Singularity

Seems a blocking high should take shape near or over the Urals starting 7-8 days from now. That ramps up wave-1 flux into the stratosphere, which for the first time this winter, GFS is predicting to head poleward and into the core of the polar vortex. Something to keep an eye on. In the meantime, exceptionally settled UK outlook continues. Interesting to see a merger of the Canada & Scandinavia troughs keep being put back or dropped completely by the individual runs (ensemble means don’t show it well due to differences in timing of lows travelling between the two troughs). Yet sufficient partitions for the UK high to gain enough latitude for more than glancing incursions of polar/Arctic air are tending to stay in the 10-16 day range. Hence the pattern looks so locked-in.

A very good point in the OT, which I've expanded on with respect to the UK & Europe next week.

Even if the northerly plunges miss to the east, Jan 2022 could become an exceptional month... just not an exciting one unless an impressive run of high pressure gets your pulse racing. This befits the Pacific v Indian Ocean standing wave standoffs (I still see it as that; changes in amplitude have been too slow to fit typhoon interference in my view). Then again, it did so in Dec and in reality the high moved on much sooner than the models predicted in the week+ range. That might have been due to the MJO actually being more in phase 8 than 7, though the jury’s out on it. In Dec phase 8 promotes a -NAO that soon turns west-based, a bit like we saw in the last week of the month. Now, the MJO-like signal has been around so long that I don’t think MJO composites help is much - but it does make sense for it to force an exceptionally stagnant Western Hemisphere pattern in terms of the trough and ridge positions.

UK details aside, interesting that GFS has again created a new, quite large cut-off low near the Azores in 6-7 days time. This allows the high to keep trying to build north even without the most helpful of polar patterns in force. We even see a bit of an omega blocking formation with a shot at a cold north-easterly via the polar jet cutting back on the SE flank of the high. A rare way to see a bit of continental cold without either of a -NAO or Scandinavian High. The 00z runs had no such development (GFS and ECM shown as examples). The UKM 12z wasn't interested, but that model is often one step behind on new developments (for some reason!). All eyes on the 12z ECM to see if it adds it. I'm not saying it will, mind.

I'm not gonna lie, that was surprising from the 06z GFS. Significant changes are present by +120 hours (16th), not just with the storm over the eastern USA, but the heights over Greenland. There's a cross-polar bridge, which stems from slightly more separation of the low height areas across Canada & NW Asia on 15th. Due to being relatively short range, it was bound to affect GEFS a fair bit. This also makes it hard to judge the merits of the change - whether it's a genuine correction, or a spurious adjustment caused by changes in the supplied observation data amount. In this situation we can only wait and see if the 12z retains the adjustment - especially the one for Greenland. More often than not, the 06z proves to be a flight of fancy, but you never know.

Among other things the resolving of how the USA/Atlantic interaction unfolds this weekend into next week will provide an excellent case study for how important even a small cut-off low’s position and timing can be for the broader scale evolution. It seems that the further south it is by Friday, the more the Atlantic ridge becomes oriented SE to NW prior to toppling, leading to a further west position early next week as a low drops to our east. However, as usual, it’s not the only major moving part. The timing of the USA low remains to be resolved too and it’s interaction (however much) with the cut-off low is also important. Being such a vigorous system, it has a larger than usual impact on what goes on across the pond. Then there’s the the stretching, focus-shifting polar vortex to factor in. No wonder the models have some wide spread going on!

Considering the trend to re-amplify the Pacific standing wave during week 3 of Jan, I have a feeling we’re going to be teased with northerly chances for quite a while. Maybe even most of the rest of January. Who knows, one mid-Atlantic ridge build might find a space to get its foot in the Arctic door and produce more than a brief glancing blast of cold air for the UK - but which one and how soon that is is anyone’s guess. For now, best just to sit back and keep one eye on the ensemble clusters rather than pay much attention to individual runs.

Well noticed. While the tropical forcing looks more important, it does make for a notable departure from seasonal modelling. If only a negative AO had a strong correlation with cold UK weather - forecasting would be a bit easier on the mind!

MJO composites are misleading really - what it really does is puts us ‘in the game’ when otherwise we wouldn’t be. It doesn’t mean we win! For that we need a conducive polar pattern for the ‘push’ from the MJO to head to the Arctic latitudes rather than stall in the mid-latitudes... and that has to be when the MJO is traversing the Pacific. So much has to fall in place for more than a fleeting HLB result. The MJO composites are even more of a red herring when the observed state is very slow moving or quasi-stationary such as we’ve seen for a month now. The composites are dominated by past cases of steady eastward movement, which has different implications - more of a sharp kick to the pattern than a prolonged nudge.

