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Tamara last won the day on September 5 2018

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  1. Just a one-off summary before disappearing once more.. I freely admit I have no interest whatsoever in attempting to fit patterns to preferred outcomes, despite my own preferences which much more closely align these days in terms of this thread at the opposite time of the year - so this approach will be in stark contrast to the overwhelming one adopted in this thread and likely disregarded due its later conclusions I have only done a bit of speed reading through most of the posts on this thread, and the majority of them are rooted in my honest opinion within confirmation bias circles anyway. But despite so many claims to the contrary, this is in truth a very typical late autumn low angular momentum driven pattern that features an anomalously amplified wavelength as dictated by a strong NE Pacific ridge leading the downstream configuration of ridge/trough/ridge/trough These low momentum phases in autumn are often punctuated by an intra seasonal higher phase tropical cycle which augments the underlying natural downstream Atlantic ridge profile that the default La Nina type circulation imprints. Forget the actual ENSO base state here - it is the tropical/extra tropical circulation that the atmosphere is adopting to the base state that matters in terms of wind-flow inertia to determine the jet stream behaviour - and it remains likely in my opinion that a Nina type disconnect will persist through at least the first half of the winter So the current intraseasonal phase tropical signal is attempting to pass the Pacific over the coming period and this is providing an added boost to the amplified ridge/trough profile. Much of the seasonal modelling is in excellent agreement with this signature for November, so a continuation of trough disruption likely to continue this side of the Atlantic for a while yet based on the meridional aspect to the wavelength. The CANSIPS, as one of the various majority of models in agreement with the longer term evolution of the pattern is a very close matching example indeed of what is advertized to persist well throughout November As the tropical signal ultimately heads back towards the Indian Ocean following this period, and returns alignment to the default circulation, the period beyond this looks accordingly highly susceptible to a renewed collapse of angular momentum and occurring very conceivably at the same time as the seasonal wavelength change occurs as November progresses closer and into December. Such a renewed drop in momentum implies increased energy returning to the polar jet and re-alignment of sub tropical ridging as trade winds re-dominate the Pacific. With this in mind, and with the the stratospheric vortex uptick to over 40 m/s prior to upcoming temporary (relative) reduction in speed - it means that the November planetary wave poleward momentum providing some perturbing of the tropopause has to be taken into context of longer term angular momentum trends related to the tropical cycle - and also related to the transition of the QBO which is still within its transition phase to easterly. This transition unlikely to complete till late winter. On that basis its hard, at present at least, to see how a disconnect will sustain beyond the tropospheric support available over the coming few weeks and this in my opinion would point in such circumstances to an elevated chance of quite a pattern change during December I am very aware that this suggestion will not be taken seriously, but the context of a chilly November therefore, has (in my opinion anyway) to be treated in sensible perspective and *shouldn't * be extrapolated forward into winter as part of the huge bias preference in this audience. Similar principle to warm late Springs that often are featured as part of low angular momentum wavelength regimes - but, with a few exceptions, more often than not give way in accordance with summer wavelength changes to cooler and more unsettled summer patterns If a warm May was in progress, I would be enjoying it, but still cautious of prospects beyond this time -, so as far as I am concerned the same applies to below average Novembers( for those enjoying that if its your thing) and the portents for winter. And yes, before the usual response(s) appear, I am perfectly aware of all the arctic feedbacks that are chattered endlessly in these threads every year with cast iron guarantee and which always are supposed to lead to the *big one* - but these still require a conducive tropospheric/stratospheric pathway to engage them and in turn provide a sustainable pattern to make it snow... I am not a forecaster, and don't pretend to be able to second guess the weather - but continue to differ from the popular bias view and can see support for the seasonal modelling suggestions of a milder much more south westerly driven type ultimately taking over heading into/or during the early part of the winter. The seasonal models can be wrong, and will surely be wrong again at sometime - but that doesn't mean they are always wrong and especially when it is clear to see from a bias free diagnostic approach that less appealing solutions are indeed also possible than the one most want to fit to suit their preference. Those *other*solutions might entail a NW/SE type of split, with reduced air frost risk and the wettest and windiest weather transferring accordingly away from more southern parts and some hope perhaps for some quieter drier weather here after so much rain this autumn - and at least for the benefit of a very small minority of those who visit this thread, also a much welcomed chance to reduce heating bills at least during the day. With that in mind, the CANSIPS model closely reproduce this prevailing thinking with the seasonal wavelength change showing a much less amplified Pacific/Atlantic ridging regime during December - and a European downstream ridge replacing the disrupted trough. At present at least, most of the other main models have close variations on this same theme Time of course as always will tell. The winter as a whole of course remains anyone's guess Best wishes for the remainder of 2019 and for the start of 2020 . A very few of us will wait as patiently as possible for Spring and the longer days spending more time outside once again..
  2. First I wasn't presenting a seasonal forecast ( just a paraphrased post) and am not a competitive forecaster anyway, and don't subscribe to competitive 'methods' either. If you read the post properly (which you didn't) then you would see that I illustrated this by saying that I don't have the technical where-with-all of the person (person(s) to which I referred. Being honest isn't a weakness and doesn't need to be compensated by tub-thumping statements that proclaim what the 'talk of the weather community is these days'... This is an an internet site for people of all levels and hopefully of managed ego's - , not a platform for parading 'method manifestos'. There is too much of this grandstanding in our outside world as it is... Arctic amplification was mentioned - I referred to it briefly as part of a condensed post and simply didn't write extensively about something that is pertinent to what you want to be the peoples anthem for the coming winter. It is one factor of many, which on its own does not lead to one favoured outcome all the time (as one season to another each year bears out), but involves consideration of multi-variables that might imply more than the one solution than a majority might want to hear. You are free to do this of course, if you want to steer towards a populist outcome - but things don't always work out in reality however convincing they might appear to those who like what they hear. Indeed things don't always work out for anyone when based on attempted objective discussion - the weather makes mugs of all of us. But better to start from a baseline that manages expectation and hopes to learn from mistakes rather than be a tribal lead to populist weather preferences . The price of narcissism does have limits
