt for the greater part of what is a small geographical area (the UK) within a large Northern Hemisphere, let alone island within a large continental landmass one side and a vast ocean the other. Precise geographical alignment is required at the outset to direct this to a small island, notwithstanding 2) which is most relevant of all 2) However another take-away, and really the more important one that looks set to facilitate the lifting out of cold air trying to advect southwards across the UK without too much resistance is the solar uptick/ -ve (easterly) QBO interface helping to weaken the burgeoning heights over the Arctic and hence the blocking structures towards Iceland retrograding towards Greenland have been made weaker than they otherwise might have been. At the same as the polar cell is weakened, a boost to the Hadley cell occurs and subsequent expansion of sub tropical riding. The interest, which I have repeated looking ahead, is whether the adjustments occurring within the Pacific to alter the rossby wavelength feedbacks and hence keep an unstable GSDM profile (which adds further amplification into the atmospheric circulation) can override the ongoing dampening down of blocking structures trying to establish further poleward as further momentum transport occurs, or whether the countervailing equatorward processes prove stronger than this and instead sustain strong sub tropical ridges amidst the backdrop of the lowering of pressure in the middle and upper layers. Its very much one set of forcing coming up against an opposite set of forcing and really quite fine margins between a strong Hadley cell and weaker polar cell or vice versa, weakened sub tropical forcing and increased propensity for momentum transport to migrate anticyclonic wave-breaking to higher latitudes. In this respect a floor to angular momentum is required additionally through active tropical convection sustaining deeper into the winter- as collapsing AAM entails greater polar jet strength which would add further weight and attritional forcing to the solar uptick/-ve QBO forcing lowering pressure at higher latitudes. Its not straightforward one way or the other. Equally, the -ve QBO, on the basis that the solar uptick is not sustained could on the other hand dovetail with buoyant tropical convection and lead to the opposite regime and evidence of other winters with similar parallels to now has seen January and into February have quite different blocking structures to the ones advertised for the end of 2021 and into 2022. I am happy with the current evolution and some further balmy warmth down here in Portugal, but from a weather pattern point of view and suspending personal preference, it is also very interesting to see how these conflicting tropospheric/stratospheric factors play out and which gains an upper hand over the extended period.

I'd suggest otherwise. Some do bother - and all year around. For my own part that means low key in terms of contributions at this time of year, but the same keen interest applied in the weather patterns behind the scenes, irrespective of it not being my own season of preference choice. Applying the same diagnostic approach, all year around, means looking objectively at all probabilistic patterns - irrespective of preference. A good way to see model weaknesses all year round and in that way, an aid to stepping back from them and questioning the model interpretation of of the diagnostic rather than getting sucked into the numerical model product itself (including very much what is and what isn't pleasing to the eye) That is deemed boring of course by the majority who just want weather preference excitements fulfilled. Its just not possible to have cake and eat it in the sense of wanting a 'chase' exclusively to seek out a weather fad, but at the same time expecting accuracy by attempts to fit numerical models and even the diagnostics driving the numerical models to the outcome of choice. As soon as human bias (and more than it is evidently true, obsession) comes into play then the margins automatically start to skew very very quickly. Which leads to the sort of frustration and introspection that inevitably fills up page after page afterwards, time after time, year after year (at this time of year) .All of it quite unnecessary - but that leads back to the cake and eating it. Have the cake by all means (enjoy the 'chase') but don't always expect to be able to eat it (savour the end product to the chase) Most of all though, little or nothing gets learned that way as means to avoid the same issue next time. Some people take issue with this sort of dispassionate thinking, even get angry towards it when bias is spoken of (and I know that from my own experience in receipt of it) But there is nothing clever, or preachy involved - it is simple common sense and also means that a lot more time is freed up to do other things in life besides watching every computer model and every frame of every computer model. Which means coming back with a fresh mind and a fresh eye. Observing and learning about weather patterns all year round, slowly starts to forge an understanding of sub seasonal periodicity of patterns which can be of very useful assistance over the longer range. Very much not a hindcasting format.. In many ways the atmospheric circulation is exhibiting the same imprint of late October time. with the addition of westerly winds amplifying the flow, and this can be traced through the periodicity of the tropical and extra tropical feedbacks cycles which move in up to 60 to 90 day timelines i.e in this case a similar wave-breaking pattern in the Pacific though this time around the West Pacific forcing is stronger still and also a more sustained feedback that looks set to keep an unstable atmospheric circulation vs ENSO base state (GSDM measured) right into 2022. However it is evolving and should re-deploy convergence zones further eastwards along the tropics, re-configuring the rossby wavelength in the Pacific consistent with an east based La Nina. Including therefore changes to the stoical high pressure in the North Pacific which has been both propagating the tropical signal in the Western Pacific but also inhibiting movement of the standing wave due to the convective suppression exercised by the presence of the Pacific high through wind-shearing propagation eastwards of the convective westerly wind bursts (in association with the MJO signal). Changes to this though, and according to a couple of previous posts which touched on it, are highly interesting to watch for the opening month of 2022. Hated as they are by the majority (but most appreciated by me in Portugal) those Iberian height rises in the modelling are just one component as an opening part of that process. A probabilistic envelope as always, but for this thread anyway (and putting aside my own ambivalence to cold weather prospects in the UK) worth repeating that the atmosphere remains very primed and pre-disposed to amplification and greater chance of model thread reward is well within that amplification envelope.

