Singularity

One of those situations where longer range confidence shatters in the mid-range. The culprit is a ‘heat low’ development within the plume, which may or may not be picked up by weak south-westerly flow aloft and steered toward the UK. If it is, high pressure isn’t able to build as strongly or much across the UK and we see an unstable run of days with intermittent warmth, maybe a hot day or two (E.g. 00z ECM). If not, the heat low stays to our south and supports the high instead, making for a much more settled, warm or very warm outcome, in the style of the UKM 00z. GFS takes the first path but less emphatically than ECM or it’s own preceding two runs. GEM is more akin to UKM out to day 7. No clear resolution for now.

For anyone seeking a longer lasting unsettled spell that includes the far south, there’s a chance that 4th week of May could deliver as AAM drops back down after the higher spell of the near future. Its when AAM drops substantially that high pressure is most inclined to locate west of the UK. Having said that, this tends to be least pronounced in May-June, when it can just as well prove to be cool but manly dry, 2010-style. However, the +NAO tendency established in the past fortnight makes that less likely, instead favouring a changeable westerly interlude which tends to be wettest in the north, not really what we could do with. At present, there’s little sign of a big push to flip or at least neutralise the NAO and make the 2010 or 2012 outcomes more likely. A +NAO can self-reinforce when the North Atlantic SSTs are coolest in the northwest, as we have presently. The pattern weakened a bit during April’s -NAO but should restrengthen in the next week or so as cool cyclonic conditions dominate the north-western N Atlantic.

Caution would be advised either way, especially with such an outlandish proclamation as a 60-day heatwave…! In any case, my goal there was to discourage interpretation of posts like Summer Blizzard’s as indications of a frequently cool & unsettled summer to come. I know that’s not necessarily what they putting forward. On a related note, the high pressure building across next weekend could feasibly be sustained for more than a brief period via a +NAO with sufficiently north-shifted jet stream. Such has been the emerging theme in recent weeks. It’s a worrying one for the far south & southeast of the UK, where rainfall has been extremely lacking since mid-March, following large deficits overall last Nov-Feb. We might have a shot at some useful rain Wed-Thu though, as a small low nips through - it’s path has suddenly adjusted further south on the 00z runs, especially the ECM which is slower with it too.

Some important detail to add: The enhanced westerlies of -AAM in summer tend to apply July onward & even then aren’t always directed far south enough to bring the UK much unsettled weather. In fact sometimes the opposite results, as the westerlies lock in high pressure across the UK for long periods at a time. This is the reason why La Niña in general has only weak correlations with UK weather as a whole. The exact characteristics of the La Niña (such as how long it’s been going for & whether it’s skewed toward a particular part of the Pacific) matter, as does interaction with other key forcing, such as the North Atlantic SST pattern & sea ice to the N and NE of the UK. On which basis, I advise against drawing any assumptions for summer 2022 based on the La Niña and low AAM alone.

It makes a real difference, as the climatological zonal wind reduces - the balance genuinely shifts in favour of northerlies or easterlies when compared to winter. So it really is my thought that this one hangs in the balance and we could well see a notably cold few days for the time of year to start April 2022.

I’m having haunting memories of the December fiasco… but it’s spring this time so all bets are off! Fascinating how dogged one model can be against the majority when it’s underlying algorithm is either at fault or to be congratulated.

UKM has no local advantage because our weather depends so much on what happens across the thousands of miles of ocean to our west and then land beyond that, in fact even the Pacific often plays a significant role. Complex low interaction Sun-Mon still to be resolved. GFS seems to have had an 18z from the most easterly end of the spread and a 00z from near the least easterly end, in terms of next Tue-Thu. ECM is close to a middle ground outcome, just a little toward the more easterly end. As for UKM, not clear how it would unfold past day 7. There’s a bit more of a southern jet stream arm evident, increasing odds that the next Atlantic trough stalls and disrupts, but it’s finely balanced.

18z GFS has shown that what goes on in the 4-5 day range is crucial for determining the strength and orientation of the Scandi High - hence its ability to force full trough disruption - next week. Its a complicated interaction of three lows to our west that appears most important. So the situation is even more difficult for the models than usual. Some saga this could be!

Another variation from the GFS 06z. It's been a long time since we last saw a full rendition of the cult classic 'models initially way to keen to blow away a Scandinavian High'. While I dislike cold weather in March, I do find it fascinating to watch the knock-on effects of a first low becoming more disrupted, leading to the blocking high having more of an impact on the next low, raising the odds of that one disrupting too much to push a maritime flow across the UK... and so on. So my eyebrows are raised this morning and I'm increasingly struggling to be convinced by the strong UK trough signal in the modelling for the middle third of the month.

The simultaneous shift by multiple models, following key adjustments in the 4-6 day range (more of an initial HP build northern UK into Scandinavia), give this proposed easterly more credit than many may be inclined to assume at this time. The split seems to have some impact but the question now is whether it will actually be along the lines currently indicated. The easterly could feasibly be dropped tomorrow… or it could become more substantial, for all we know. It’s a wildcard situation. I’m not a fan of cold easterlies in March down here, but would still just about prefer that to another barrage of lows, which is the main alternative outcome we’ve seen modelled in recent days. So no big wins on offer for me!