  3. Just as a very brief addition (before properly retiring to the sidlelines until next Spring) to the previous post made about 5 weeks back which alluded to the escalation of confirmation bias wrt weather preferences which.rises to its highest levels in autumn and winter Its based on a conversation I have had with someone who is a very well respected observer elsewhere, with a level of knowledge and perception that many would aspire to and of a higher technical level for sure than my own. Most importantly they suspend their own natural preferences and make judgements neutrally and based on the merits of all variables present and not selectively. Such observers I think are the ones particularly worth listening to and trying to learn from. Anyway, relevant to the here and now situation but extrapolating that forward in terms of future possibilities - here is some of that conversation based on my own contributions. A few further additions have subsequently been made to make it most relevant to the UK and Europe Yes for my own purposes, a defacto traditional Nina regime looks increasingly likely to persist in the months ahead. The question being where any Pacific convergence may set up to interfere with the regime and create scope for less tropical and extra tropical disconnect to the base state. In that sense the much less disconnected SOI plays a bit part if any westerly wind compartment is isolated, so to speak, due to a wider overriding -ve momentum inertia High amplitude Phase 3 GWO could well be set to return to Phase 1 as trade winds increase across the IO following the recent extra tropical increase in momentum. Once the +EAMT wavelength fades over coming days, -ve FT in the tropics will lead MT momentum trending -ve once more and maintain the GWO well within the Nina attractor phases - providing the steer to persisting sub tropical ridging. This sub tropical ridging is well in evidence in present modelling and amplification of this ridging during autumn and indeed winter to provide short lived colder incursions is quite consistent with many similar years and not at all unusual, or any signal of a harsh winter, This, plus question marks wrt QBO transition needs to be balanced against any weakness within the polar field c/o arctic and other destabilizing factors and why I personally continue to be much more ambivalent and sceptical about the relevance of blocking patterns at this stage of autumn in terms of what it might imply ahead. Just my view of course - and which customarily tries to be detached from mainstream hype that grows each and every year, at this time of year - and when sensible weather preferences and ideals often attempt to fit too many drivers to produce that desired outcome, when probabilities often suggest alternative solutions that are much less popular to the masses. The gist of this extract is that much is yet to become clearer in terms of seasonal overview, but especially stressing that model output commentary during the early stages of the short seasons and making assumptions based on persistence of certain aspects of patterns is fraught with error when attempting to assign specially selected drivers as a means to fit the desired outcome. To all those following progress of the weather patterns in the weeks ahead, try to enjoy progress and evolution - regardless of outcomes vs preferred outcomes
  4. Its been forgotten that the opening week or so of the month was warm both by day and night and the CET was (technically) well above average and the second third of the month has simply corrected that opening warmth to average. With several days of, mostly, very warm conditions to come heading into this last week of the month, it looks highly likely that for all the negativity attached to the wind and rain of late, August will probably finish on the +ve side of the rolling average(s) I think it goes to show that so many dubbed 'poor' summer months nowadays are viewed with quite a different benchmark perspective to, say, 30 to 40/50 yrs back. Heaven knows what some of the 'poorest' summer months would have been described as back then, had the internet and social media existed.... Its interesting, if a little elusively frustrating from a weather observer point of view, that so many consecutive summers have seen either La Nina type forcing dominating or at least a trend as the summer has progressed for some countervailing -ve momentum to appear, and prevent overspill of some the mainland record breaking temperatures to the UK due to an element of Atlantic blocking intervening. NB: -Before there are the inevitable protests from a few, this is a weather related observation and interest, and not personal welfare or infrastructure based However, I think that when there is 'alignment' once again of the atmosphere during the summer, to a greater extent than happened even in last summer 2018, with a sustained El Nino type feedback encouraging extensive and prolonged 1976/2003/2006 type mid latitude ridging, the 40C barrier will be broken with a little maybe to spare. Its seems simply a matter of when, not if. The plot below is a depiction of momentum tendency that includes wave activity flux convergence (beyond just the typical individual torque processes) which is simple terms is a very useful guide to jet stream velocity and trajectory trends This summer managed its new UK summer record...just when atmospheric tendency was at a brief peak in late July...it is not a mere coincidence... This implies just what might be managed with a more sustained momentum/ridge feedback to the atmospheric circulation The crash in momentum at the start of August also doesn't exist by coincidence with the downturn in weather that occurred either soon after. So also applies to the latest uptick relating to the return of the ridging and fine warm weather...and so on and so forth... With all the above in mind in terms of momentum transport phasing - as we approach September, rather than momentum downside, there are signs that beyond the imminent African easterly wave and upturn of tropical activity in the Equatorial Atlantic, there is greater long term upturn potential in +ve AAM momentum transport than existed this month overall. ECM convection velocity potential chart forecasts identify the principle standing wave to occupy the Pacific during September - this reverses the profile that has dominated a vast swathe of August and means that greater westerly wind inertia should be added from the tropics to the extra tropics and lead to further downstream ridge development rather than inevitably the upper trough of the last couple of weeks Albeit tentative, there are also signs that deterministic MJO modelling is starting to identify a higher frequency response migrating eastwards towards the Pacific through the first 10 days of September. At this provisional stage it is not the amplitude that matters, it is the general trend of direction that is relevant. Since the last post, excerpt as quoted above, the numerical models have followed the captioned theme quite closely, with quite the struggle to gauge the parity between an impressively poleward expansive sub tropical ridging regime to mid latitudes vs an equally robust polar jet steering low pressure systems along quite a tight thermal boundary between moist tropical air flowing around the top of this ridging and relatively much cooler air to the north of the zone. Fortunately, sub tropical ridging has become increasingly expansive in modelling, and improved the micro scale prospects as we head into the B/H weekend. With this in mind I don't personally think it is worth getting too hung up about the small feature causing mischief for Monday and let it overtake the bigger picture. Yes, some eastern parts are best favoured for the latter part of the weekend holiday - but taking the weekend as a whole, it remains generally fine and warm with just isolated shower risk (if not very warm and hot for some) The period following the B/H is prone to further numerical modelling faux pas. A slow breakdown attempt of the present lovely weather seems likely and has some shorter term support - however to what extent and for how long? There may well be continued further effort in the coming few days to stall momentum and dig the upper trough further south with an Atlantic ridge present rather than across Europe. However, there will be the ongoing curveball of tropical developments which can just as easily generate warm air advection and ridging ahead of them, than get steered to our shores - and then based on the upstream tropical>extra tropical signals in the new month, any upper trough scenario could well run into trouble without tropical interference in the coming days anyway. Some extended NWP is already playing with the idea of persisting with a greater element of downstream ridging and retaining a more discreet trough further NW. So while the mood occupancy of this and other threads will likely drop interest in warm weather like a stone, and flip into hunting for the first frost and distant three month signals that this winter is going to be 'the one', a minority of others will be away watching with interest for hopes for some extended summer Au revoir for me for this year. @Scorcher If emigration isn't possible, then maybe a warm winter retreat might be a good idea (one can only hope its still possible to escape easily enough!)
