Singularity

I suppose when it comes down to it, you can’t ask for much more than prolonged Pacific MJO activity interfering with a La Niña base state as a means to initiating a HLB spell while also driving wave-2 activity to assault the polar vortex as slowed zonal winds allow the Pacific and Atlantic-Europe sector ridges to extend further N or NE. Whether that wave-2 can force a SSW remains a big unknown looming over forecasts for late Dec through Jan. In the meantime, some cold(er) weather from the east is feasible in 1st half Dec - SSW precursor patterns tend to be at least intermittently so for the UK - but it’s up in the air how marked and hence conducive to ice or snowfall that proves to be.

A combination of an MJO-driven positive East Asian mountain torque event & the polar vortex drifting away from Canada/Greenland for a while means a legit chance for high pressure to build up through the UK or hereabouts and attempt to establish a high latitude block, albeit not of the full-on Arctic variety (AO not looking to drop negative enough). However, the modelling seems very uncertain on the timing of this and hence what it has to interact with to our east. Earlier timing makes it likely to have a Scandi-west Russia high to link with, giving us a probable easterly but who knows how cold (fine detail important there). Later timing makes it more likely that the high to our NE/E has moved on with a trough in its place. Then it becomes a case of whether the high can reach as far as Iceland/Greenland as to how cold and inclined to snow the UK weather becomes. These two pathways are tied to MJO forcing, phase 7 Scandi high promotion followed by phase 8 Atlantic high with chance - but no guarantee - of a negative NAO spell. Should the earlier advance north of high pressure occur, it’s certainly plausible that we see one scenario then the other: High links to Scandi-Russia high then the merged feature retrogrades toward Iceland/Greenland. This is the rarest outcome, mind.

Something’s afoot in the modelling - the MJO is now being forecast to get a move on, then decay over the Western Hemisphere. That kicks the atmosphere toward a more Nina-like state, though AAM might not drop below neutral. To get to the corresponding mid-Atlantic ridge, Scandi trough setup requires a very large pattern shift in our sector. So, no surprise to see some dramatic changes in modelling at the 8-14 day lead times. Alone, this pattern is conducive to near to a bit below normal temps. For something colder there will need to be weakening of the zonal flow. Signs are the polar vortex could become sufficiently disrupted for that by the third week of the month so it’s worth keeping an eye out for any increase in ensemble support for a -NAO pattern in the final third of this month. Based on the extended ECM ensemble, the MJO might also rejuvenate over the West Pacific which would lend further support.

An unusually active MJO for a La Niña event has delivered us an exceptionally warm October and now shows signs of bringing further well above normal temps next week. This boosted by the warmth of the Mediterranean and nearby North Atlantic. However, this occurs via amplification and shifting northwest of high pressure over Europe. This might be just the beginning of a longer term process that brings drier, then cooler conditions before eventually culminating in some manner of negative NAO pattern by the final week of Nov. Whether that’s of the sort that brings cold weather to our part of the world remains to be seen. I’d say we’ll get a ticket to the raffle, that’s all at this stage.

Development of secondary lows on the eastern flank of the main low this weekend has been toned down a lot since just two days ago. Especially by ECM which was producing some severe gale scenarios for parts of Ireland… a shocking performance there. This leaves us with weakening fronts struggling to advance east, lacking much to reinvigorate them or push them onward. Hence the western quarter to third of the UK looks to see by far the most rain.

Having seen low 30s over a week into September just last year, I’d not be so sure of that. High 20s is the safe bet but the potential remains for low 30s more than just locally. Later temp climbs and earlier falls will make whatever heat we see more manageable regardless.

Hmm. Just watch ECM flip now but if it doesn’t we’ll have agreement between it and GFS & UKM on the Monday low exiting swiftly NE having disrupted just enough to drop a shallow low down over western mainland Europe, that then working to support a bridge of high pressure across to its north by Thu - broadly similar to the setup last Tue-Sat.

A note of caution on the UKV model - as far as I'm aware it uses the UKM as a basis, which had the most unstable outcome for Mon-Tue out of it, GFS, and ECM. Those last two have produced less unstable outcomes compared to their preceding 00z and 12z runs, with a bit of a lottery for the south for between 0 & 10 mm rainfall during Mon-Wed. UKM has been persistent with its more unstable outcome, though, so who knows, it might be onto something. Would certainly be interesting, albeit dangerous in places owing to intense rainfall hitting hard-baked terrain. I'd not want to be living in reach of a typically fast-responding waterway! Nice of ECM to demonstrate my point regarding feedbacks and climate trend potentially placing the low even further north for next weekend. Normally I'd welcome such an outcome... but these are not normal times.