Rightly or wrongly, GFS seems to have put together a quick tropospheric response to the ‘mini-split’ of the polar vortex, despite that falling short of major SSW status. Rather than a reversal propagating down, it simply has the tropospheric pattern mimic the stratospheric one for a few days, producing a cross-polar ridge. This is not implausible, but will be more sensitive to other pattern forcing than a SSW response in terms of whether much HLB takes shape to begin with. So you could say it’s a ‘fluke’ event that might happen if things behave just-so. Of high scientific interest, whichever way it goes.

We’re seeing the models begin to respond to the MJO crossing Indonesia by about a week from now. That promotes a build of high pressure initially across Western Europe, then northward and eastward from there. A bit of an easterly is a plausible result… or a south-easterly like the ECM 12z has for day ten. Seasonal models have been taking that and running with it as the main theme for the spring season. Perhaps owing to the tropospheric polar vortex imprint being very resilient even as the vortex itself is weakened by wave activity. Whether they’re right to predict that resilience remains to be seen.

Worth noting that the potential sting jet development is unlikely to be resolved by GFS or ECM. Probably not UKM either. The UKV model could do, perhaps ARPEGE too. AROME may be best placed of all so that will make for keen viewing tomorrow. Just potential of course - the cyclone type supports such a feature, but it remains to be seen whether the timing of emergence into the polar side of the jet stream boundary supports it too.

That ECM 12z had an 81 mph wind gust for 3 pm here in the western New Forest, so even that was an extreme solution. A note on ensembles: they exist to explore the question ‘what if the initial best estimate (from assimilating new observations into precious model run data) for the deterministic run is slightly off?’. Meanwhile, the control run tells us whether a change in resolution is causing much difference to the deterministic run. So generally, differences between deterministic runs result from variations in model dynamics. ICON takes it to another level by using a whole different approach to the grids and how they interact with each other… an interesting experiment. Can’t say I’ve been impressed with it overall, but it’s had a few (occasional!) golden moments over the past four years.

00z models have increased confidence in an extreme weather event this Friday - damaging winds in the south or central parts and snow in central or northern UK, depending on the track of the low. A few runs still taking the low south of this main spread with less development, mind. That lower probability scenario has a snow risk in the south but the weaker low development means less of an effective inflow of cold air so it could be sleet or cold rain away from high ground.

The longevity of this unsettled run of weather has been trending a bit longer for southern parts where the rain is much needed away from the far west. Perhaps as the models resolve the interaction of a phase 3 MJO with a strong polar vortex: Subtropical high is weakened but HLB isn’t able to establish so we just see the polar jet shifted a bit further south compared to much of Jan and first third Feb. A lot of support for the MJO to propagate eastward in the weeks ahead. This will lead to increasingly strong support for high pressure to build across Northern Europe generally. This should then try to head north but the polar vortex may well stand in the way enough that we instead find ourselves looking at another very settled month overall. That’s what the latest EC seasonal sees… if it proves accurate, then the current unsettled spell could prove very important for southern UK.

So here we go - increasing signal for the MJO to wake up in phase 3 or 4 and then propagate eastward, If it wakes up in 3, a mid-Atlantic ridge attempting a -NAO (but likely being held short by the vigorous polar vortex) is likely to feature at times in opening 3rd Feb, otherwise a high across NW Europe including southern UK will tend to dominate. Current NWP modelling seems to be caught between two minds and oddly favours the latter followed by the former. This implies that either the northerly attempt around 8th Feb will also prove to be a red herring, or the NW Europe high later next week is a coincidental feature and we'll see a mild-cold-mild sequence characterise the first half of Feb. Beyond then, if the MJO propagates across the Pacific, AAM will climb into positive territory and blocking highs will be increasingly on the cards during 2nd half Feb. Whether those are high-latitude or mid-latitude will again depend on that pesky old polar vortex. The MJO sequence and rising AAM should force an assault on the stratospheric polar vortex but at this time we can't really tell whether that will be reflected away, as have all the prior ones so far this winter, or penetrate the polar vortex circulation and bring about substantial weakening. Of late, GFS has been flipping between the two scenarios. Even if its reflected, seasonal trends will favour higher latitude positions to blocking highs in final third Feb when compared to those seen most of January. Overall, my sense that the spring warmup will be delayed this year continues to build - about which I have mixed feelings, to be honest!

Looks like the MJO is waking up in phase 3 at just the right time to prevent first half Feb being a flat +NAO driven affair. The usual Hendon-Wheeler plots aren’t capturing that well. Admittedly it’s not a strong MJO so I can’t see it overriding the strong polar vortex and forcing a sustained -NAO (phase 3 supports such during a La Niña base state in Feb), but it ought to do enough to keep HP building at least as far north as Iceland from time to time, with troughs dropping down to our east. A NW flow overall but northerly at times. Changeable for all, most rain/snow in the north. Southwest may struggle to reduce rainfall deficits from a very dry Nov-Jan.