  5. Another self quote - just for the purposes of continuing thought relevance to the way ahead. The much recently discussed mercurial set-up in the Pacific, at a time when the first hints of seasonal change are starting to appear within an unstably punch drunk arctic profile - are manifesting themselves through inchoate, and somewhat capricious numerical modelling in terms of energy flow and distribution. Endless repetition of not taking NWP at face value, and not reacting too much into each and every operational output, is admittedly a broken and probably annoying record, but with NWP caution in mind, the ensembles continue to persist, overall, with the idea of anomalous mid latitude ridging, ebbing and flowing, and consistent with the overall volatile mix and duelling of low frequency signal vs intermittent -ve wind anomalies in the EPAC. The danger, and I have seen this in some social media weather pattern commentary, is taking these countervailing wind-flows in isolation and trying to fit individual numerical modelling solutions to them that (appear) to most closely identify - and presenting a rolling 'forecast' forward based on attempted coefficient x+y assumptions about how strong each of the variables present are - when each variable is in reality, the sum of all parts and not compartmentalized in isolation. This is very much an especially blended diagnostic outlook and one where the blended solution in the numerical models is, equally, therefore the most sensible way forward at times like these Overall, with a slight erring to relative angular momentum being fractionally weaker than optimally desired for very best ridging profile for warmth and most settled conditions nationwide - - those percentages support the idea of mid latitude ridging slightly to the west of the UK and with a little more +ve momentum in the polar jet than would be preferred for that greater nationwide settled potential. The sub tropical ridge ebbing and flowing with the westerly upper flow and steering systems downstream close to Iceland The more downside to AAM, then the greater chance of ridging retracting and greater trough disruption. Up to now, this has been perceived as reasonably contained and not sustainable. However, its worth a bit of additional perspective here - Much has been spoken of the low frequency signal and the ally of having an oceanic SST configuration to assist a convergence zone . However, in the light of the persistent -ve zonal wind anomalies in the EPAC, the time is coming where support is increasingly required from the high frequency MJO signal to provide a big additional spike in westerly wind additions to properly combat the EPAC trade winds, ramp up angular momentum on a more substantial level, and in turn prevent the Pacific from more widespread cooling that would diminish the tropical convergence in the Pacific and which would have some ramifications to blend with other intra seasonal factors that evolve, for many weeks ahead (and right through the autumn and possibly beyond). The MJO itself has been quiet for some weeks and this has had some influence in supressing the upside to angular momentum, despite the weak walker cell producing a modest supply of westerly winds in the CPAC to counter balance the EPAC trade winds. As stated in the previous post, the MJO is only a part story to the whole GSDM budget. But it often is, nevertheless, a major catalyst to shaking up the atmospheric circulation and forcing pattern change at the synoptic level. Interest in this intra seasonal cyclical tropical wave phenomenon tends to be restricted to the winter, due to being ( totally erroneously) perceived soley as a magic bullet to deliver cold spells and of no significance or importance at any other time of year.. However, its influences wax and wane all year around and it can have significance for weather patterns equally all year (and not necessarily always to fit human weather preferences) In the here and now, the longer that the high frequency tropical convection signal stays quiet, then the tendency for more sustained falls in angular momentum grows, as easterly wind inertia is allowed to spread from the tropics to wider areas of the extra tropics. This would be reflected in the Global Wind Oscillation incrementally orbiting further towards the La Nina attractor phases - , - and in tandem with seasonal wavelength changes, increase the chances of atlantic ridging taking up residence too far to our west to influence better weather - and inviting a longer lasting trough signal. This doesn't mean automatically mean a La Nina base state would be arriving It simply refers to a somewhat a-typical relationship that the atmospheric circulation would adopt to the (neutral) base state at such a given time (if such a circumstance evolved) Based on intra-seasonal factors (and related to the end of the Indian Monsoon summer cycle) the high frequency signal may well pep up once again over the coming weeks, but hopes of late seasonal warmth and an Indian summer for those like me who would highly welcome it, will more and more rest on this type of eventuality .
  6. Excerpts from last Saturday post as means to try to help link and document progress since that time Since that post, NWP has duly responded by focussing away from any notion of a persisting trough between the UK and Scandinavia, and towards return of ridging next week - as the upstream changes in wind-flow pattern discussed in the previous post(s) create our downstream improvement in conditions, initially most especially further south. As proposed in the quoted post, it remains the case that no instant heat looks likely as the changes get underway, but every sign that by the bank holiday the cooler air left in the wake of the weekends deep trough will have been mixed out with time - and also with warmer air starting to be drawn up from the south as the High migrates close by. Using Atlantic/European views, both ECM and GFS day 10 means are very closely aligned - look set fair for a weekend that hopefully will be some compensation for the two decidedly poor weekends that are preceding it. With the present unsettled pattern in mind, its worth looking at how the GSDM diagnostics represent the changes. On this 2 day lagged data, notice how far down -ve torque tendency ( strong -ve wind-flow anomalies at 20 to 30N) has led to considerable -ve momentum transport in the extra tropics and supressed the present pattern in our downstream location - due to sudden amplification upstream. The Global Wind Oscillation, a key plot phase indicator of extra tropical wind-flows, has responded with a rapid aborting of the Phase 4 +ve tendency orbit late last week to -ve tendency Phase 8 through the weekend and into this week. This type of rapid orbit momentum phasing, is a signal of an unstable GSDM momentum budget within the atmosphere, and often red flags high impact weather events due to the rapid shifts they force on the Jetstream. as the laws of conservation of angular momentum rapidly re-apportion wind-flow inertia to fulfil Mother Natures need to fill a vacuum. Its not a surprise that this spooked the models in their longer term programming - before seeing that the mechanisms responsible for it are set to hit reverse afterwards.... In the very extended period - as for any hint of retrogression (which is 2 weeks away on operational GFS output)….very little point in speculating at that range, much as just recently has been illustrated with the modelled over extrapolation ahead of the UK/Scandinavian trough - and very slow to dissolve it right to the Bank Holiday period itself as analysed above.. For what little it is worth discussing at this time, a late summer/early autumn 2017 full-on La Nina style collapse, in angular momentum (as opposed to this temporary -ve momentum phase) would steer towards a major retrogression outcome. GFS -ve momentum tropical bias would be catalyst for tendency to default to retrogression in lowest resolution (and quite possibly its a trend one or two observers would like to see such as an outcome which is a factor here as well ) The upstream Pacific pattern this August is different to 2017 and with a +ve PDO pattern in evidence that was not there then - but regardless, at this range, and based on the persistence/re-occurrence of the low frequency convergence zone in the Pacific, such sustained relapse, at present at least, is highly questionable in terms of support. Notice the strong suppression signal over the Maritimes (shaded orange) on the velocity potential chart....this is hallmark of return of a standing wave in the Pacific.. …so ensuring the entire Pacific basin is not dominated for the time being by trade winds, as occurred heading into autumn 2017. Instead, classic wind anomaly convergence conducive to deep thunderstorm development,occurring at the dateline with -ve wind anomalies east of the dateline and +ve westerly winds at, and just to the west of the dateline. All c/o the split SST distribution of warmth to the west of the dateline and (relatively) cool to the east. The latter influenced by trade winds upwelling cooler sub water as triggered by intermittent -ve phasing of the South Pacific Oscillation and associated high pressure zone ( cool SST anomalies in South Pacific below the Nino 1.2 ENSO zone). Its important here to differentiate between the low frequency walker cell circulation and the cyclical high frequency MJO cycle. Effects on momentum transport and related angular momentum tendency require key considerations of different proxies.. A reliable assessment of tropical convection and its potential effect on synoptic patterns cannot be made by simply referring to an MJO forecast alone. Extratropical propagation of momentum transport from the tropics has key variables, especially at a time when the Hadley cell is being skewed poleward by the latent heat processes of convection created by additional warming in the tropical oceans. Then, factor in the amplification mechanisms created by the disturbingly low arctic sea ice, and it is no surprise to keep seeing some unprecedented scenarios cropping up within extra tropical hemispheres from one season to another. More than enough to dwell on....At this stage its a case of seeing out the current unsettled spell and seeing in a highly welcome change to more settled conditions through next week