Contemplating analogous years for possible weak MJO activity in the western Indian Ocean with AAM falling but probably not very far, I’m of the broad impression that the ECM 00z was a decent effort for later next week while the GFS 00z & 06z runs positioned the big low some 1,000 miles too far south. The 12z GFS was nearer the mark but still went awry soon afterward… classic GFS perhaps? I’d not rule out feedbacks & climate trend placing the low a little further north than even ECM has it, mind. p.s. GFS has lately been exhibiting a positive max temp bias of 1-2*C for most areas that have a long overland fetch of wind - currently much of the West Country & CS England, for example. It’s also tended to reduce that bias to within 1*C at the last minute (12z of the day in question).

For the far south of the UK the ECM 12z offers some heavy showers and thunderstorms moving north-eastward on Monday with a suggestion of organisation to them. After that it’s mainly dry and warm for the region, so not a great outcome for tackling the drought. GFS managed to bring a small disturbance across the far south Tue-Wed with some longer spells of rain, although they mainly miss the very far south and southeast. Thankfully this is all far enough ahead in time that the details are likely to change, hopefully for the better!

Interesting that we still have the low dropping well to the southwest - that change seen in the 00z runs has persisted - but a stronger jet stream has provided a counter on the 12z model runs, most of all UKM, least ECM. It drives cooler air aloft which allows the heat low to develop more as its own entity during Mon-Tue rather than becoming a mere ‘lobe’ to the south-western low. So, now we wait to see if that change is held onto.

The key is in the upper pattern, if you have a surface low but no upper low then the air aloft tends to be relatively warm and dry, meaning cloud bases are high and even where they manage to produce rainfall, much or all evaporates on the way down. This has got me wondering - are the model charts we usually see predictions for rainfall that reaches the surface, or that falls from the clouds?

With the hot air sticking around instead of cooler air overrunning, we’re largely reliant on surface heating plus sufficient moisture concentration, which will produce much more sporadic results. Each day, some should do well while others miss out. Convergence caused by sea breezes could prove important . That is, if this new direction of travel prevails. The critical point is at the 5-6 day lead time so we should gain a good idea if it will during the next 3 full sets of runs.

Sheesh, this is quite the time to be having one of those classic westward adjustments to a diving low that we often see in the winter or spring months. Not often you see 3+ days being added to an already week-long heatwave. More runs needed indeed to see if it’s a trend or mere fluctuation, but seeing 3 of the main 4 move in the same direction and with ECM having been toying with the idea for a few days now… it’s ominous.

Dry air being lifted as pressure falls chokes off a lot of the shower activity on the GFS 06z which then brings high pressure back in swiftly, leading to another lengthy run of very warm to hot and mainly dry weather for southern UK. A nightmare scenario from some perspectives, hopefully got a bit carried away with the high pressure dominance (did I really just write that about a model prediction in August?!). I suspect the reality will prove wetter in at least some parts of the south. Ideally we need a proper frontal boundary to take shape without moving on quickly. Yesterdays 18z GFS did that to the extreme. 06z GFS just lacked a bit of shove from the west to get the convergence line going.

A familiar breakdown tale of this summer from the GFS 00z, which I can’t help thinking gives it some credence. The Atlantic low passes to our north and quickly lifts out the heat low, keeping rainfall low for much of southern UK. ECM and UKM offer more hope with the Atlantic low dropping down to merge with the heat low instead, although ECM complicates matters by leaving part of the Atlantic low behind to the northwest. What follows is very wacky indeed in terms of the high latitude developments so best viewed with some skepticism at this lead time. I still see grounds for high pressure to return to southern England in some way or other before long and then be a frequent visitor going forward, so IMO it’s important to get as much out of next week’s breakdown as possible.

Important to be specific when talking of a breakdown. What is of? In this case a lengthy heatwave, by sometime Sun-Tue. The troubling aspect for southern UK water reserves is that modelling generally indicates a traditional wettest north, driest south pattern to follow, as the Azores High remains close enough to keep extending across mainland Europe frequently while southern UK gets involved at times too. By contrast the north may see quite a lot of rain, increasing the sense that we could do with a monumental emergency water transfer pipeline running from Scotland to England. If only!

For CS & SW England I can see this heatwave being a bigger test overall than the one of 17th-19th July. That event was relatively short but sharp, with many parts short of 30*C on 17th before hitting mid-high 30s 18th-19th. This upcoming heatwave looks likely to bring at least 4, potentially up to 7 days into the 30s with 3-4 reaching above 32*C. Despite shorter days and cooler nights at until at least Thu, the greater longevity should lead to structural heat buildup on par with that of the July event but for an extra 1-3 days. I for one am not looking forward to taking that on while working from home without air con.