Well, that embarrassing lurch by ECM has brought it in line with GFS regarding the pattern across Asia & N Pacific - one which that model has been persistently using as the trigger for a significant wave-2 warming that sets up a split the polar vortex sometime in the middle third of Feb. So that appears to be how it’s going to end up - another late winter HLB opportunity driven by stratospheric developments. Remains to be seen whether the tropics cooperate. Some hints that it might by 2nd half Feb.

You know, it really intrigues me that even with the polar vortex flexing its muscles and tropical forcing heading through a more La Nina-like phase, the models are continuing to put out charts that aren't classic +NAO, zonal flow situations, even in the 12-16 day range when that sort of thing is usually expressed very readily. Of the 00z runs, the ECM was the closest and even that had the polar vortex imprint focusing to our NE as of D10. The 06z GFS has reverted to a fully HP-dominated outlook, with no breakdown from D12, and in the late stages has charts that wouldn't look out of place in April. The very compact nature of the cold trough around Greenland seems out of place for early February. The cut-off low less so, but still not a common sight without a high-latitude blocking pattern at this time of year. Seems to be that the interaction between an east-focused La Nina, expanded Hadley cell, strongly negative PDO, exceptionally warm (overall) N. Atlantic, and unusually strong polar vortex for an eQBO winter, is pushing the N. Hemisphere weather patterns in ways rarely, if ever, seen before. Fascinating from a broad perspective, whatever you think of the UK's resulting weather.

Depending on how you feel about late season cold weather, a possible silver lining to the emergence of a strongly +AO pattern is that it provides a good pathway for vertical wave activity flux to reach the mid-upper stratosphere and put the polar vortex under greater pressure than it has been so far this winter. Of course this depends on high pressure being in the right place i.e. across the Urals. GFS is keen on that but support from other modelling isn’t strong at this time.

For the southern half to two-thirds of the UK, the outlook until further notice is colder than average - even against the 1981-2010 baseline - but with the nights doing most of the work except when & where fog lingers well into the day, perhaps more widely during one or two brief polar incursions. Its just not an exciting colder than average. It is however delivering some exceptionally springlike weather in places for January with crisp mornings and warming sunshine by noon, which for those at the receiving end who appreciate such conditions, is most welcome after an extremely dull December.

On the theme of reversions - the ECM 12z has bounced back quite a way compared to the previous 12z. Puts it close to GFS, but with the low south of Greenland having a slightly positive rather than negative tilt to its eastern flank. Appears to be headed for further strong HP centred right over or slightly SE of the UK.

GEM 12z is a partial reversion back toward a less flat outcome in the 7-10 day range. I've had a look to see what might explain the sudden switch to flatter outcomes from this model and ECM. The tropics offer little guidance, with not much organisation to deep convection to be found and frictional torques (FTs) having already hit a 'floor' at about -2 sigma, having been heavily countered lately by positive mountain torque events. The net result being the GWO analysis falling gradually through phase 0, within which it has relatively little steering power with respect to the broad-scale weather patterns. That suggest that the flattening of the pattern is stratosphere-driven. The GFS 12z instead keeps the focus of the lower polar vortex divided between Canada & Scandinavia. There's lately been a trend to put back any cessation of that setup, but of course, it can only go on so long. Still, given that the GFS 06z very much saw an end to it, it's interesting that the 12z has reverted (with GEM also doing to to a lesser extent). Looking further ahead, with FTs having floored, the groundworks may be laid for a new AAM rise by the closing days of this month. However, the models are all over the place when it comes to what role, if any, the MJO plays in that. Then there's the matter of whether the polar vortex will give HLB formations a hard time regardless. GFS is hinting that it might not, but GEFS and EPS only show partial interest, with a wide spread by a fortnight from now. The view becomes about as foggy as the recent weather in parts of England!

We certainly look to be under more of an East Pacific focused La Niña influence this month, with the +NAO regime heavily distorted. If it wasn’t for the unusual polar vortex intensity - which may have some tie in to climate change via increased blocking of troposphere-stratosphere radiative energy exchange - I expect we’d be experiencing a -NAO by now, fitting the EP Nina historical composite for Jan-Feb. You see, while the AO analysis shows a ‘disconnect’, that’s mainly with respect to the Arctic region. There’s still a considerable imprint onto the troposphere, divided between Canada and Scandinavia. If the possible wave-1 warming materialises and weakens the polar vortex while the tropics continue to drive big mid-Atlantic ridges, a negative NAO will become more feasible. The MJO casts a pall or uncertainty over this though - we can’t be sure whether it will align more with a La Niña base state by late Jan, or begin another trip from Indonesia across the Pacific. Both outcomes are on the table at the moment, though modelling seems keener on the latter - which would support a continued EP Nina influence.