  7. Autumnal synoptics and increased mood music of summers demise, but the premier bullfight is a sneak preview of this years provisional winter message with its traditionally perspicacious advice to 'forget the teleconnections' (aka its going to snow) There will be plenty of time for the building blocks for a redux of that 'beau ideal' of winters, 1739-40, to be weaved and plotted. Just leave the computer models to follow the carefully laid trail of selected drivers, as well as those deselected and inappropriately not fit for purpose, and then simply put on the central heating, put your feet up and wait for the first river to freeze over.... Yay! Job completed. First of all back to the mundanity of the kindled bonfire that MOD thread HQ has tossed summer 2019 onto - without its required 3 week notice Having had help to make the summer gazebo safe today in that wicked 'autumnal' wind, it might require some patience before it can be fully utilized a next time, but it hasn't been fully dismantled and put away. Clear signs now of the ongoing reversal of strong E'rly trade wind momentum close to and east of the dateline from midmonth - and with convective suppression retreating away from the Pacific. This is concordant with return of the low frequency convergence signal in the CPAC. The w/QBO and presence of Nino type walker cell circulation together continuing to regenerate westerly wind bursts in the Western and central Pacific belies the macro scale adjustment through the summer to an ENSO neutral status in the Pacific. As previously alluded to the unassuming neutral status disguises some stark contrasts in SST's across the Pacific which looks set to throw curveballs to modelling now and through into the autumn itself. In the here and now it continues to question the retention of La Nina type downstream flow advertised by NWP in the medium/longer term range. Albeit there is an eventual very slow relinquishing of the trough solution between the UK and Scandinavia. There is no route back to heat that can yet be justified, but any lingering cool air advection looks most likely linked to the transitional relaxation of the upper trough as ridging takes over from the south. This in turn though, an eventual bridge to warmer and settled conditions to return as thicknesses rise and any cooler uppers get mixed out. Prior to the present trade wind burst override, atmospheric angular momentum in recent days has been returning close to average... …...with the extra tropical measure of aggregate wind-flow, the Global Wind Oscillation, reflecting a small increase in momentum flow with a very weak amplitude orbit in Phase 4. This is a two day lagged plot and just ahead of the trade wind burst which will be returning the GWO back to lower momentum phasing while its effects remain. It explains why the current low was not able to secure a more westward track attendant with a stronger downstream ridge response which would have arisen with a higher amplitude and longer lasting GWO Phase 4 orbit. Minor, but significant margins for why the UK narrowly missed a plume yesterday. and instead witnesses an 'autumnal' style deep low... However, based on the subsequent return of the low frequency signal in the CPAC and further westerly winds added back to the atmospheric circulation, there is not really support for a persistent Nina type low angular momentum response long term c/o of a Scandinavian/UK trough and amplified Western Atlantic profile either... The suspicion remains that NWP is being too slow in the extended period to acknowledge the upstream pattern and obfuscation to a ridging transition is overly procrastinated. One word about any MJO/GWO composite inference here. Live proxy data, including wind anomaly, convective velocity potential, OLR data, and polar field profile in relation to the troposphere should always be used to help interpreting synoptic patterns to wind-flow changes across the hemisphere. Much as NWP interpretation, any teleconnection based composite should not be taken at face value without sufficient diagnostic proxy data support. Especially important with so much superimposed heat stress skewing responses within tropics, extra tropics and pole. On the other hand, myopic and parochial dismissal of the role tropical and extra tropical momentum transport processes via torque mechanisms play in altering rossby wavelengths - and hence changing the velocity/ trajectory of the jet stream and providing such a useful advanced insight into how NWP may evolve weather patterns ….is not an act of perspicuity
  8. I agree very much with the premise of this post, and especially its conclusions Signs that the tropical stratosphere is staging a warming ahead of typical seasonal transition. Seasonal warming in the SH implies laying the pathway towards the start of seasonal cooling in the polar stratosphere. The precise increasing differential of temperature between the poles and the associated transport of ozone/assessment c/o Brewer-Dobson circulation to determine the potential strength of the new season vortex still some weeks away of course. However, based on the legacy of a final warming of the stratosphere very late in Spring and into the early summer, that overstayed so many meteorological predictions this summer in terms of its dynamic tropospheric shock waves - then there is going to be increasing pressure to flush out remaining warm anomalies within the troposphere, with the cooler anomalies that have been pent up higher in the stratosphere descending thereafter as the highest levels cool further So in essence I think there is something of a further -ve zonal wind anomaly precursor in the troposphere/stratosphere boundary ahead of a more neutral AO regime(ultimately +ve) under way that makes things 'worse' in the short term wrt summer blocking and supressed Jetstream. Add in the interruption to the weak walker cell low frequency signal circulation in the Pacific at the expense of increased trade winds - -,and in the absence (to date) of any high frequency tropical signal (MJO) to compensate this by helping adding extra supplies of westerly winds to the atmospheric circulation during mid summer, then its easy to see how angular momentum has been trending slightly below average for a while. This -ve wind-flow anomaly assists the upstream pattern becoming increasingly amplified, while this side of the NH at the expense of the higher latitude heights supressing the jet stream in our locale. Much of this has been discussed at length recently. On the other side of the equation however - disparity of SST anomalies shows sharp contrasts in the WPAC and the EPAC. Considerable warmth west of the dateline contrasts with neutral/slightly -ve anomalies east of the dateline Why does any of this matter so far way downstream? This disparity supports the weak walker cell re-emerging sooner or later c/o CPAC tropical convergence and ingredients for convection/deep thunderstorm development - and this translates to questions over the longevity of the amplified upstream pattern as momentum transport increases due to westerly wind bursts returning to the Pacific. At the same time as the AO begins a slow trend towards neutral and also alters the jet stream path To add to the mix, the high frequency tropical signal is starting to show signs of activity following its mid summer slumber . The slumber c/o quite an active Indian Ocean monsoon season. The I/O intra seasonal signal assisting keeping a lid on atmospheric angular momentum and the trend towards the more La Nina-esque pattern that is spoiling Augusts middle third fortunes. There is discrepancy within the deterministic MJO modelling, with the GFS keen on amplified progression to the Pacific whilst the ECM exhibits what can be a problem bias to Maritime convection - but these are quite unreliable beyond 5 days or more anyway. However the velocity potential (VP200) anomalies, a more consistent source of inspection for the purpose, continue to point towards propagation of a more active regime in the Pacific replacing the trade wind suppression of activity from after mid month. Note the movement of suppression across the tropical Atlantic and towards Indo/African Western Hemisphere as this is predicted to occur. . So there is increasing pressure from within the polar field to neutralise heights with time, and for a rally in angular momentum c/o a return to Pacific rossby wavelength control as summer season (not necessarily weather) draws towards its official close. This does not present a clear cut prognosis at all for NWP evolution the further the month progresses. Having spent the last few posts cautioning against taking each NWP suite et ensembles at face value - and especially extrapolating them too far forwards in time, I appreciate the difficulties in the context of this thread of that successfully being heard But from my own point of view, and being naturally sceptical of NWP interpretation of diagnostic drivers, the outlook heading into the last third and especially last week/and end of August period is far from certain. Such scepticism includes the likes of the EC weekly, and its own suggestions of timings (and that of any other long term numerical modelling product). Unsettled and not especially pleasant at all to mid month and shortly after? Yes, alas very much so. But as it stands the upcoming very volatile pattern, further augmented by an astonishing amount of ocean heat north of 30N in tandem with hot continental landmasses to sharply define thermal boundaries, nevertheless risks in its dramatic context, to obscure and risk overdoing a signal that appears to have longer term drivers acting against it. Something eventually gives in my opinion - its a question not of if, but when? That is the most difficult question.