Those iberian heights programmed around the 10 day period make complete sense to me based on earlier discussion wrt poleward momentum that is set to keep recycling, Active tropics> active extra tropics and a disrupted polar profile sets a highly intriguing early part of 2022, irrespective of the froth over the festive period in the UK As an aside, the 18z GFS is a lovely thing for my part of the world. 21C by day with a high pressure profile that is carved out of summer than the depths of winter. For the UK folk though, this is setting up quite a season to watch and enjoy for quite different reasons to me. Boa noite

With that in mind, some attempt not to 'hunt' for anything at all but simply a bit of analysis among the fast reactionary NWP frenzy 1) A lot of talk continues about the MJO. At the risk of further repeat, the MJO is just one part of the wind-flow budget within the atmospheric circulation and so an assessment has to also be made of the wind-flow inertia within the extra tropics so that in turn, conclusions can be made about how both the tropics and extra tropics are interacting with one another. This is especially important at this time because what is happening within the troposphere is influencing the make-up of zonal wind inertia within the stratosphere. In other words, the polar vortex within the stratosphere is under the control of the tropical>extra tropical circulation within the troposphere 2) With that clarified and in order to assess the wind-flow budget within both the tropical and extra tropical wind-flow circulations, we can turn to the GSDM (Global Synoptic Dynamical Model) plots and break down the components individually to come to some conclusions: 3) Looking at frictional torque tendency within the tropics, this is heading upwards as surfeits of westerly winds are being added into the atmospheric circulation c/o the high frequency tropical cycle (MJO) heading eastwards through the Pacific. This process is leading to large scale deep thunderstorm development and the creation of convergence zone outflows and associated wind-flow eddies within the jet stream (expressed as the torque itself). 3) With time, these wind-flow eddies and surplus westerly wind inertia can propagate into the extra tropics as a manifestation of this momentum induced turning force on the jetstream, and rise in angular momentum tendency (See the AAM tendency >T/time plot with depicted lag marker added to account for extra tropical feedback ) Taking a look at both the frictional torque plot and also the angular momentum plot , the westerly inertia (shaded orange) is propagating through the tropics through to 30N along the convergence zones of the eastward moving tropical convection. According to the laws of the conservation of angular momentum, there is compensating negative easterly inertia (deceleration of the jetstream) along the mid latitudes (currently) circa 50N to 60N. Hence the latest anomalous blocked pattern across much of Europe . 4) The outlook period is predicated on the further poleward momentum transport of these westerly winds c/o mountain range torque acting to similarly create eddies which are forced upwards into the tropopause and lower stratosphere layers. It is not a given each time that westerly wind propagation from tropics >extra tropics will be a co-operative process. However on this occasion that does look to be the case - explanation given:. 5) . One major factor that determines extents (or failures) of momentum transport into and then through the extra tropics (from the tropics) is the relationship between the atmospheric circulation and the base state, and whether this is a stable relationship or an a-typical unstable relationship. Presently there is an evolving highly unstable relationship due to the low frequency La Nina standing wave being 'destructively' overridden by the highly active high frequency cycle and this is acting to accentuate the trend to weaken the walker cell circulation and adjust it further eastwards than is usual within a traditional La Nina. The consequence of this is to create longer term feedbacks within the tropics and extra tropics and perpetuate an unstable GSDM atmospheric circulation relationship to the base state, and in turn also alter the tropospheric >stratospheric pathway as poleward momentum transport disrupts the lower stratosphere on a sustained basis as the displacement eastward of the low frequency standing wave alters tropical convergence feedbacks from the tropics to extra tropics and perpetuates forward moving and poleward momentum. 5) What do all these complex interactions mean synoptically? As stated, the further outlook is based around these poleward momentum processes and the present blocking mechanisms at mid latitude amplifying at higher latitudes as the troposphere feeds the lower stratosphere with planetary wave disturbances and 'warms' these echelons of the polar field. In that sense, and as alluded to in a previous post recently, blocking structures are likely to continue to dictate the numerical model number crunching for some time to come - with the confidence now that perturbing of the polar field has a feedback to take proceedings into the early part of 2022, at the very least and so amplified blocking equally looks set to ebb and flow well into the extended period I am (deliberately) not going to get into the main discussion about how much snow, where and when etc in the UK. That is not my own purpose and interest in proceedings. Suffice to say, a suppressed jetstream through the festive period final week of the year looks set for a lot of wet and windy weather at my latitude, but with temperatures anomalously warmer than they have been overall (night-times through much of November and into the first part of December) have been unusually much lower than average) North West Europe mostly on the polar side of the jetstream with considerable cold air advection potential trying to head southwards - but with the very tight margins between wintry conditions and cold rain as being discussed endlessly within this thread. Of more interest personally is the longer term implications and the annular mode quite conceivably coupling very closely with the deepest seasonal and monthly switch of wavelengths and for those of you 1200 to 1600 miles and more further north a good chance that the wintriest conditions may well be seen through January, especially if the GSDM feedbacks and troposphere>stratosphere feedbacks leads to a SSW event much as happened in, for example, January 2010 following a troposphere led opening part of the winter from mid December to mid January. Boa tarde, boa sorte.

