Singularity

Those signs of high pressure arriving from the west later next week are largely tied to a new MJO event which might cross Indonesia, trouble is it’s yet to be nailed down so we can’t depend on it yet. Thats either good or bad news depending on the water situation in your area. I’m leaning toward it happening though - and keeping a eye on the heat to the south, suspecting GFS is taking the ridge a bit too far north in its latest two runs.

Tomorrow the air a little above the surface will be considerably hotter - indeed record hot widely - prior to vertical mixing bringing it down. Sitting here in 37*C heat, I’m struggling to comprehend it being 1-3*C hotter tomorrow.

East Anglia looks to me to be the region to watch, clearest skies for longest and a big overland fetch to the wind. Hawarden a possible exception due to the foehn effect, could yet see todays highest temp.

Agreed, that’s what I’ve observed as a general tendency over the years. Boundary later is modified too rapidly in largely or (most of all) entirely sunny conditions which leads to temps rising then levelling off too soon. Even in mid-summer, it rarely predicts much of an increase between 12-3 pm and 3-6 pm when looking at the 3-hourly max temp parameter. The reality is typically a 2-3*C rise with a peak sometime between 4 & 6 pm. Today with the very hottest airmass still arriving through the evening, maximums could be at the late end of that range, similar to yesterday.

Regarding the lack of much rain during the breakdown, perhaps the cool air undercuts the hot, dry air meaning instability is all elevated and where moisture is severely lacking? I’m on holiday without my usual resources so if somebody could check out a few soundings (skew-Ts) that would be much appreciated

It depends on the model resolution, the highest ones such as UKV and AROME capture the finest detail with even small cities and valley formations being resolved to some extent, while GFS for is relatively course but can still resolve urban heat islands for large cities like London or Birmingham. ECM is a little higher res than GFS but I’d still use UKV when available.

That chart shows pretty much the long-term average late July pattern for NW Europe hence the anomaly over the UK is very slight. This is possible even with the positive anomalies to the west, as they represent the stronger than usual Azores High rather than one displaced northward (hence no negative anomalies near Azores), while ones across the Arctic aren’t strong enough to suggest a -AO. So, a standard westerly setup is suggested to me, mostly settled in the south, unsettled at times in the north. I’m not sure how much I buy into it though, given the modelling difficulties I mentioned yesterday. Could be interpreted as ‘no clear signal’ from NOAA.

I’ve long observed off GFS diurnal temp cycles, where it heats up too quick and tends to level off temps too early… and in my experience this doesn’t usually cause higher max temps then reality, possibly due to the two biases cancelling out. In such extreme conditions, though, I’d not be surprised if it overshoots by about 1*C, at least on Tue when rapid climbs may be a bigger requirement depending on how quickly things move along.

Impending inferno aside - and I’ve just travelled to Norfolk of all places for a holiday! - that was a very insightful post from Tamara earlier. Not all La Niña events are equal in their degree of impact on NW Europe’s summer weather, in fact if you analyse them all you find no significant correlation. Slight variations in the position of the strengthened Walker Cell’s western edge can have major implications for the way that La Niña encourages the enhanced subtropical high (Azores high) to move (the intensification is the most reliable La Niña impact, where it goes is another matter). A more westward edge encourages focusing in the mid-Atlantic, while further east favours extensions and sometimes wholesale displacements across UK/W Europe. More akin to Nino impacts but with a stronger high. Forcing from closer by or right on our doorstep can also distort the pattern and I agree that with the changing climate the latter has become more of a factor. Feedbacks occur that models often overlook at the longer range. The imminent event was assisted by a big AAM climb that was well anticipated, hence the long lead time on it. That’s not the case for what may occur late July and in August. The models are clearly struggling to resolve that period at the moment, with lot of run to run variation and wide ensemble scatter. EPS haven’t been able to resolve more than one catch-all cluster (essentially just an ensemble mean) for the 11-15 day range for a few days now.

For what it’s worth, UKM is in my view like an ‘old hand’ that’s not faring so well these days, tending to be most erratic and prone to spurious development of disturbances - including very shallow upper level features bringing cloud and sometime rain - which are there one run and gone the next. Does anyone know when it was last updated?

EPS have raised an eyebrow as I dare to contemplate the later stages of July for a moment. Just one cluster, usually meaning huge spread when at 11-15 days range, yet it manages to somewhat resemble another case of a cut-off low leading to a big heat build over France & potentially the UK. I know where @Tamara & @Man With Beard are coming from regarding potential for a pattern repeat (but not precisely, of course). Sitting here in a near 30°C room at gone 10 pm with many more heatwave days in the outlook, I'm not best pleased by that signal to be honest, even though it's at low confidence.

Today, we're (at long last) reaching the point where the all-important cut-off low is actually forming, giving some true situation data to work with. The impact on the GFS 12z compared to 06z has been to shift the low even further south, by about 50 miles as of 9 am GMT Sunday. At this rate it really could stay cut-off right through Tue... but that will depend on how the N. Atlantic profile shapes up, with is another major source of uncertainty - and where the 00z ECM raised big questions with an approaching low instead of high on Monday.

ECM 00z shows what may happen if an Atlantic low merges with the heat low over the UK. However, most modelling has other ideas, so it’s best viewed with a pinch of salt.

Key here is that this is a surface based ‘heat low’ driven by rising air that hasn’t yet aligned with an upper low to uncap the atmosphere and allow more than isolated (but powerful) thunderstorm initiation. It sits awkwardly between the polar jet to its north and the upper low formerly west of Iberia. ECM is keener to align the upper low yet even it didn’t produce much rain in the south due to prohibitively dry layers of air.