  9. Quoted a further time simply for purposes of continuity and in the face of some advertised less than desirable computer modelled weather through to mid month This sticking trough solution is now, not unexpectedly, coming very much under the radar in the 10 day period - but (for what it is worth) am still very much of the view that it needs to be seen as just another phase of weather, rather than anything sustained very long term. The greater problem in the coming days, than the synoptics themselves, continues to be the risk of taking each individual ensemble data at face value wrt this trough solution and extrapolating it too far forward. More of that shortly. For the near mid to end of week term, the models did not seen the slowing trough solution until the resolution came within the 7/8 day period either. Until fairly recently the ensemble means saw a seamless phasing of the departing mid week trough and the advancing one at the end of the week and a flatter pattern with no warm air advection evident. It is only a transitory downstream reinforcement of warm air advection, and unlikely to much assist those further NW - and does not prevent a continuation of unsettled conditions, and flatter solution beyond this in the medium term,. But it is a reminder of how surface patterns can take on different shapes to the suggested interpretations of pressure anomaly charts at distance. Irrespective of the likely somewhat cooler and remaining unsettled sequence in the following week, it means that temperatures for the next 6 days at least, despite showers and rain about, look set to remain in the average to warm category (warmest further SE). From later next weekend and into the following week comes that suggested cooler incursion as the upstream pattern amplifies in the Western Atlantic with a holding UK/Scandinavian trough downstream. The tropical diagnostic support for this points to blame c/o -ve zonal trade wind increase in the Eastern Pacific as a result of suppressed convection and interruption to the low frequency signal in the central Pacific (that has supported ability of downstream ridges to develop between Atlantic troughs for quite some time this summer).. The increased trade wind burst close to and east of the dateline very evident on Hovmollers (shaded deep blue) and reflected by the strong suppression in the same location(shaded orange) on the VP200 convection anomaly chart The convection anomaly forecast is now just starting to focus on the eastward propagation of this suppression phase and return of dateline active phasing of convection convergence, soon after mid month. Despite a neutral Nino 3 zone, there remains considerable warmth to the west in Nino 4 - and this suggests that central Pacific convergence still remains a valid default signal. In tandem with the return of the low frequency signal, the MJO 'mini ENSO cycle. convection pattern also perhaps re-cycling back to the Pacific for the last third of August and early September period. This would add even further support for a warm end to official summer and leading into September. NWP cannot, this soon, be relied on to 'see past' the suppression phase and associated programmed amplification upstream which holds the downstream trough in place. So simply not worth agitating about the inevitable under par operational and ensemble suites in the coming week and little sense at all in drawing conclusions longer term about them. However, with time as suppression focusses further east in the tropics across the Equatorial Atlantic and Africa, and active convergence of convection returns across the Pacific - then this cuts off the supply of -ve trade zonal wind close to the dateline . The consequent increase in upstream momentum from the Pacific>US as this wind-flow switch occurs should flatten out the upstream pattern and blow a hole in the ridge in the Western Atlantic and across W/Greenland, and as a result greater sub tropical ridging forced downstream instead - so in this way lifting out the downstream trough from SW>NE for the last third of August and retreating it away across NE Europe. On this basis, continued (and increased) supportive reasoning, in my opinion, for an improved scenario with warmer and drier conditions to see out the last official days of the summer
  10. Further self-quoting from the other day, but it has the well meaning intention to reiterate the theme of 'snapshots in time' instead of taking every operational/ensemble/cluster suite at face value, These also extend to the associated upper air pattern representations which also are subject to change as numerical modelling keeps reading the diagnostic elements (comprising fluxing tropical and extra tropical wind-flow phases) over time. Extrapolating all of the numerical modelling representations ahead for unduly long periods of time is fraught with error when margins of change can sometimes emerge over periods of days at a time, rather than every 6/12/24hrs at a time The bolded extracts in the quoted post are simply for particular emphasis. An interruption to the low frequency tropical walker cell signal in the Pacific that has dominated much of July (and that propped up the Atlantic trough/our downstream ridge) has been in progress and is set to persist into the new month with the renewed increase in trade winds over the Pacific clearly evident since the end of last week (shaded blue in and around the dateline) ,...This will be helping propagate -ve AAM anomalies to higher latitudes based on a slump in global atmospheric angular momentum . So essentially this slump is now catching up on us - and as a consequence helps re-invigorate blocking at higher Greenland latitudes and this side of the Northern Hemisphere inviting a path towards a supressed Jetstream and low pressure phasing between the Atlantic and Scandinavia by next week. Therefore a trough close to the NW of the UK has support in this respect. However, temperatures throughout this period look quite respectable and warm at times - especially perhaps this coming weekend before the pattern flattens out (as expected) so nothing in those respects that is poor by summer standards. (Also see the conclusion of this post). Personally, for what it is worth, I'm not attaching confidence in a Scandinavia/UK trough as any long term sustained solution encompassing 'the rest of summer' - and continue to think that the numerical modelling risks over dwelling on this signal. For me anyway, its a case of sitting on hands, and letting the scenario play out. The numerical models are not going to drop it immediately within a day or two, which further envisages in my mind the moans thread busier with standard writing (or repeating) of summer obituaries - but that is where the risk lies in interpreting/making assumptions about longevity. Summer 2012 style, did not materialise following the first two thirds of June 2019 after all....And whilst the last third of summer beckons immediately, there remains plenty of time for summer-like weather So as was the case in June, its also too soon to summon the meteorological priest at this time It remains likely that the pattern will keep retrogressing, as suggested previously, so that ultimately at some stage the jet stream will be forced to lift a trough further NE and invite a further ridge response towards Europe - and irrespective of any trough signal over Scandinavia. This is especially true if tropical activity in the southern Atlantic is created by the actions of a convectively coupled kelvin wave activity passing through Africa from the Atlantic - such a CCKW is occurring during the first half of August. This type of activity often throws curveballs in modelling at short notice during many late summers and as an interruption to mid Atlantic ridging and Scandinavian trough scenarios and cuts short their attendant less than desirable cool changeable north westerly winds, besides which, are often over modelled at distance. Those - that are one of the culprits for summer obituaries There is a lot over the coming 10 days that could,and conceivably will, alter the apparent prognosis for August as a whole,. Whilst an unsettled spell is supported and it may also turn cooler for a while around the second week and towards mid month (pending tropical developments) it makes sense in my opinion to compare ensemble suites and clusters over that opening week to 10 days as an evolutionary way forward, rather than taking each at face value individually, on repeated intra day intervals and drawing conclusions/extrapolating the snapshots in time too far forwards Notwithstanding the above suggestions about not taking individual modelling suites at face value, even if I break my own rules at this early time (as subject to the further evolution as discussed) there is nothing exactly terrible anyway about the latest temperature ensemble set assessed against the implied (and fully expected) flatter pattern phase for the second week August period. Its maybe forgotten just how very much cooler than this the early to mid June period was.