Posted about 3 weeks back in a now locked thread : The present amplified pattern, as stated early on in this review, is wholly in keeping with approach to winter La Nina seasons that have gone on to produce stable relationships (and hence less likely to produce cold air advection blocked patterns). Equally also from present evidence, it is conceivable that an a-typical relationship Nina base state could evolve which sustains amplified tropospheric patterns into the greater winter and increasingly switches the emphasis of the position of these from the Atlantic as time goes on. This then destabilises the GSDM profile and in turn is an indicator of greater tropospheric >stratospheric destabilisation. Which itself is still not a guarantee of anything, even if the probabilistic envelope is shifted. Answers to these particular question will come in during the coming few weeks. But my own interest in this is not from a cold weather pattern starting point, personal preference (or because geographically its a case of lack of likelihood of significant wintry weather) but simply which direction the diagnostic pendulum swings over time. Much the same approach as any other season - again irrespective of personal preferences. 3 weeks later and the very same perspectives apply and have proved very consistent with the watching brief offered at the time. The probabilistic envelope has certainly shifted - and towards an unstable GSDM profile. However, as alluded to within the italicised text,, this still cannot (and should not) be fitted into one one bias solution - though it is understood that this suggestion is admittedly likely to fall on some deaf ears An unstable GSDM profile does for sure greatly enhance the prospects of blocking - but the problem is that the definition of blocking on this thread, that immediately translates to a deeply cold air advection block heading for the UK, is not the same definition as within the dispassionate realities of the diagnostic envelope. But for the moment, back to that destabilising GSDM profile. Westerly wind additions have been whipping up within the tropics c/o a high frequency intra-seasonal signal that is 'destructively interfering' with the low frequency (La Nina) standing wave, attempting to align that further eastwards from the I/O and Maritime continent and create a rossby wave feed-back which is (or would be) essentially less La Nina-like. A big medium and longer term question mark that remains in my opinion whatever any long range forecast or prediction might assume or hope. The Global Wind Oscillation, a phase plot depiction of wind-flow inertia (determining path and strength of the jet stream in simplistic language) has jumped into a high amplitude Phase 4 - concrete evidence of the (presently) developing a-typical relationship to the La Nina base state (as identified by attractor phases 1,2 and 3) Total angular momentum, naturally influencing the movement of the GWO, has edged slightly above average. Again, a clear signal of the destructive aspect presently to the La Nina standing wave in the Pacific. As the tropical convective envelope propagates eastwards, these westerly winds create hugely large scale linear deep thunderstorm development along convergence outflows. This in turn causes wind-flow eddy inflection which, with time, often propagate from the tropics to the extra tropics and translate in synoptic parlance as a big red flag for amplification at mid (and sometimes) higher latitudes. NWP is, self evidently, jumping all over this amplification signal. However, at the same time, a wall of -ve easterly inertia persists at the dateline (shaded blue) and the question remains over the persistence of these trade winds and how much resistance will be provided to the active envelope of the convectively coupled kelvin wave attempting to propagate eastwards. Irrespective of what RMM and other tropical convection forecasts might suggest and which cannot be relied on longer term. Wind-shear has the effect of nibbling away at the eastward propagating convective wave and this translating to loss of amplitude of the convective envelope itself as the westerly wind additions are wiped out of the atmospheric circulation Far too much MJO Phase 7/8 = HLB and snow chatter comes from twitter sources and weather forum alike and far too little examination of the wind-flow relationship between the tropics and extra tropics and which is the bigger picture in terms of where any blocking regime might set up c/o the ramp up of angular momentum tendency that is underway. The GWO tells much more of the complete story of the whole global wind-flow profile than any obsession about the MJO as a kind of stand-alone magic bullet that is ignored two thirds of the year but is the centre of attention for 3/4 months to deliver cold weather. Seasonal wavelength changes also complicate the picture. The obvious attraction of this thread, and the audience at large, baying for a cold outcome is to latch onto every stratosphere forecast that shows a hint of destabilization. Westerly wind propagation into the extra tropics creating poleward eddies from troposphere to stratosphere that destabilize the polar field are much harder to sustain this early in the season based on any assumption that the high frequency MJO is going to be successful in creating a sustained adjustment of the walker circulation. The slightest shortfall in these assumptions will incrementally lead to quite stark adjustments in numerical modelling and time after time such corrections have been seen, but quickly get forgotten - until the next time... Later in the winter tends to be more conducive to this kind of tropospheric>stratospheric 'game-changing' - but even that cannot translate to hope-casting the micro-scale pattern. Each situation has to be taken on its merits and all the analogues and composites in the world cannot compensate for this. Hence the use of the sub seasonal periodicity cycles of the GSDM which is far from some kind of hind-cast but a very useful tool to provide this kind of periodic updating analysis. Kind of a continuity check on progress. Hence so many self-quotes in the posts that I make. The long and short of it is that a lengthy period of blocking is very likely for quite some time. What remains highly uncertain, at least in my opinion anyway, is whether this time around it will lead to any sustained cold air advection pattern to NW Europe, which obviously includes the UK. From my own point of view, here in SW Europe, the interest is very different anyway. But apart from that, as echoed in the italicised post, it is the movement of the macro-scale wider pattern that is priority. However that plays out.

Thanks sweetie - that wrong footed me for sure. If it was actually true I can't offer much that will help and be of much use because it comes from an angle that is not consistent with thread purpose and wishes. Also I don't profess to be an expert, just take an impartial interest in weather patterns and try to understand them - irrespective of my own preferences which like everyone I do have but have learned to suspend much, much more than I admittedly used to do at one time. So to the long convoluted detail. There is indeed a very strong blocking signal - a genuinely referenced one as well Suffice to say though, not one which will necessarily make it snow in the UK. Jam side down or up?

