Singularity

A high centred in line with central Greenland is not west-based. Say the 12z GFS verified 100% (imagine that!), it would be a very frustrating run of weather for me in the far south... but the key details are all 6+ days from now so I frankly couldn't give a flying fish at this stage. For all we know, the airmass over Scandinavia could correct a degree or two colder or less cold in the days ahead, which would significantly affect how much leeway we have with the path of the trough and interacting lows. I will become a bit concerned if the 12z ECM is anything like the 00z, though. I've seen enough very wet and windy weather for this year!

However... unlike when we saw such outcomes cutting off most of the cold drop to begin with, we now look likely to have quite a bit of cold air to the east. Hence we've started seeing a few runs bringing a new influx of cold continental air from over that way the following week.

GFS 12z is similar to today's UKM runs. Cold air is spread out a bit further west which is not ideal, but no escaping how low the 0°C isotherm and 500-1000 thickness becomes across the UK by late Thu. Being an Arctic airmass, it's freezing cold until very near the surface where a shallow modified layer (due to North Sea crossing) is the only real resistance to low elevation snowfall. On this particular run I think that would be overcome by anything more than slight precipitation. For me, it's notable that this is still a long way from the 00z ECM in terms of what goes on to our west. Also, the strong low near the Azores has been adjusted slightly further west again as of +168, reducing the likelihood of it visiting Europe as an intense feature.

To be honest, when I see an 850s mean like this ahead of an approaching south-tracking low, I think 'will the fronts even reach the UK from the south?'. Funnily enough what the ICON 12z does with the first runner low that most models haven't been developing much (GEM 00z a notable exception) is more or less what I could see the following one doing based on past experience, unless there's one or more breakaway lows to our west Thu/Fri, which is notably not so with the ICON 12z. (00z ECM on left, 12z ICON on right) Note how much further south the green shading reaches in the N. Atlantic on the ICON run due to the lack of small lows west of the UK. This makes it more likely that the next low from the Azores will run into the base of the cold trough by or over the UK, rather than replace it entirely. A key detail to track across the 12z runs as they roll out.

On reflection, it's hit me that the way the 00z ECM went was not just just about how strong the low by the Azores peaked, but the little breakaway low that I've labelled 'L' on this chart. It peels away from the Euro low and provides a 'positive vorticity path' (not a technical term) toward the UK. This allows the intense low to both sustain for longer and take a track further north. This little low isn't there on the UKM 00z or GFS 00z and 06z runs, so it's either a key feature being overlooked by them so far, or a spurious feature steering ECM the wrong way. The 00z EPS control run was similar to the det. although the little low didn't venture quite so far west. I don't have time to check all the individual EPS members but of the first 14, 5 of them have that low or at least a notable extension of the Euro low in that direction. So it's a substantial contributor to the overall EPS direction of travel... whether rightly or wrongly, we can only wait and see.

As little as individual runs really matter at the moment, it's interesting to see the trough extension across Scandinavia from Siberia maintained a bit longer on the GFS 06z. This implies that the changes to the orientation of the high that we saw for the 00z run are prone to reverting at least a little bit. There's a connection to the (sub)tropical cyclone being modelled near the Azores; the 06z forces it to peak further west before extra-tropical transition & disruption of its circulation by the cold air to its north. Hence it's a fair bit further south as it heads into mainland Europe compared to the 00z.

What makes the ECM 00z suspect later on isn’t that it develops a deep low, that’s reasonable if such low heights make it so far southwest. It’s that the low becomes extremely intense, putting it right in the uppermost reaches of what’s physically feasible… that makes it an inherently unlikely outcome. There are plenty of ensemble runs with deep lows heading east into mainland Europe with a flat northern flank. As usual it’s a game of chance for the south while in the north it’s more a question of how much will fall from the sky to begin with. Still, I do wonder just how much more the blocking pattern might continue to be adjusted, altering how close high pressure is to our north.