  11. Looking back at a previous post last weekend, and some thinking expressed back then, its clear that the lesser progressive solutions did lead the way in some erratic modelling that was forced into some impressive westward corrections. The closer detail reveals that this weekend is turning out wetter and therefore cooler than could have been anticipated at such a range, and across what is a microcosm of an area in relation to deciphering the Atlantic and European pattern. However, this rather distorts the bigger picture which shows that the very hot airmass has proved every bit as difficult to shift eastwards as these lesser progressive solutions suggested. Indeed it is this same slowness that has resulted in the stalled front and associated rainfall in evidence today and into tomorrow for quite a few of us. The next thing to say, is that very many contributions and updates from a wide range of posters has made this, and other threads, following the modelled progress of the heatwave, an excellent read this week - and so in my opinion a lot of credit and thanks is due to a large number of members for some very detailed discussion and debate Its been an astonishing week - not just for the heat itself, but also ( just for example) the outflow winds from thunderstorms which actually exacerbated that heat on Thursday evening associated with active and prolific 'dry lightning'. These strong outflow winds were picked out by high resolution models - as the EML theta plume (Elevated Mixed Layer) very gradually destabilised set against the very high upper air temperatures that had been 'capping' it - and have been taking so long to displace eastwards Looking ahead, in my opinion a main theme of this summer stands a good chance of repeating itself. The recurring 'spasmodic' blocking of this summer previously discussed c/o a highly unstable polar profile keeps duelling with a very organised low frequency walker cell in the tropics that equally persists in renewing sub tropical ridging to our mid latitude. At the same time according with this, and with our SST signatures helping, an Atlantic pattern fails to sustainably send a procession of lows under any block for a length of time, as one might expect with recurring higher latitude blocks, and instead sees follow up low slow on approach and give the sub tropical high every opportunity to ridge into the void ahead of it. The modelling has shown itself this summer as susceptible to wanting to keep the pattern too flat for too long with the phasing of these lows and instead the sub tropical ridge proves more resilient ahead of it. This is something to watch in the snapshot in time ensemble suites and not take too much at face value from the more immediate suites - but maybe instead judge them from how they evolve during the coming days ahead. Persistence factors alone do not of course guarantee past trends will be mimicked in future trends. Next weeks blocking programmed to the north is likely to migrate westwards with time, in tandem with pressure falling over Scandinavia and also tending to migrate that low pressure west to phase with approaching troughs in the Atlantic . This initially looks set to flatten the pattern to create the illusion of a longer term procession of lows. But as the pattern retrogresses, then this increases the likelihood that a follow up trough will, ultimately, like previously seen this summer, deepen in western/mid Atlantic and so a re-occurring downstream sub tropical ridge response accordingly gains traction, in time, with the modelling. On that basis, the present fall in pressure over Europe (created by a displaced arm of the jetstream digging southwards, elongating the trough that was to the NW of the UK and gravitationally forcing its new centre of low pressure into the continent) represents the conclusion stage of a repeating cycle that commened at the start of summer, progressed its second cycle through July to its present end stage, and over the coming 10 days is looking set to commence its third cycle. The differences between the first two cycles has been that seasonal wavelength changes vs the tropical/extra tropical windflow circulation on the jet stream (the low frequency walker cell and convectively coupled kelvin wave activity in the Pacific) have helped incrementally back the pattern westwards. This trend takes us to the third cycle. So that : 1) In June we saw the trough directly over us, and then immediately following up were still subject to cool air advection influence with the very high uppers that followed. 2) During July we have seen the trough resisted close by to our west. Hence the least progressive solutions verifying and the very warm/hot air advection managing to stick long enough to share the noteworthy conditions that mainland Europe saw in June 3) As August progresses our third tropical/extra tropical cycle of the summer, there is a reasonable chance that we may see a trough sequence further displaced to our west and a more prominent late summer anticyclone influence from Western Europe/Scandinavia. Especially perhaps if some activity in the tropical Atlantic assists in the process. At the moment, the modelling is best 'seeing' the phasing of falling of pressure across Scandinavia (from its latest downstream ridge peak) with the trough(s) approaching in the Atlantic next week. The caution is in extrapolating and making assumptions too far ahead from this - and so not a bad thing to be mindful of the risk of NWP persisting, in error, of this synoptic stage for too long. Too soon to suspect August may follow any alleged underwhelming tradition, even if the ending of July and heading into the new month might appear to be getting off us off to a (relatively) unremarkable start. But temperatures still look respectable over the coming period and perspective has to be set against the spectacular events of last week
  12. A bit of balance might be required, not unusually, for this thread: 1) Starting from round about now, the weather is on a fast track improvement for many and one thing is certain is that it is going to become very warm, and decidedly hot for some. Excepting of course those furthest north west who are ones to maybe have some justification for feeling a little left out and have reason to moan. 2) Whilst the odds on an extended heatwave scenario continuing from late this coming week have clearly reduced over the last 24 hrs, the latter part of this week nevertheless remains an NWP minefield - and there is not yet consensus on precisely how things are going to pan out. 3)The 'worst case' scenarios still look to manage low to mid 20s for many next weekend even when the greatest heat is 'swept away' These sort of temps have graced much of the month so far, again for quite a few (if not always all) and are a far cry from the dismal first two to three weeks of June. 4) The latest ECM operational is a least progressive and one of the warmest favoured solutions, but it is not far fetched in its evolution (as I commented with this type of scenario the other evening) and even its ensembles still suggest (currently) something very warm persisting into the following week - with the uppers remaining above 10 right throughout next weekend for many central and eastern parts. Yes these temperature numbers are for London, in the most favoured SE region, but they illustrate some perspective of how much the 'cool down' might be 'Normal service' ie low 20s is not suggested until well into the following week on these - though as a snapshot in time they are of course subject to change....either way.. 5) Its worth bearing in mind that the FV-3 ensembles have performed a spectacular volte face in 18 to 24 hrs and moved the heat zone more than several hundred miles in that time later next week. So whilst caution is required with any snapshot in time NWP ensemble suite, it certainly applies also to the FV-3 here. Starting with that secondary low from midweek... This caution equally applies to the other modelling, including the UKMO at day 6. 6) The diagnostic approach, which I favour, cannot help further with a scenario like this because the full range of micro scale numerical solutions on offer for the UK all fit within the macro scale range of the diagnostic at what is a relatively short distance for such non numerical evaluation. Just a 100 to 200 miles lee-way, either way in this sort of situation, could make some very big differences when trying to dissect the fortunes of a tiny island at a crossroads to a huge wind-swept ocean one side, and a hot and humid landmass the other. 7) With that in mind, maybe a good idea to enjoy the coming days (if like me who are a big fan of this type of weather), rather than already be thinking its over before its barely begun. And especially when specifics later this week still are not wholly certain. Living in the moment doesn't come easy with weather pattern watching, and especially for quite a few on this thread - but weather enthusiasm is also about life itself and appreciating the here and now, when so much worse can happen in the real world than how long a heatwave lasts. Again, in weather terms, those furthest north and west will not gain much consolation in that - but for the UK there are always winners and losers as it is rare to have a whole island witnessing the same conditions at any given time with our maritime climate.