Based on yesterdays attempts to provide some balanced grounding about the MJO and the wind-flow budgets on the broader scale - your analysis persists in this stand-alone magic bullet concept that MJO x+y = desired solution. I am not a prophet, and do not profess to have the ability to second guess the diagnostic, even though its (real) purpose is invaluable and gives some very helpful insight looking ahead as to what *might* happen in terms of *ranges* of probabilistic solutions. That as opposed to fitting to one bias solution.. But I certainly do know that this persistence in trying to fit to bias is flawed, other than the broken clock analogy where it comes right eventually and appears to vindicate the bias confirmation. As for the time-lags and 'signals seem to look good for another round of amplification' - . Quite apart from the extra tropical wind-flow budget not accounted for in favour of taking the MJO in isolation because it is believed to deliver an arctic flavoured happy pill - it makes it further impossible to start to extrapolate what *might* happen in mid December. On the other hand if the GSDM is considered in respect of total global wind-flow, and objectivity can resist trying to tie analysis down to a pre-conceived ideal - then it starts to become a worthwhile discussion and probabilistic outcomes can begin to be reasoned.

Option 2 for 16C to 18C warmth and sitting outside for a festive meal (rain permitting) Boa tarde - respeitos do Ribatejo A fairly standard late autumn La Nina pattern established across Europe as a whole. In that context, the models have not struggled at all in the evolution of the wavelength resulting in a classic mid Atlantic ridge with wedges of polar maritime air flooding south and east around the perimeter of the high. Ironically the only issue has been the degree of initial amplification which was always destined to be modelled too strongly, and backed the pattern so far west that it hinted at a west based -NAO . This would have been a good outcome for my location and (from my own point of view) with low pressure south west of Iberia cutting off cold air advection as far southward and by contrast pulling up air from NW Africa and the Canary islands instead. In the event, the cut-off low provided some vigorous elevated thunderstorm activity to the south of the Algarve region early in the weekend before the mid Atlantic ridge gained ascendancy and pushed the cold air advection south to arrive across Portugal and Spain through yesterday.. So sometimes heading into and during winter (not every time) a higher latitude block *can* be better (for this part of the world) than an amplified mid latitude ridge if some out of season warmth is your thing. A very different perspective of the current pattern from anyone else no doubt, but a real one just the same. The ultimate concluding destination of this particular mid latitude wavelength still likely remains a more traditional flatter Azores High as the amplification, eventually, relaxes upstream. Less chilly nights and warmer days (back closer to average) for SW Europe and less cold, less (relatively) wintry for NW Europe. The question is what happens from then: This post has a very different purpose to the vast majority, and purely seeks to look at the diagnostic vs the numerical model interpretation of the diagnostic and try to gain some thoughts as to a range of probabilistic outcomes and not try to fit the diagnostic to any given partisan outcome. Same as at any other time of the year. With all that in mind, its worth reiterating (yet again), a few things: 1) The MJO. This intra-seasonal tropical convection phenomenon is largely forgotten about and serves no purpose to the majority between about March to November. But when it does become de rigueur at this time of the year, it is then perceived as a stand-alone to aim to achieve Pacific propagation and the putative magic bullet for high latitude blocking (to help make it snow). It is for sure a very instructive driver of patterns and its movements do give a highly valuable heads-up to pattern changes. But this is the case at any time of the year and not just in winter. And the MJO is just one tool to examine all probabilistic outcomes but not to try to fit to desired biases. Aside from cold weather preferences, it does have an instructive value in winter in how QBO and polar field proxies in tandem with ENSO forcing can give clues to how active the high frequency tropical signal may be looking ahead over periods of time. An active MJO will indeed initiate a more amplified hemispheric profile, according to where tropical convergence(s) are most active - but these are not automatically always cold air advection driving patterns to mid latitudes.. Equally important though, how and when the low frequency dominant walker cell circulation might be overridden due to high amplitude MJO cycles that can, temporarily, reverse the ENSO standing wave convection wavelength that produces the default global synoptic pattern. So in turn the default pattern is overridden, for a time. NB: This will again be important this winter according to how La Nina aligns in the Pacific and whether it is of a traditional stable nature or behaves more a-typically due to eastward any shift in the standing wave position and a a consequence changing the rossby wavelengths from upstream. However, this is still not the whole story. 2) The GWO. The global wind-flow budget (determining jet stream patterns in simple terms) accounts for the atmospheric circulation in the extra tropics as well as the tropics - so therefore it is counter intuitive and prone to inaccuracy to only examine the tropical wind-flow budget incorporating the MJO. This is where the use of the Global Wind Oscillation (GWO) gives a more accurate fit to the diagnostic because it also measures extra tropical wind-flow (which is Inbuilt influenced anyway by the ebb and flow of torques according to evolving convective convergence zones within the tropics). In this way the Global Synoptic Dynamical Model (GSDM) provides breakdown plots of tropical and extra tropical wind-flows and tells a lot about their whole relationship and in turn from that, how the troposphere as a whole might interact with the stratosphere.This particular aspect does more especially apply in winter due to the presence of the seasonal polar vortex. The coming season will, again, as usually the case in the winter when the polar vortex is present, revolve around points 1) and 2) and whether GSDM global wind-flow is stable to the ENSO Nina base state, or whether it is unstable (a-typical) to it. A stable outcome will be much more likely to produce a traditional Nina wavelength of early amplified Atlantic ridging giving way to a flatter more zonal pattern (within the Atlantic and European hemisphere) according to a tropospheric/stratospheric pathway that is not conducive to destabilizing the polar vortex this side of the pole, with only temporary relatively minor displacements from time to time. This is where the GWO is a barometer of GSDM stability (or instability) according to the relationship of the atmospheric circulation to the base state. A stable outcome will see a tight restrictive orbit, based on a traditional Nina forcing, within GWO Phases 1,2 and 3. The La Nina attractor phases. An unstable GSDM profile, to the base state, will see amplitude progressions much wider between transitional phases 4 and 8 and hence at something of a 'destructive' aspect to the predominant standing wave setting the pattern from upstream. This barometer indicator lends itself to tropical convergence propagation occurring further eastwards along the tropics than under a traditional La Nina, and this results in altering the tropospheric pathway through engagement of westerly wind propagation and increased momentum transport c/o rising torques between tropics>extra tropics which in turn also create greater destabilisation of the relationship between the troposphere and stratosphere through rossby wave-breaking from extra tropical mountain torques ascending the tropopause layers. The current situation sees something of a stand-off between the tropics and extra tropics- with, in a way, each cancelling the other out. On that basis the GWO is relatively incoherent low amplitude and hence the fairly traditional late autumn patterns - albeit angular momentum is average (mildly elevated to the Nina base state). This isn't significant at the moment and as in previous years doesn't say anything about what may come - but with changing wavelengths will become more and more instructive as to whether the atmospheric circulation stays where it is, or whether the relationship with the base state tightens up and a more traditional low angular momentum Phase 1,2 and 3 resumes. Despite the present wider evidence that is available, and some of which is still rather at odds and countervailing in parts, as far as I am concerned it is still too early to come to any conclusions about the coming season - and also coming from a neutral point of view. The present amplified pattern, as stated early on in this review, is wholly in keeping with approach to winter La Nina seasons that have gone on to produce stable relationships (and hence less likely to produce cold air advection blocked patterns). Equally also from present evidence, it is conceivable that an a-typical relationship Nina base state could evolve which sustains amplified tropospheric patterns into the greater winter and increasingly switches the emphasis of the position of these from the Atlantic as time goes on. This then destabilises the GSDM profile and in turn is an indicator of greater tropospheric >stratospheric destabilisation. Which itself is still not a guarantee of anything, even if the probabilistic envelope is shifted. Answers to these particular question will come in during the coming few weeks. But my own interest in this is not from a cold weather pattern starting point, personal preference (or because geographically its a case of lack of likelihood of significant wintry weather) but simply which direction the diagnostic pendulum swings over time. Much the same approach as any other season - again irrespective of personal preferences. Obrigada. Muito cumprimentos. /