GFS 18 even find a rebuild of low to mid-30s heat 17th-20th Aug, something the ECM 12z wasn’t far from setting up. I think the problem for those seeking an emphatic breakdown by early the week after next is that the tropics keep acting up relative to what would usually be expected for a La Niña. Forecast models try to silence the East Pacific and wake up the Atlantic but this keeps being delayed. As a result there is repeatedly only weak forcing to try and force cool air into western Europe via mid-Atlantic highs & break down the well established heat dome. Its going to take something stronger to do the job. I keep thinking it must surely come along sooner or later with La Niña expected to strengthen substantially… but as we go into autumn seasonal wavelength changes increasingly favour a closer position of high pressure in response to La Niña forcing with temps near or above average. It does at least tend to be more rainy in Sep via brisk westerlies, before a drier trend emerges during Oct (last year a notable exception though, took until Nov).

It’s an unusually hot easterly setup next Wed-Sun/Mon due to hot air arriving from the southeast & then descending into the surface flow which looks to be somewhere between north-easterly and east-south-easterly depending on whether we follow a UKM 12z / All GFS runs of today so far route or an ECM 00z route. As a result we could see a rare case of highest temps being in W/SW and being in the low to mid-30s. Given the west has lower heatwave thresholds than the east, this is a truly extreme prospect and could prove to be the most intense heatwave across a 4-5 day period that the likes of the West Country have ever recorded.

I must say the 12z GFS & ECM runs are extraordinary in terms of the pattern that takes shape late next week, a sort of slack blocking regime but devoid of any well defined highs or lows. Plausibility feels rather low though! Clearly something’s shaken them up a bit. The tropical Atlantic is trying to wake up as it often does at this time of year but the Eastern Pacific is being stubborn with its own activity and the two sides rarely coexist in active states very well. All this is overlaying onto the La Nina forcing, resulting in a scrambled mess of an overall signal. I wonder if even whole ensemble suites can be trusted at this time. The deterministic GFS & ECM followed day 6+ pathways barely represented in the previous ensembles. Is it safe to apply the usual logic and consider them unlikely because of that? Chaos reigns!

The GFS 06z and 12z runs are very similar in terms of the jet stream, best consistency in several days and much more like the UKM 00z. No surprise, then, that the UKM 12z is more like the GFS 00z . Huge difference in midweek N. Atlantic jet stream strength between the GFS and UKM 12z runs. The level of sensitivity to how one Atlantic trough behaves is about as high as it gets. Ensemble clusters suggest that whichever way things go by next weekend, will have a strong bearing on what pattern dominates through the middle third of August.

Broadly I believe it could be summed up as: The Pacific wind patterns have mostly been a bit eastward of the typical La Niña position, which has allowed for more high frequency interference (E.g. MJO events) lifting AAM while also shifting the typical pattern response a bit eastward during the low AAM spells. Its actually pretty rare for La Niña to fully lock AAM into low orbits in summer, which is why correlation with UK weather is weak overall. La Niña summers have featured some of the hottest, driest months on record, along with some of the coolest & wettest.

Having been well negative for over a week now, sustaining the ‘unsettled-ish’ weather regime, AAM is expected to climb toward neutral in the next 6-10 days. It’ll likely hit a ceiling before reaching positive territory but even so, the climb should be large enough to force the typical pattern response in August, which is high pressure across Northern Europe generally. The envelope is wide enough to encompass more UK or Scandinavia-focused versions of that. Models are exploring this rarely well, though we’ve seen some likely over-progressive scenarios (ECM 12z of yesterday and GFS 06z of today). The GFS 00z also fits as a low anomaly over Iberia / France features in the composites, so we can’t rule out a more distinct feature there shoving some very hot air across southern UK especially. I for one hope no such thing comes to pass considering the blow it would deal to wildlife, though I’m not sure dry easterly winds will be much better - it maximises the wildfire risk. Historical composites suggest this should persist to some extent even once AAM starts falling again around mid-August. Only once AAM is back to the very negative territory is high pressure likely to focus more west of the UK, giving us the best shot in ages at widespread unsettled weather, though there is a risk of the west being too close to the high to get much from it.

ECM 00z has significant differences to the other deterministic model runs to the southwest of the UK as soon as day 5, with a weak disturbance added into the mix. At this stage it seems likely to be spurious but a couple more sets of runs are needed to confirm if that is so. As of D10 it still has a blocking high that would slowly drift east and perhaps a bit south due to a continued positive AO and neutral to positive NAO setup, but it would be a gradual recovery from a relatively cool and cloudy - yet still mainly dry in the south after tomorrow and far south throughout - route to that point.