Singularity

In a bid to try and figure out what's messing with the models - it seems there's two adjustments taking effect; The first is a reduction in the strength of the jet stream crossing the Atlantic, with the deceleration zone now (i.e. on the 06z) occurring before it reaches Europe, instead of over France as was the case on yesterday's 12z run (shown left). The second is an extra small trough being picked up by some model runs over or near the S. tip of Greenland. It's a very shallow feature - indeed still only just evident on the 00z GFS shown middle - but it's enough to reduce the ability of a ridge to nose in between the Canadian trough and the one trying to become cut-off over the UK. So it is that during the weekend, the jet is a bit flatter between the mid-Atlantic and Scandinavia, which as a result means that instead of the progression being determined largely by a fairly strong NW-SE section of jet stream that would push the low SE, there is more of a balance against the poleward-returning jet on the eastern flank of the trough, with the combined force keeping the low near-stationary while positioned N-NW of the UK. There's even the chance of a weak ridge building over the SE as there is a net deceleration of upper winds over there. This, then, is an explanation for GFS' behaviour - but clearly UKMO has not applied any of these adjustments, and ECM has swung from applying them in the extreme to, it appears, retaining the weakened jet stream but also backtracking to produce greater ridging in between the two main troughs. So it's not the best of times for making any weekend plans . Next week we have been seeing another round of mid-Atlantic amplification being suggested but, as MWB has well documented, there has been a trend away from this toward a flatter pattern. Hints are that envy of southerners nabbing the best of the weather may remain a major theme of this summer. For those up north to stand a better chance of seeing fine summer conditions, the upward AAM swing that has been increasingly advertised in recent GEFS output may offer some hope but it does depend on how those C-Pac trades behave and also the MJO. GFS keeps the latter quiet while the trades slacken off again and this (combined with the AAM trend) is probably why it's recently started to take on a more positive mood in the 8-16 day range. Really though, those trades could do with returning to at least neutral anomalies.

I seem to recall some fairly promising looking charts being around a few days ago - just not full-on 'entirely settled'. Being at 8-14 the signals are usually more muted anyway but I can see that they were being more cautious than they could have been - wisely so it would appear!

Ah thanks for reminding me. Looking at the 'guidance used' it does appear to be all-American so that must be their default.

We must have the strength to face up to this sort of thing. Fact is, with so many recent and indeed current runs painting this sort of general picture, the guidance was bound to look something like this. A run like the ECM 12z would need to be repeated on the next 00z, and by more than just one model, before adjustments toward it can make much sense. Not sure why they've gone quite so far toward GFS over UKMO but... they have their reasons and they should be well-founded.

I had to double-check that I wasn't looking at cached charts with the ECM 12z! That's really put the cat among the pigeons Perhaps it works with a slower propagation of the signal from the C. Pacific, such that the jet stream does not ease off quite soon enough for the trough to escape and become cut-off. Imagine how edgy we'd be if this was a winter's evening with a sinking low interacting with an easterly incursion having suddenly been put back up for questioning! Has to be a 'hmm, really?' for now, but perhaps the way GFS shifted the initial low NW with a bit more interaction with the mid-N. Atlantic jet stream segment was a more restrained version of ECM's response to some tweaking of the propagation rate of the tropical signal? Making a call for next weekend has become a whole lot trickier again. Amazing to think it could still end up fine and warm in the south... oh how we'd look back and laugh. Or maybe cry, I don't know... /

As to be expected when it's a cut-off low we're dealing with, there's quite a bit of variation at 6 days range in terms of the position and intensity of the system. UKMO has remained steadfast with a position more to the NW as an increasingly slack feature. Showers fairly few and far between for the southeast where it could still feel quite warm in the afternoons. Progressively cooler and more showery as you head NW from there - but never terribly so. GFS has adjusted the position of the original low NW since the 00z which is of some interest, but also added a secondary feature swinging in which lacks much support from other sources at this time. Not a good thing to happen as this not only brings a spell of more persistent rain and suppressed temps, but 'tops up' the trough with cooler, more unstable air - so helping it to then hang around well into next week. GEM remains the odd duck with the low making more eastward progress - but at the expense of it being deeper on Thu-Fri due to the stronger jet stream required to achieve this. So cooler with a greater number of showers during that time, but slowly trending drier through the weekend, albeit with some fairly cool air in place so struggling to feel very summer-like except perhaps in the far SE. So yes - a variety of some note. UKMO offers hope that the weekend could continue to feel reasonable enough away from the NW, but GFS conjures up some unwelcome secondary shenanigans while GEM would rather have a considerable bash at the positive monthly mean temperature anomalies. If the ECM 12z could side with UKMO, I'd at least feel a little more positive than has been the case for several days now.