I saw the same, at least in terms of appearance this morning and it was drizzling and very overcast. Looking out the window, It looked like late autumn in the UK if you ignored the exotic plants - palms, banana. bougainvillea etc. Steeping outside though it was close to 20C at 9am with the full overcast cloud cover. Inside the house it was still quite dark and needed a side lamp on. The thick cloud gradually thinned a bit eventually to occasionally allow very brief filtered sun but it stayed very cloudy pretty much throughout the day.. It still got to 24/25C this afternoon despite the thick cloud and felt like sup tropical velvet

With all the usual time of year discussion that is attempting to match high frequency tropical convection forecasts to try to find and fit analogue composites with cold weather preference outcomes, its worth re=posting this real-time analysis. Real-time analysis doesn't seek to find desired outcomes but instead ranges of all probabilistic outcomes as means to be realistic rather than ideological. Therefore whilst obviously human error can and does occur with the diagnostics involved, there is less chance at least of being prone to larger error due to the lack of attempting to fit a diagnostic to a bias. As suggested previously in the quoted section, the numerical modelling has duly latched on to the evolution of an amplified longwave trough edging eastwards across NW Europe towards Scandinavia as the upstream wavelength re-sets the Pacific ridge and downstream sequence to an Atlantic ridge and an upper trough over N Europe. Angular momentum tendency set to fall with -ve torques scrubbing westerly inertia out of the atmosphere and adding more -ve inertia easterlies. This, as a result of the MJO high frequency tropical convection beating a retreat due to the ongoing La Nina trade winds roadblocking at the dateline and the point of inflection where easterlies replace westerlies being where re-amplification of the Pacific wavelength takes place. On this basis alone, simply taking the MJO in isolation and fitting it to composites as a kind of magic bullet to find desired outcomes is fraught with error because the total wind-flow budget in the atmospheric circulation also comprises extra-tropical circulation and how that responds to forcing from the tropics. A much more reliable guide is to look at the Global Wind Oscillation (GWO) which comprises aggregated tropical and extra tropical forcing - though even here the composites that can be created from the total wind-flow budget has to be accorded to each situation on its own merits with no one rule fits all synoptic outcome. The GWO has attempted to break out of the La Nina attractor phases, as part of the temporary interference from the eastward progressing high frequency tropical convection cycle, but it is to be noted how weak this has been, ahead of tropical forcing becoming weak and incoherent and hence the signal for another fall in angular momentum to cement the base state heading into late autumn (and early winter). This means that the GWO is set to head back through Phase 8 into the Nina phases 1,2 and 3 as part of the upcoming upstream amplification. Caution needs to be applied with making assumptions about NWP indications of amplification at longer ranges. Not necessarily the amount of amplification, but the fact that the consolidating of La Nina forcing in the Pacific, without any mitigating means any time soon to re-interrupt that wavelength, increasingly points to a Pacific ridge dominated stranglehold profile and with time more wind-flow added to the polar jet and the polar vortex re-assigning itself towards the pole. Yet another means to re-state that there is nothing whatsoever unusual about the current NH profile, either taken in conjunction with the spring and summer pattern or extrapolating ahead as means to hope for a cold winter. There is a long way, in weather terms, as to what the coming season has to offer - whatever the numerical weekly or seasonal modelling may, or may not indicate. For a small minority at least, the main interest is enjoying what continues to be on offer, at the same time as periodically reviewing things according to weekly//two weekly analysis rather than the whataboutery of two/three months ahead.