  13. The ECM 12z operational might be an outlier of this evening, and indeed may prove by next week to have been just that - but it fully exemplifies (to the greatest extreme maximum!) the scenario I outlined yesterday, and which is also present in sections of the FV-3 GFS suite as well. NWP model suites are simply snapshots in time, and each one develops the themes of the previous suite according to the diagnostic probabilities. So its quite conceivable for even upper air patterns to diverge from one suite to another when an essential part of the jigsaw in terms of the upstream pattern shows a large spread degree of uncertainty. And from memory someone posted a substantial degree of spread of the eastern Atlantic yesterday which illustrated the conundrum of how energy is split with the trough responsible for ushering in the initial (very) warm air advection from the start of next week. Climatology favours the more progressive solution, and it could be that the expected two/three day plume = fresher air to follow does indeed verify, but I wouldn't personally be astonished to see a curveball. This based on the diagnostic probabilities that suggest and support some deceleration of the jetstream across the Atlantic round about the pivotal time that heat advection is pushing north through the UK. This interfering with the distribution of energy in the eastern Atlantic at the key period - and which leads onto that Atlantic ridge late next week to fill the void left by the slowing pattern to complicate things further and help stall the eastward shunt script. And which is clearly modelled on the ECM 12z as well The ECM 12z might turn out to be some viewing entertainment in the fulness of time, but as a few have already hinted at, there is nothing about its modelling within the day 4 to 6 period which is not credible - even if the chances of such sustained intense heat as it shows are likely (relatively) moderated. Or maybe not as the case might be...
  14. This is a good question - I agree with the response that @Singularity has made earlier. For those who dislike very extended wider discussion detail, this post eventually 'gets there' in terms of its relevance to the here and now model discussion context - but there are benefits from a bit of analysis in terms of the increasing difficulties of 'pattern matching' and why it is increasingly fraught with potential error if too many x+y= assumptions are made about composite analysis synoptic responses. This in turn affects those who attempt forecasts, short and longer range, using diagnostic vs traditional NWP methods. I'm highly grateful that I merely offer observances and reflections such as these in posts, than attempt (or want to be involved in) any competitive forecasting element itself Caution is required when referring too literally to composite analysis of the tropics (MJO) and extra tropics (GWO) when trying to decipher synoptic NWP responses, as ocean/atmosphere and polar/troposphere disconnects and a-typical synoptic responses become increasingly common - no doubt at all ever more skewed by the modern climate context.. That doesn't mean in any way that the GSDM is invalid - quite the opposite and far from it. Simply that the vital wind-flow (jet stream) diagnostic offered, requires to have the minds that utilise them, (and are open minded enough to acknowledge them) adapted to full acceptance and lack of denial of the changes that are happening out of human control (it would seem!) and which the science has harder and harder challenges to keep up with While the arctic sea ice stresses and subsequent amplification feedbacks are obviously well known enough on sites likes these, and as one major element of this macro scale step change shift, there are also feedback stresses within the tropics where superimposed, additional anomalous heat in all the tropical oceans, above the norms, is also creating ever more skewed ocean/atmospheric responses. A very good example of unusually strong disconnect was the strong El Nino event of 2015/16 which managed to manifest starkly a-typical -ve forcing aspects due to the unusually high SST's that persisted in the I/O at the same time as the very warm ENSO event was present in the Equatorial Pacific. This anomalous extra superimposed ocean heat across all the ocean seas in the tropics is very much a factor of the regime shifts being seen and which have accelerated ocean/atmosphere feedback distortion further this decade. This blog entry, at the time of that event, remains a very good discussion pertinent to the post question under discussion https://blog.weatherops.com/global-warming-and-enso-a-helter-skelter-atmosphere Also, very much part of the jigsaw connected to the increase in Nina-esque disconnects to Nino phases, we have being seeing persistent feedbacks since last year from relatively cooler southern ocean waters, as mentioned in the post the other day, that create a -ve SPO (South Pacific Oscillation) and hence a quasi -permanent sub tropical high pressure propagating zone which acts 'destructively' on warm ENSO events. This means more prevalent trade wind phases than might be expected during an El Nino are restricting relative angular momentum tendency to lower levels than might be expected with a traditional event. More on this further down wrt frictional torque mechanisms. The atmospheric divergence created by greater, overall, trade winds, creates wind shear that is hostile to eastward propagating tropical convection, and this stabilising suppression of the atmosphere transfers into stronger sub tropical ridges which, according to seasonal wavelength changes, periodically push northwards towards mid latitude. Hence prominence of poleward amplified anticyclones in both Pacific and Atlantic during any -ve (Nina-esque) forcing. This particular link refers to the analysis and effects of -ve SPO regime. https://www.climate.gov/news-features/blogs/enso/look-south-enso-forecasters The inference in GSDM terms is that negative SPO feedbacks create negative tendency of frictional torque in the tropics. In the here and now, notice the persistent trend of -ve FT within the sub tropics below 30N as indicated by the -ve (easterly trade anomalies). The largest anomalies, circled that are in blue, naturally coincide with the greatest departures in frictional surface torque. It is these -ve easterly wind anomalies and associated -ve torque mechanisms that underpin the sub tropical anticyclones and create compensating westerly polar jet momentum at mid latitudes on their poleward flank, according to the laws of conservation of angular momentum, and so impacts on jet and pressure profiles downstream across the US and in the Atlantic Stronger trade wind bursts, as well as reducing the eastward propagating effects of convective convergence in the western and central Pacific, assist upwelling of cooler sub surface waters in the EPAC, In terms of my own 'at home' observation as a fascinated pattern watch observer, there is no doubt in my mind that these sub tropical ridging patterns, whilst always a feature of greater trade wind phases, and typically subject to seasonal wavelength changes and Annular Mode variations, are becoming more and more dominant as part of shifts in long term global changes. With the tropics forcing one way, due to anomalous extra heating, set against the destabilised polar profile creating larger swings in AO responses as the sea ice patterns distort feedbacks there, a new way of interpreting composite analyses for