Yes this is what I meant - thanks for the interpretation Nouska. Essentially, there is a mechanism known to exist, but studies disagree on the extent of influence on extra tropical patterns. This may be due to the role of AAM cycles, with studies tending to look at the overall picture rather than against the cycle. We look to be poorly placed in the cycle on this occasion which adds to the level of influence being seen on this occasion. So it becomes difficult to gain enough certainty to apply a significant modification to longer-range model guidance. In this light it's a shame that work on the Global Dynamic Synoptic Model (GDSM) was abandoned some years ago, as it made some promising steps toward better understanding of the role of the AAM cycles in connection with other forcing. Also in my last post I should have clarified that I meant 'significant' in the scientific sense I.e. there is enough variability in results that the impact of the C-Pac signal befomes muddled unless the picture is broken down further. This being where I believe further research is much needed to facilitate more confident application of signals from such events; in particular weighing up against other sources of tropical forcing.

I too question the longevity of a shallow trough sitting over us. The Central Pacific upper wind anomaly should too localised to persistently force the pattern so it would not be surprising to see a kick of westerly momentum crop up around the weekend time. That could lift the lingering low out, but will struggle to be enough to bring the Azores High very far toward us. GFS 00z a fair reflection of this with a few drier but still not all that warm days early next week. For a better recovery next week we could do with the wind anomalies becoming more neutral again as there is a window of opportunity for another upward swing in AAM. I have seen the models overdoing those C-Pac easterlies on several occasions in recent months so there is a chance. Odd thing is there is barely any literature supporting a C-Pac upper wind influence of any significance on Atlantic-European weather patterns, yet recent observations appear to suggest otherwise. The lack of strong theoretical grounding may be why the Met Office have left it late to move away from the drier and warmer stance despite models predicting those anomalously strong C-Pac easterlies for quite some time now. In fact, they were predicted before the output even moved away from settled conditions, so I'm not convinced that they have been more than half the story here. The rest may quite simply be hard luck given the extraordinary coincidence of three LP systems in the right region of the N Hemisphere at the right time. Only relatively small time variations could have produced a less undesirable placement of the resulting shallow low; GEM illustrated this yesterday evening. Yes, it hurts and some may feel downright cheated out of sole nice weather. Its a bit like when we had impressive easterlies snatched away in Jan-Feb... this sure has been a terrible year for raising and dashing expectations - though it could have been worse given the abundant dry weather and warmth in spring and, for the south, summer so far.

Just to really get on peoples nerves here is a comparison between the sort of model consensus output we were looking at yesterday morning for noon Monday 17th, and that which we have now, in which we can see how little has actually changed at this timeframe. Yep - there's just been a bit more of a kink added to the jet stream over S. Greenland as a shortwave trough makes itself known. This brings the jet closer to the low tracking just N. of the Azores and so that feature finds itself with some upper level divergence to work with. This causes pressure to lower between it and the shortwave trough, establishing a connection that leads to a merger of the two features. Because one was so far north and the other so far south, very cool air clashes with very warm, generating a fairly vigorous trough (though worth noting these that occur with steep thermal gradients are often overdone by the operational models until at a few day's range). UKMO is the only model offering a 12z run that avoids this interaction, with much shallower lows and so much less of a pull of cool air from the NW toward the UK. In the grand scheme of things, cool air and showery weather making its way over for one or two days before what remains a strong impetus to settle things down by Friday evening does not seem too bad, and for the south it really isn't, but those up north are bound to find this a frustrating interruption to the long-advertised improving trend for mid-late July given the lack of much fine weather in recent weeks. I do feel that 9 times out of 10 we'd not have had at least one of the two troublemakers out to our west early next week and things would be looking to go much smoother, so this really is a harsh deal of the cards. The best I can offer regarding the middle part of next week is that GLAAM trends are positive enough at this point in time, and ensemble guidance spread-out enough, to give at least some hope that the combined trough - however deep it ends up being and presuming it does form in the first place - won't move so close to our shores as some of this evening's output is suggesting, so reducing the duration of cooler and showery conditions. Oh and you never know, we might get a good light show on Tuesday or Wednesday night!