This is a general response and not a personal one. Its simply based on responding to numerical modelling forecast extrapolations for very long distances ahead that trigger expectancy. I am sure that Week 6/7 will not be anything exactly the same even down here where the now villainous Azores High (in the UK) still lurks close by to its natural home. The stratospheric forecasts at x or y range simply echo these standard Autumn rossby wavelengths under a given forecasted tropical > extratropical propagation regime and attempt to give lagged and timeline related responses - but all numerical models predictions are subject to large error on the basis that tropical convergence patterns and poleward momentum flux feedbacks between the troposphere and stratosphere cannot be predicted with any degree of accuracy several weeks/months ahead.

Fairing infinitely better than the folk have had to endure across the other side of the Mediterranean in Italy, It continues glorious in this SW part of the Mediterranean with a cool down to about 20C on Sunday with some rain a.m - warming back up through this week to 28C yesterday and already 30C this afternoon with stunning deep blue skies helping to ripen the new citrus loads on the trees. Each evening here at this farm with its chickens and various semi-rural wildlife is velvet warm with starlit skies. The outlook continues the same In the bigger picture, the continued warmth in my part of the world is further helped c/o the strength of sub tropical ridging augmented by the warm air advection processes of the remnants of Hurricane Sam. Equally it is helping the UK recover from the drab wetness of recent days and typically underwhelming temperatures. The persistent African waves across the Equatorial Atlantic, based on the alignment of the low frequency tropical convection standing wave across the Western Hemisphere, Africa/Indian Ocean - often favourably create a strong anticyclonic downstream ridge extending from sub tropical ridges at this time of year when such tropical storm activity recurves sufficiently to interrupt the Atlantic ridge dominance of the entrenched low angular momentum La Nina state. The GSDM plots clearly depict the low angular momentum Nina state, with the GWO endorsing the upstream amplification momentum transport processes at work with no orbit for quite some time out of the Nina phases 1,2 and 3. A sustained deficit of -2SD (standard deviations) is very firmly indicative of La Nina The described upper air and surface pattern changes as the tropical activity season comes to its close and the more commonly seasonal La Nina default resumes. Though unlike a fair amount of commentary to the effect on this thread, it means little or nothing for the following season on its own to sate the very strong bias preferences of the majority of the UK audience. The high frequency MJO is tracking eastwards through the Maritime continent towards the Pacific ..... ....and this will act (temporarily) to quieten the African wave process but at the same time provide some extra holding to the downstream ridging as momentum across the Pacific (temporarily) increases c/o westerly winds (shaded red) west of the dateline trying to push up against a wall of easterly trade winds (shaded blue) at, and to the east of the dateline. As wind shear c/o the wall of La Nina trade winds weakens the MJO thunderstorm development attempting to encroach the Equatorial Pacific, then tropical convection suppression occurs and the advancing westerly wind burst gets stopped in its tracks. The low frequency walker cell signal re-emerges and its then a case of seeing how the Atlantic ridge evolves to take over the downstream pattern as the wavelength is re-set from renewed amplification in the Pacific. In broad weather terms this translates to a pleasant fairly warm (by UK standards) spell of weather and continued very balmy summerlike weather down here in SW Europe with more days of circa 28C to 30C to come. Thereafter, a signal for the downstream ridge to break down and some kind of amplified longwave trough crossing west to east across N Europe and Atlantic ridging behind it with wet and windy weather followed by chillier showery conditions with NW winds. Further south west in europe, a good chance that this trough will repeat the recent sequence and provide a day or two of (relatively) cooler weather with some rain before renewed ridging from the nearby Azores settles things down again. An alternative scenario is a cut-off low heading down towards the Azores/Madeira providing some humid thundery weather with the Atlantic ridge overlaying the cut off feature. At the same time, this would probably give chilly anticyclonic conditions for the UK and other parts of NW Europe. Either way, it would suit me fine and don't miss the UK descent through autumn one bit

Summer gloriously sizzles on in my new homeland Snapshot for the week ahead (and todays recently taken reading after 2pm) are quite typical of the summer, more especially this second half. The first half of the summer was technically by SW European standards 'below average' when I left the UK in very early June. The few coolest/lowest maximum values were about 22C to 25C when the dominant Atlantic ridge/NW European trough dug the 'coolest' upper air this far south . But as an absolute minimum, so to speak, this provided what would be constituted as very pleasantly warm English summer days. Since the approach of mid summer it has been much more typically 'Mediterranean' with max values in a range between about 27C/ 28C as a lowest and 37C/ 38C as the hottest maximum so far (locally) though into the low 40sC the further south and east into the Alentejo region. Velvet evenings and nights between 19C and 22C around midnight and briefly dipping to 16C to 18C around dawn (6am now towards 7am) There is definitely the Atlantic influence in this region which moderates the heat under the prevailing NW wind-flow - and that in turn also moderates the stifling overnight temperatures seen closer to and over the Spanish border. September, as previously suggested, based on the wider -ve inertia tropical>extra tropical wind-flow patterns, intuitively would point to the prompted return of the Atlantic ridge after this anticyclone sitting just to the N/NW of the UK(cloud or no cloud!).As a benchmark for future prospects, the GWO, subject to 2 day lag and mired in low amplitude circa Phase 2, reflects the relative total angular momentum below average and underpins the latest witnessed difficulty in getting a ridge established at the expense of a weak trough as the UK saw, again, just this weekend. . There is some assist from sun activity just awakened (within low activity phase in general) to promote anticyclonic rossby wave breaking, but still not the momentum looking ahead as summarised previously, to sustain a ridge sitting just to the east of the UK. However, there is a chance that the Atlantic ridge takes on more of an eastward extension with less trough influence than during the earlier summer dominance that supports a lot of generally fine, if not especially very warm/hot weather. Aside from sun activity, its also interesting watching the brand new polar vortex establishing in this respect which might help scupper greater amplification towards Greenland and so with cool air advection better restricted- especially though as intra seasonal high frequency tropical convection likely favours a return of activity from the Maritimes into the Western Pacific after the first week to 10 days of September. Boa sorte para todos vocês no Reino Unido