H500 patterns is certainly required. Innovation in forecasting science is great, adaptability to undeniable forcing change on the weather patterns, and the benefits that follow of greater further understanding is even better. All the more reason, in my opinion, to pay greater attention and scrutiny to what are dubbed on this thread the 'background signals' because they present greater and greater challenges to deciphering NWP, as they equally test the performance standards of NWP modelling itself Which brings us back once more to the here and now and that mercurial mix of low frequency tropical forcing in the Pacific vs the South Pacific Meridional Mode - both acting against each other. A twin low frequency signal acually persists over both Africa and the Pacific. This presents a disconnect element when the African wave predominates over the Pacific wave, and its associated equatorial easterly wind anomalies propagate across the Equatorial Atlantic and create wind shear in the EPAC which suppresses convection. This stable suppression helps poleward promotion of that South Pacific ridge and in turn downstream strengthening of the Azores sub tropical ridge in our sector. However, the presence of another standing wave due west to the dateline and separate low frequency signal in this region creates resultant tropical convection convergence zone where the African equatorial easterlies meet the Pacific low frequency westerlies. It is this convergence that prevents a more full blown Nina-like response in the Pacific - and which equally therefore prevents the Azores ridge retracting, with added polar jet flow over the top of the ridge depositing a downstream trough in our vicinity for any sustained period of time With such countervailing factors evident, not unreasonable to expect a combination of upstream pattern that presents a neutral enough PDO regime to enable downstream split energy of the jet stream that allows downstream ridging to occur as troughs deconstruct, but also at the same time persists with a recurring strength of sub tropical ridging in both the Pacific and Atlantic (Azores ridge) due to the equal presence of the African wave supporting the sub tropical ridges. Final outcome probabilities, in synoptic terms, allows the type of scenarios where cut-off lows result as trough deconstruction occurs, and in the wake of this deconstruction, the sub tropical ridge upstream re-asserts a renewed eastward ridge from within the Equatorial Atlantic to reinforce the existing downstream ridge, trapping that cut-off low beneath the ridging, and potentially sustaining heat ridging in place for longer. With meandering and sluggish jet patterns, choreographed through a mix of highly unstable tropical and polar forcing, which in turn create destabilised and often disconnected GSDM profiles, heat trapping scenarios are bound to become more regular summer time features it would seem, to me at least as much as implications of various kinds can be drawn for the opposite times of year.. The UK's fate, in terms of tapping into continental increases in heat, rests as usual in the actual wavelength that the sub tropical ridge amplifies. All tied into that primary wavelength dictated by frictional torque tendency within the Pacific, as just discussed according to the controlling tropical wave(s). But also the polar profile determining the latitude the ridging sets up. Late June, with greater downwelling of -ve zonal wind anomalies from the final warming legacy also a main factor than is the case now, was a myriad of Nina-like sub tropical ridging in the North East Atlantic, set against the struggling Nino low frequency standing wave in the Pacific, with the result that cooler air advection was able to come around the retracted stronger sub tropical ridge in the Atlantic ( the weaker less dominant sibling ridging extension over Europe could not prevent this) and off-set the furnace level upper and lower temperatures from mainland Europe manifesting themselves at ground level here in the UK beyond a day as the dominant Atlantic ridging briefly weakened This time around, the downstream pattern is allowing trough progression to be able to force greater sub tropical amplification of ridging ahead of it - instead of what occurred in late June when the wavelength favoured earlier retrogressive amplification. My own interpretation from the 12z thus far is quite consistent in inclining towards the ECM ensemble consensus this morning. The consolidating downstream European ridging is also a good sign of +ve poleward momentum transport taking place - and which according with the increased maturity of the +QBO phase, also suggests intuitively that a more +AO and +NAO regime may be forthcoming for this second half of summer.
  15. If I might interject here The lack of MJO forcing in the Pacific, with emphasis of activity sticking in the I/O, is being dampened/offset by the low frequency signal which remains firmly in the Pacific- and a reminder to us all that the MJO is not always the be all and end all when it comes to pattern change dictation from the tropics. Its early days, but there are some similarities developing with the latter part of the summer in 2016 developing. That is not to say it is going to pan out exactly the same as the second part of that summer, but a Pacific low frequency wave train is present at this time as it was then. These persisting westerlies showing up orange shaded close to the dateline here: The latter part of summer 2016 and indeed well into September produced a re-cycling pattern of plumes interplaying with eastward displacing Atlantic and Azores ridging that alternated cooler more unsettled phases with these warmer plumes. The prominence of the low frequency signal is echoed by the SOI (Southern Oscillation Index) which ironically is more -ve now than at any time this year that an Nino SST signature has been present in the Pacific (SST's overall now trended neutral - warmest to the west, coolest to the east) So despite the fact that angular momentum has slipped to levels not seen for over a year c/o loss of westerly momentum in the extra tropical circulation (and is also at a similar level to late summer 2016), the weak walker cell circulation and associated convective convergence zone is still enabling something of a downstream synoptic response that is more reminiscent of El Nino than the extra tropical wind-flow Global Wind Oscillation might suggest. Hence the ability for some a-typical sub tropical ridging downstream, as a reprieve from occurring too far upstream and allowing a UK trough dominating instead as one might expect from a lower AAM extra tropical response. Its still only mid July, and clearly still rather early to say for sure, but this tropical forcing disconnect, if it can prevail/re-cycle may yet produce a better second half of summer, than the increasingly Nina'ish extra tropics implies Much as happened in August/September 2016. As long as the western/central Pacific walker cell circulation keeps producing convergence close to the daleline (as it encounters countervailing trade winds c/o divergent/stable relatively cool waters in the Southern Pacific) then there is no automatic assumption to be made that a more typical Pacific/.Atlantic Nina-esque circulation should take over (such as it did after mid July in 2017 as a good example) and which resulted in a retracted Atlantic ridge with enhanced polar flow and troughs steered close to the UK.
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