UKMO offers some interest with shallower lows and little in the way of consolidation into one feature that sweeps the heat away. Looks more of a very warm but thunder-showery setup... Much as per latest Met Office guidance

I'm not allowed to go into much detail for NDA reasons but broadly, the wave train that typically occurs in response to Indonesian MJO activity is almost the reverse of that for eastern Pacific activity across most of the Pacific Ocean, yet the wavelengths are also shorter and this happens to mean the downstream ridge/trough position over N. America is similar between the two MJO phases, and so on across the N. Atlantic and N. Europe. This is as far I dare describe it and may not be entirely precise as a result, but what matters more is the response in the Atlanitc-Europe sector, the most significant of which for both MJO scenarios is a northward displacement of the Azores High which then tends to ridge east although not always persistently. We're seeing this feature in current model output, with the usual dramatics as smaller scale features play in - these being the reason for 'not always persistently' and, as it happens, many disgruntled faces on the MOD thread this morning .

Bottom line is, background signals continue to strongly support ridging of Azores in from SW so that remains most likely as the overall trend with time. This midweek madness is just plain bad luck if it unfolds as the 00z op run consensus depicts. A smaller scale development interrupting the settling down tendency (Mon & Tue look largely settled).

ECM has the quick recovery too so that's something. Worth noting that plume timing on that run looks right for a 30+ day Wednesday and with the heat reaching well north so at least taster for most. Still... it seems very harsh to have to make do with that for midweek.

Like some malicious plot line, the 00z runs from all but UKMO (and it's perilously close) see a shortwave trough passing close to Iceland perfectly timed to interact with the little low trying to take a trip to Europe. Very painful to watch unless you dislike heat and enjoy quick breakdowns with sporadic thundery conditions. Those seeking fine weather for mid-next week will have to hope that the timing of one of these features adjusts away from the latest consensus. Not that the sweep-away mid-week should really be the end of the story for a fine spell; it really should shove back pretty quickly so within a day or two. GEM 00z captures this notion.

Yes... Nice little adjustment west with the dropping low on the 12z runs giving us a wider margin of error for trapping the heat low in place S of the UK with warm or hot dry air wafting into England and then (later) remaining countries from the continent .

If I didn't know better I'd swear the operational runs have a built-in 'nudge' toward taking small lows across southern UK at the mid-longer range . Such a path is a possibility, sure, and one that leaves us looking for a nice clean realignment of the jet stream to stay far enough NW of the UK that the Atlantic troughs are unlikely to move close enough to open a doorway for lows over W. Europe to move north. The GFS 06z keeps the jet more wavy and this is not helpful unless the S-tracking low stalls out well west of the UK in which case it may simply pump more warm air into the ridge aloft and help it to become/remain strong across the UK. This is a more common outcome for fine summer spells in the UK but the resulting as settled weather doesn't tend to last as long. The trapped Euro 'heat low' is more effective - and while harder to achieve, it does usually depend on something the models strongly agree upon; a deep quasi-stationary trough across Greenland. So we have a good shot at it but as usual, no guarantees - so best keep those fingers crossed!

Given the similarities being drawn between the mid-to-longer term model output and July 2013, it's of considerable interest to compare the two Junes in terms of preconditioning for high temperatures; On the one hand, June 2013 was much, much cooler over western Europe than 2017, but on the other hand, it wasn't as wet, particularly across France. On balance, though, the far greater mean temperatures outweigh the wetter conditions due to being both more widespread and capable of driving extreme rates of evaporation and transpiration (i.e. evapotranspiration) which quickly remove moisture from the surface soil layers. Even in 2013 this will have mitigated the restraining effect on heatwave preconditioning, but still, I expect 2017 is leaps and bounds ahead of that year in most if not all areas of Central and Western Europe. In fact not only is this the case on land, but also out at sea; I very much recall thinking about how July 2013 could have brought day after day reaching into the 30s *C across a wide swathe of the south had it not been for those well below average SSTs (largely a result of the exceptionally cold March). So it would seem that all that remains is to achieve a similar pattern again this year and things are going to get very toasty this year. Well, I say all... Plenty of ground to cover before we can really start concerning ourselves with the possible impacts - but I figured this assessment would be entertaining anyway .

So the ECM 12z really did stick close to the GFS 12z! Odd how that makes it three 12z ECMs in a row with this general progression - but improved this evening - with both the 00z runs between being more aggressiveresive with the Atlantic low. I daresay the latest effort is setting up a mighty heatwave. Notable how a bit more of the higher uppers finds its way in compared to GFS - a typical difference with GFS often undercooking this factor.

I was a little concerned by the continued presence of the deep low at +144 on the ECM run, but less so than I could have been thanks to the ridge looking stronger - closer to GFS than the 00z run was. Happily this has proved sufficient to fend off that vigorous low and keep the run tracking satisfyingly close to the GFS 12z - though a more straightforward NE motion to that system in the first place is actually more important here. Less and less support for trying to dig that trough down close to the UK so that's encouraging .