I would recommend reading this for example : A good post which I agree with and very much fits well with the diagnostic viewpoint expressed by some of us of late. Also worth reading this: My own comments from the diagnostic viewpoint accord that numerical modelling of tropical convection becomes increasingly unreliable from about 5 days out in time. The GFS, especially has difficulties with amplitude and propagation biases. All that said, trade winds have awakened once again across the Equatorial Pacific and based on the changes as briefly alluded to yesterday in relation to upwelling of cooler water to the surface, then the loner term trend is clearly more and more Nina-esque in direction of travel. This supports the amplified Pacific and downstream sequence to corresponding amplified ridge in the Atlantic and the jet stream riding around this across NW Europe. However, a default pattern is still subject to intra-seasonal factors and the monthly ENSO 'mini cycle' and as we have seen already this summer, can and often does assist temporary interruptions to the main theme. That brings us back to the MJO forecasts and the caution adhered to those. I suspect that the high frequency tropical wave will propagate to the Western Pacific and promote a temporary rise in angular momentum sufficient to get the GWO to Phase 4. This, before the MJO likely beats a retreat c/o -ve inertia trade winds across the central Pacific and the GWO responds by returning to La Nina orbit phasing during September. But within the limits of official summer itself, and with the above suggestions in mind -based on the rock solid aspect of the Azores high modelling within the context of the long term Nina-esque backdrop, it would nevertheless not take too much upturn in momentum transport to secure a more defined ridge from SW to NE and make the blocking structures advertised by the models to the north relatively less dominant than they appear in current NWP. modelling. The cooler air advection processes driving the troughing around the eastern perimeter of this high cannot be relied on to be accurately modelled, and it wouldn't take much to alter the alignment based on more forcing eastwards (for a short time at least) from the Azores High. I think, for NW Europe prospects, an end of official summer fine and warm spell cannot be ruled out - even if longevity of this is to be questioned. For me here in SW Europe I continue to rely on the irrigation watering pump for my Mediterranean garden

There is nothing to change this thinking. Not even the bizarre 'law of sod' methodology which overtakes this thread. So, to the diagnostic.The GWO, subject to 2 day lag, has been orbiting to low angular momentum phases, circa Phase 2, following the downward momentum drift that began in late July - not coincidentally the time that the fine, very warm spell broke down in the UK and other parts of NW Europe. The 40 day phase plot clearly depicts the ascendency of momentum during the first half to two thirds of July, and the steady improvement from the unsettled start to the month to that late month breakdown. Despite this, there remains the opportunity for a third fine summer spell c/o ridging into the last phase of this month for NW Europe to see official summer out in a way that it began. As suggested in the previous post, its a case to what extent the high frequency tropical wave can, temporarily, diffuse the underlying Nina-esque standing wave which continues to develop as the Pacific maintains upwelling effects of cooler water to the surface. The positioning of the ridging clearly important to how much warmth can be generated through pulling up air from the south (its the hottest part of the summer locally here in SW Europe) There is not a large window for potential heat in Northern Europe- based on the inherent tendency this summer for retrogressive upstream forcing. In this sense, shortfall of tropical forcing would readily see the ridging adjusting to retract once more to the Atlantic - and with a lot more La Nina to come in the remainder of 2021, the coming autumn looks to shape up maybe along the path of one like 2017 which featured a lot of Atlantic ridging through September. Sending some summer vibes to punch through the grey skies x : :

The -ve mountain torque wavelength, impacting AAM tendency, we were discussing the other day is fully advertised in short to medium term modelling (up to about a week) which fits the envelope for periodicity (timespan) of such events if we include the time since its inception. You correctly observed recently how numerical models can get spooked by these events and their associated lunges in momentum (downwards in this case) I expected continued gyration of the GWO around Phase 8 and back into the null phase based on the -ve inertia of the upstream pattern - though the amplitude of this event, as indicative of the deeper Phase 8 orbit that has occurred is clearly greater than expected. So, within the 7 day period to come, the advertisement of the anomalous ridge across the Atlantic and downstream trough across Scandinavia is fully concordant with the upstream -ve inertia within the Pacific creating the downstream retraction of the pattern - but equal to the actual diagnostic assessment( to the even greater extent than anticipated) With this type of cross-analysis in mind ,I think its important to tie diagnostic wind-flow to the interpretation made by the representative upper air pattern of numerical modelling to illustrate at what point that parity is reached at any given time, so that people can see where the two tally, but also can get some idea how the numerical modelling can also misread or under or over extrapolate such a given signal and which means that the two do not always tally quickly. With the nearer term upstream pattern in mind, it puts the context of your latest quoted post into good focus relating to the anticipated recovery of momentum between days 8 and 10 and how enduring it might be and whether the models will be equally spooked by this event in the opposite direction of travel. As the high frequency tropical cycle propagates eastwards, then the elastic band effect will come into being with quite a shunt to the downstream pattern as suppression wanes across the Pacific and de-amplification occurs here and transfers instead further downstream. As previously suggested, a dash of the GWO from Phase 8/0 across to Phase 4 would endorse the present indicated modelling at day 10 hence. For the late August prospects for NW Europe, much hinges on both the strength and durability of recovery of momentum - with the anticipated prospect as stated previously that the low frequency walker cell will resume ascendency thereafter. and all that ensues in return of Atlantic ridge dominancy. For me down here, always under the grip of intense sub tropical ridging right on the doorstep - its a case of just how hot, not for how long. However for NW Europe its very much a case of how (very) warm might it get and far how long in relation to the transfer of the ridge from west to east, allowing warm air advection from the south, before the pattern breaks down once again much as it did in July. This type of anticipated rinse and recycle was mooted in a previous post. But at present, a similar reprise of the July fine spell for the UK looks a reasonable expectation.