This HLB spell is starting to look quite peculiar, perhaps rarer still than a ‘typical’ strength Greenland High (but not a very strong one such as we might have seen). I would put ECM days 8-10 in the ‘suspect’ tray as it pretty much develops a tropical cyclone over the Azores which has the effect of steering the jet northward over Europe. It’s not impossible but…! Generally, as others have noted, we’re now seeing a consensus for a battleground setup across the UK generally. There’s been enough change to the broad-scale in the past 48 hours to leave me wondering how trustworthy that is past day 5, though!

The AAM drop will initiate a poleward propagation of westerly momentum anomalies and they usually overpower blocking highs... but when one of those effectively extends well up into the stratosphere (in terms of positive geopotential height anomalies), that momentum can be diverted southward, resulting in a strong, flat jet stream that's well south of the UK. That's what we're seeing in the modelling for later next week. The thing is, this also encourages retrograde motion of longwave features to the north, such as blocking highs. So, we're left looking at whether that happens in one smooth transition, or 2+ steps that keep the UK in a cold weather regime for longer. I expect the latter becomes more likely the stronger the stratospheric support for the initial blocking feature is. This month, we also have the added possibility of a cold surge out of East Asia across the Pacific initiating new MJO activity, or something similar to it, over the tropical West Pacific sometime in the 2nd half. So, there exists a possibility that the HLB pattern gets reinforced, in which case it would become a very big deal how much the pattern has retrograded. At least some retrograde is pretty much inevitable, so I daresay the question would actually be whether there's been enough retrogression to place a blocking high close to our northeast (or even due north of us) by then. Got way ahead of myself there, best concentrate on next week for a while yet!

In the midst of everything I hadn't noticed the +168 UKM with its sneaky little low near Iceland. As you can see below, EPS are mostly less cyclonic than the ECM 12z for next Tue-Thu, which suggests little appetite for UKM's scenario, as that would be another path to a cyclonic situation in the vicinity of the UK. Even so, I've seen enough of those sneaky lows to the northwest cause trouble in the path to know that it can't be dismissed. It should become clear by tomorrow's 12z runs whether it's a rouge feature or not.

ECM 12z wins the ‘most interesting to experience’ run of the day award in my book. The UK has been through a fair few more brisk bitter easterlies than slack cold cyclonic setups in the past 2-3 decades. The surface cold establishment would be quite something, especially above lying snow. Brutal for heating bills but I can’t deny that the excitement would offset that pain a bit.

Well then - I can’t say I was expecting ECM to produce the coldest det run chart we’ve see. so far for next Wed but there it is… goes to show, we really can’t take individual runs very seriously at the moment. Too many moving parts in a highly abnormal pattern.

Took me several minutes to identify what sent GEM down its own path (it's not like the ECM 00z - markedly different across N. Asia). Look at the low south of Iceland on the 12z GEM run (left) at +120 hours. It's considerably further west than on the 12z ICON (middle) and UKM (right) runs. This causes the high between Iceland and Greenland to become fully cut-off, changing its orientation toward west-east rather than southwest-northeast. The GFS 12z has also moved the low further west compared to its previous run, but not so quickly, which means cold air reaches the UK before the high re-orientates. Then it has the re-amplification which tweaks things again - we'd otherwise have been looking east as of day ten, like with the 12z GEM. Still, that low heading so far west in a hurry is a feasible way in which things could turn out chilly rather than cold for the UK later next week. Can't say I'd back GEM over UKM at this lead time, but stranger things have happened and that low has been giving the models a hard time.

GFS finding another wave of amplification on Friday, with a bigger trough drop lining up to our NE then E. More residual over Hudson Bay is one way to prevent a marginal cyclonic setup akin to the ECM 00z but of course raises the odds of the ridge never getting so far north to begin with. Cut-off highs at the Iceland latitude have supported some notably cold UK weather in times past, mind. Usually of the drier sort away from the far south, though.

I don't usually give ICON the time of day but it is interesting to see where it sits relative to the 00z ECM and GEM runs as of +180 hours. It's a little more like ECM on the Pacific side, yet far more like GEM on the Asian side. Judging by the ECM 12z of yesterday, the Pacific side seems of relatively low importance, as it was more like today's 00z ECM than 00z GEM, yet the run panned out like the latter. On this basis, it seems to me that what sent the 00z ECM on such a different path was the additional amplitude to the ridge through Greenland. The high was thrust across so vigorously that it forced a trough partition on the Asian side. What if it was just an example of ECM's tendency to over-amplify at the 6+ day lead times?