The major models so far this evening, from least to most desirable (if you like very fine summer weather): It's curious that GFS keeps on making a lot more of the second push of high pressure from the southwest Sun-Mon, and GEM a lot less, while UKMO is happy to sit in the middle ground. When it comes to the behaviour of that low S of Greenland, it's not clear whether UKMO would take it NE to pass by the UK with little direct impact such as GFS goes with this evening. It does seem though that the idea of it cutting S/SE has lost favour with the det. runs. GEM is now almost as aggressive as the ECM 00z was with the feature so that makes one wonder. If we do take the halfway house that is UKMO then prospects look reasonable enough in the south and then - at last - in the north by around Tuesday evening (though this second part is another case of extrapolation!). GFS remains the best fit for the background signals but there is a risk that it's being too clean/simplistic with the Azores-Scandinavia ridge setup - this relating to the usual overly smooth jet bias (not enough kinks). Something to bear in mind, although it doesn't explain the enthusiasm for the ridge being so strong to start with; this comes from the Azores High itself simply being 3-4 mb stronger (but why is that, I wonder? Different handling of tropical forcing on the Hadley cell?).

Quite a range of scenarios with respect to that low passing S of Greenland early next week: A) The very progressive 00z ECM which moves it quickly NE and intensifies it into a fairly deep system which is able to push close enough to the NW to provide a quick exit route for a shallow low that forms on the periphery of the heat plume. B) The less progressive 00z GFS and (given similarities at +144) UKMO runs which still take the low NE but as a weaker and slower feature, allowing more of a ridge to build across the UK with the low on the plume periphery unable to find an exit route (despite a couple of days of trying in the case of the 00z GFS). C) The 00z GEM and 06z GFS which take the low E then SE, dropping it down well-west of the UK on Wednesday. At this point GEM displayed it's usual strong enthusiasm for advecting the plume and associated peripheral low northward, leaving us waiting for a reload of high pressure from the SW on Friday following a warm (very warm in south) but thundery Wed-Thu. GFS is a different story, with little in the way of northward plume and low movement. This is due to a much stronger ridge of high pressure through the UK which is in place thanks to the Atlantic jet stream being orientated more SW-NE than W-E. Personally I feel the balance of probability is toward heights more high than low just NE of the UK so a more SW-NE jet orientation, but if that low heads NE instead of SE then there will be enough of a kink in that jet toward the UK to cause some trouble (opening the door at least briefly for lows drifting N/NE from Europe) anyway. So there's a lot that has to go right to get a lengthy fine spell out of this. It just so happens the background signals favour just that, but as usual this cannot rule out the smaller-scale details that can interrupt the adjustment of the pattern toward what the signals are encouraging so we do need the usual dose of luck if we are to experience a nice smooth ride .

The GFS 12z did a decent impression of the ECM 12z of yesterday, albeit with the usual issues modelling the associated 2m temps. The ECM 12z of today feels like a slight variation on the previous 12z after this run's peculiarly aggressive solution regarding the Atlantic lows; the consensus is for these to become separated from the upper westerlies much as has been the tendency so far this summer, but most likely with the low staying west of us on this occasion. Great building blocks for a lengthy warm/very warm spell for much or even all of the UK... Perhaps even hot at times later? Getting way ahead of myself there though!

Watching the mean trough position Sun-Tue adjust away from being not far NE of the UK. UKMO 12z paints an all-too familiar picture this weekend of a weak trough grazing past the north while a weak ridge and warm airmass affects the south, resulting in a north-south split... Hopefully the last for some time as HP becomes more influential starting early next week.

The emerging picture is one of a weakening front getting caught between two ridges this weekend, with the western one poised to move right across the UK early next week. GEM was first to suggest the latter and now ECM has followed suit. Ensembles may be slower to shift of course, and that's assuming they do, which given the stubborn GFS is not a given, although so far this summer this GEM-ECM-GFS sequence of picking up on very warm or hot weather potential has proved oddly dependable. This from ECM at day 10 really is the Azores-Scandi ridge scenario painted by longer-term guidance in recent times. It may well be coming together during mid-July after all but as always I must remain cautious until such developments are within 4-5 days range.

Just possibly, given 00z ECM and 12z GFS, the movement toward the longer-term Met Office outlooks has begun... still need to see a shift in EC Ens though. Interesting to see an ex-tropical storm featuring in the GEM 12z after its debut in the ECM 00z. It could become a set back, or a means of boosting temps further upward and (unfortunately) adding yet more fuel to the European burner. Or it may well not come our way at all - tropical exports are notoriously difficult to a pin down.