Olá, tudo bem This extract from previous post still seems good for continuity purposes. That intra-seasonal signal discussed in the post looks to provide another phase of eastward temporary migration of the Atlantic ridging and a rinse and repeat of the mid to late July pattern as angular momentum tendency responds to eastward propagation of tropical convection through the Maritime continent in the last third of August The GWO holding around weak Phase 8 to no discernible signal (Phase 0) in the short/medium term which retains greater Atlantic ridging element..... ....but with a westerly wind burst beginning across the Indian Ocean and propagating towards the Maritime continent this should start to correspondingly adjust the rossby wave pattern eastwards as active tropical convection and thunderstorm activity replaces suppression in this region - and then on into the Western Pacific. The net product of this likely to orbit the GWO back to Phase 4 (greater downstream ridging) before the low frequency walker cell reclaims ascendancy and Pacific re-amplification re-sets the wavelength back to the downstream ridging in the Atlantic. However, before all that, a good chance for some fine, very warm later summer weather for NW Europe during the last third of August. In the immediate term, heat is building down here across southern Europe. 30C locally today and tomorrow (around the August seasonal daily maximum average) rising through the latter part of the week with 36/37C likely by Saturday - edging back to 32/33C by Monday. Further inland still ,towards and over the Spanish border and through the Portuguese Alentejo and Algarve regions, values of 40C or more look very likely ECMWF 14-day ensemble weather forecast for Fazendas de Almeirim | Meteologix.com Screenshot

The very few cloudier days get a discussion going down here! The last rain that fell was about five or six weeks back, and with a few thundery showers before that earlier in June, so close enough to the theme of a Mediterranean summer even though there has not been any true sustained heatwaves of 35 to 40C (and upwards) beyond a couple of brief spells in July. Its no guesswork involved that the two periods, so far, of this summer, that have seen a departure from the changeable cooler and sometimes very wet pattern that UK and other parts of NW and central europe have seen have coincided with momentum shifts upwards between the tropics and extra tropics. The first being in the opening ten days to fortnight of June and more recently the mid to latter part of July. The downturn from mid June to the first part of July, and the latest one on-going, being a result of the high frequency tropical cycle returning to mirroring an underlying increasingly Nina-esque standing wave once again and associated generally rather lower than average atmospheric angular momentum tendency. The Global Wind Oscillation phase plot, (a depiction of wind-flow inertia within the atmospheric circulation which gives a useful diagnostic guide to jet stream behaviour) and also the angular momentum anomalies themselves (that the GWO mirrors in its orbits) both illustrate those two relatively short upward cycles within otherwise supressed tendency. Its a deliberately simplified illustration (with rather pedestrian hand-work) but it does provide a mirror for correlation purposes. In synoptic terms (for summer wavelengths) this ebb and flow pattern revolves around the movements of a stronger than average Azores high sub tropical ridge (the one that gives the stable sunny and very warm/hot conditions to southern europe). However, with tendencies for this to retrogress its influence across more northern parts of europe due to upstream wind-flow inertia decelerating c/o rossby waves amplification occurring across the Pacific when tropical connection suppression takes hold across this section of the tropics and sub tropics. The net effect of recurring Pacific amplification is to keep re-setting the downstream pattern to a trough/ridge/trough pattern - with the final ridge /trough arrangement being across the west/central atlantic and north western european sectors respectively. As suppression briefly wanes across the pacific, c/o eastward passage of the high frequency tropical cycle increasing wind-flow across the Pacific and sending the amplifying rossby wavelength downstream, then the trough over UK/NW and central europe is replaced by ridging for the temporary period that this interference with the background base state exists. Its hard to see a break to this pattern cycle rut at present, and with signs that a La Nina-esque hold on the wider hemispheric pattern may strengthen again heading into autumn. This makes it likely that Atlantic ridging will continue to dominate proceedings overall, though as per the previous post, its worth watching out to see how tropical storm activity intervenes if, following any recurving tropical storm (from within the Equatorial Atlantic) another intra-seasonal kelvin wave crosses the tropics and a further temporary rise in angular momentum occurs. This maybe more likely late this month and perhaps into September. Other than this, any hotter spells and more southerly winds (for UK/NW Europe) seem harder to come by and restricted to breakaway ridging transferring eastward ahead of main troughs and associated advection of brief plumes. Though again, this sequence could be further amplified or augmented if any tropical activity gets caught up in such a sequence. Lots more lovely sunshine and customary warmth to come down here ( nothing likely to top up the water bore-hole for some time yet), but a case of making the most of the drier, warmer and more settled spells when they come for the majority interest of the forum in the UK