It's an absolute classic: GFS continuing to trend toward where ECM was at until the most recent run. Question is, whether that means ECM was in the right place yesterday, or has been right to continue trending in that direction overnight. Not one even the brightest of minds can reasonably answer with confidence. My instinct suggests ECM & EPS 00z was an over-adjustment situation but I'm far from certain of that.

As far as the drop of low pressure down the east flank of the Greenland high goes, ECM is at the extreme end of the spectrum while GFS is at the subdued end (yet still gets there eventually). Logically, that makes the most probable outcome somewhere between the two, nearer GEM or UKM for example. The one significant caveat is: As long as there isn’t a trend toward a more extreme outcome. That’s what needs monitoring today.

These are the 12z set for the SLP means, admittedly the 2nd cluster is borderline come day 15 with the mean low position and orientation only just maintaining a continental feed across the UK. Probably contains some runs that turn conditions milder while others stay cold for at least a little longer with a snow event. As you say though, way too far ahead for this level of analysis anyway. On the other hand, it's reasonable to anticipate that the blocking high will retrograde over time, given the falling AAM prior to the possible resurgence led by a cold surge out of east Asia (which then kickstarts new enhanced rising motion over the West Pacific, as a means to a fast MJO reboot - AM kindly elaborated on this on Twitter this afternoon & it seems logical enough to me).

Only the third cluster on the 11-15 day range would bring southwesterlies to the UK and even then not until late on. Remember, those aren’t sea level pressure isobars, they’re for well up above our heads. We know have the consistent ECM & EPS support that I required to call the arrival of notably cold air by 10th Dec (at very latest) as being more likely than not.

Regarding 850s, worth bearing in mind that air sourced from the high Arctic gives us a bit more leeway versus North Atlantic, due to lower dew points. Warm seas will counteract somewhat but on balance I can see around -5°C 850s being a reasonable benchmark for the setup. Maybe 1-2°C higher if & where winds fall light, away from coasts.

Story of a GFS cave-in to ECM, it would seem. More & more of a cold trough drop down the eastern flank of the high pressure over Greenland. UKM 12z is even closer to the 00z ECM than its prior run was. Also continues to keep the low to our southwest from edging northward, which in my experience has been the more common outcome in HLB setups of the past decade. GEM is fully on board with the 00z ECM as of +144 hours & stays that way going forward. In my view, we're a consistent ECM 12z & sizable EPS support away from calling the transport of cold air across the NE 'more likely than not', despite the arrival time being at least a week away. That's a very rare position to be in.

Alright, so not a complete cave-in, rather a significant step toward the other modelling, via being a little slower (only by about 12 hours!) to sever the Pacific side connection to the high pressure over the Arctic. Yet even that sends enough additional cold air toward the UK that the polar boundary is forced to locate further south, with opportunities to source cold from the east down the line. So, a significant development in my opinion, though as I said, we need to see how the next couple of GFS runs behave first.

Well, this might be the moment that GFS caved in, depending on how the 12z and 18z runs behave.

Come to think of it, a chart exists that demonstrates what I'm talking about. As I suspected, mean polar boundary likely somewhere over southern UK (-5°C 850 hPa isotherm is usually a sufficient loose guide). In fact the mean low pressure axis is arguably through the Channel. Overall, support from the ECM det. pathway has increased this morning, though it's still not enough to make a confident call in my opinion. N. Hem views at +144 in order of support level from least to most. Notably, UKM & GEM are very close and GEM goes on to be almost on par with ECM, which suggests that despite not being nearly as advanced with proceedings as ECM at this point, UKM would follow a similar path during the following days. Typical that the model would chose today to slack off putting out a 00z +168 chart! Rouge UKM chart removed - thanks to @Blessed Weather

The 500 hPa anomalies are skewed negative by colder airmasses. With the coldest air being on the north side, that means the surface lows will be further south of the upper trough centres than usual (in any case, surface lows typically travel within the southern flank of troughs and then swing into the centre once matured). I'd estimate the polar boundary to be somewhere over the southern half of the UK on that mean.