You can get a lengthy extension of the Azores High across southern UK with a mean westerly flow aloft - a 'flat' pattern isn't necessarily changeable. That's where anomalies can help with diagnosis. The 8-14 day indicates a north-shifted polar jet which translates to an increased frequency of high pressure across much of the UK. How many interruptions are likely to occur and how widespread they are (the south could be left high and dry) is better assessed using analysis of ensemble clusters... or at least they are when those clusters exist to begin with - the 00z EPS is lacking in that regard - too much spread!

Focusing on France, the GFS & GEM det. runs are considerably different to ECM, with much more of a cut-back of cooler air down the eastern flank of the high over the UK Fri-Sat. As a result, the former two unfold in more of a traditional heat plume fashion, albeit a slower-progressing one than we're used to. ECM takes more of a 'heat dome' pathway, which is substantially worse for France with intense heat setting in sooner and lasting until at least as far into next week. That it wasn't extremely hot in the UK is down to the ridging across northern UK from the west, rather than the airmass being moderated. UKM sort of sits in between but has a completely different Atlantic-N. Europe trough-ridge profile for Fri-Sun so all I feel I can do there is shrug & grimace! Anyway my key point is, we have important disagreement regarding how the hot air shapes up before it starts to get directly involved with southern UK. On top of that, we can't be sure about the Atlantic layout either. Still so much to be resolved! Best just sit back and let it play out for a few days.

Makes a change to see an overall trend toward a less progressive heat plume arrival - so often we’ve seen it go the other way instead, like with the June event. Of course that means we can’t reasonably rule out a trend reversal before we get to the weekend. UKM is certainly a head scratcher this morning with a small low nipping across the northern half of the UK on Friday, a feature absent from all other deterministic runs. Probably off one one… probably.

From reading around, the airmass we may import from the south undergoes huge adiabatic descent over Spain, causing to become extremely hot and dry, typical of what’s seen over the Sahara under the subtropical high pressure belt. This implies legit potential to deliver temps into the 40s with relative humidity below 20%. Damaging to many indigenous plants if it happens. However, I’ve also seen mention of something called the ‘Bowen Ratio’ which reportedly GFS makes too high for extremely dry airmasses, leading to some excess surface heating, so it may have a positive bias of 1-2*C for the peak of the heat. Alarmingly, when applied to the 06z, this still allows for 40*C in some spots on at least one day.

I can’t recall seeing anything like that, for coverage of 20+ 850s, in an ensemble mean for 8 days time before. A spot of 24 in France… they’re in for a torrid time seemingly whatever goes on in the UK.

Funny how things go sometimes. For several days ECM was insistent that the low west of Iberia would drift nearer the Azores before any interaction with the Atlantic low(s) but GFS was having none of it and UKM slowly drifted toward GFS from ECM. Yet here we are now with agreement on the low drifting nearer the Azores. Hence we now see more of these 'heat dome' scenarios appearing. Finely poised for the UK, GFS was a welcome sight in some regards although still not a full escape and with lingering hot weather through Tue in the south. UKM appears to be headed for a relatively quick but intense punch of heat on Sunday, akin to yesterday's GFS 18z but without quite such a vast area of 20*C+ 850s. ECM is not a pleasant sight for the far south unless you're okay with the health impacts including some loss of lives.

The max temp variable is actually the max of the preceding 6 hours (so probably at 6 pm in this case) - something that could do with being specified on these charts considering how often I’ve seen them misinterpreted. Says something about the extremity of the airmass GFS is modelling that even as it’s on the way out on the Monday on this run, the noon temp is close to the record max here (33*C versus 34.3*C set in early Aug 1990). Or at least what would have been the record had it not been shattered the preceding day (this run has 39*C here in the Sunday… mind-blowing)!

Tamara’s earlier post regarding the potential for pattern re-sets springs to mind. In this way, high summer patterns do tend to be more persistent with internal feedbacks self-sustaining, so it would not be surprising if the GFS 00z proved closer to the mark than the subsequent det. runs of today. Such did occur in 2018, as an exceptionally positive summer NAO pushed ridge after ridge toward Scandinavia even as AAM fell a long way negative during 2nd half July. The -AAM forcing eventually gained the upper hand in the final third of August. Not saying there will be so much lag this time, but recent extended EC runs have showed considerable interest.

After perusing the 12z EPS the main question I’m left with is: Is the deterministic run (ECM) over-complicating things with too many LP systems in the Atlantic, or is it spotting important details? Its the presence of so many systems alongside the west of Iberia low still being taken further west for a time (before lifting north) that leads to the ‘conflicted’ look for next weekend.

For days now, ECM & EPS have been resolutely in favour of the low that’s west of Iberia in 5 days time journeying a bit further west until Friday and then starting to lift north, which is critical to the big plume scenarios for 16th-20th. Meanwhile GFS hasn’t been interested for the past two days and GEFS have been about evenly split on the matter. GEM’s been on and off with it, while UKM has slowly drifted from being more like ECM to being more like GFS. The path of this feature must be resolved before we can take any charts for 16th onward at all seriously. Let’s see if ECM & EPS stick or twist later.

An even flatter ECM 00z rolling out, which is surprising given it was already at the flatter end of yesterday’s EPS 12z spread. This of course keeps the very warm to hot temps south focused but it may also have the effect of further delaying a breakdown because it still has the west of Iberia low staying well south and west compared to GFS and with a flatter pattern there’s less or a ‘reach-down’ by troughs to try and lift it northward. Thats bad news for mainland Western Europe, where heat then persists even longer. This could also affect the UK but as usual we’re peripheral, looking for overlaps. Still, re-amplification can’t be ruled out and as the ECM 12z of yesterday showed emphatically, it won’t take much to set up a high-end plume event.