ECM 00z is meteorologically very interesting later on with a stagnant run of days giving rise to a bit of cold air pooling over the UK. I’m not convinced by how slowly it moves low pressure across Greenland though. Zonal flow across the Atlantic sector looks to become low - but that low without stratospheric forcing...? Still sensing that we could end up with a colder than average yet low snowfall month overall.

As I wrote earlier, the Pacific v Indian Ocean forcing balance is key. GFS/GEFS are favouring a weakening or total cessation of the Pacific forcing, so the Indian Ocean forcing reigns supreme with inflated subtropical highs. ECM & EPS are leaning the other way in the 8-14 day period but have some weakening of the Pacific forcing before then. So, the subtropical high is bolstered next week, but then there’s a tendency to try and move it fully west or northwest of the UK. For those in England & Wales who like it colder and brighter, it’s fortunate that the Sun-Mon lows disrupt just about enough to set up a varied, light surface flow Tue-Wed with some breaks in the cloud, facilitating inversion development with later-week Atlantic air riding over the top. Honestly, there’s no really confident way to call what happens in the 3rd week of the month. Using persistence from recent weeks promotes the weaker Pacific forcing outcome but the alternative can’t be ruled out. Maybe 1 in 3 chance at this stage. Even if it happens, it’s probably not going to be a long cold spell, unless the MJO-like signal becomes surprisingly strong by late Jan.

The MJO location is traditionally defined as being between two opposing anomalies, so currently a little west of Hawaii. Phase 7/8 borderline, almost as you interpreted. Except due to the extreme duration of the signal, I’m not convinced it’s an MJO event anymore. Hence my standing wave analysis - which is a lot like an MJO feature but stationary for weeks on end. It has been eroding the La Niña from the west and should continue to do so until it fades. Unclear just how long away that is - there’s been a slight trend to delay it.

A tale of two standing waves is my current take. Theres one over the Indian Ocean which is trying to force a classical La Niña Jan-Feb pattern with +NAO etc... but then another over the Pacific, initiated by the MJO some time ago, trying to force the opposite. The balance between the two is a modelling conundrum - and one the MJO RMM plots won’t capture well. Seems to be that lately the Pacific wave hasn’t been as strong as the plots suggest. Recently though, zonal wind analysis indicates some strengthening of that wave. The balance versus the Indian Ocean wave may be nearer equal now. From that comes a shot at some mid-Atlantic northward ridge building. Key questions are, will that time well with the lower polar vortex shifting toward Siberia, and how complete will that shift be? Various possibilities for both are being explored by the models with a wide variety of regional outcomes for the UK/NW Europe. It appears that cool or chilly weather is more likely than mild, but cold and snowy is a long shot. If skies are clear enough, cold nights could lead to a below average month overall for temperature... yet it could be lacking in much interest for snow seekers!

Longer-term shenanigans aside (albeit nice to see my ‘what if the tropospheric imprint splits? for a time?’ musing explored), a slightly interesting shorter-term development caught my eye: The Sunday low slides a bit more cleanly on the 12z GFS compared to prior runs, which leads to more of a high to our east-northeast, which sets up a chillier feed from the near continent into southern UK next week. Goes to show, fine details can sometimes turn an outlook colder... let’s see if it holds.

I don't see the tropical forcing and AAM trends being the main problem for the next few weeks. The anomalous westerly winds around 120E to the dateline (180) are making a bit of a comeback at the moment, so frictional torques appear likely to turn positive again for a little while. Those should lead another round of positive mountain torques, probably the 'last hurrah' before the MJO takes a nap and leaves us looking to the east-focused La Nina for any chance of the troposphere supporting a cold UK pattern in February. I wouldn't even mention that, but historical east-focused La Ninas show a distinct HLB tendency in February, so I can't ignore it, despite no obvious pathway being evident for 2022 at this time. Anyway, one more positive AAM cycle during the middle third of Jan is probably behind the NWP models exploring some mid-Atlantic high scenarios. The big question relating to strength and duration of cold weather is whether the polar vortex will relent enough for its tropospheric imprint to be at all partitioned between Canada and Asia / Scandinavia. In the near future, the lower polar vortex looks to be stretched between the two regions. That means a weakness exists over the Arctic, reflected in the vertical AO plots showing an anomalously negative AO in the lower stratosphere. There lies what I believe to be our only remaining shot at a HLB setup this month. Failing that, waxing and waning of mid-Atlantic ridges is likely to be the main story until late Jan / early Feb. We could well see some cold conditions at times, but probably nothing dramatic unless a rare cut-back of the polar jet brings a disturbance across the southeast at just the right time relative to a cold air incursion. Talk about a long shot!

ECM 70 hPa charts show the gradual eastward drift of the polar vortex in the lower stratosphere that has been giving rise to some mid-Atlantic high pressure build scenarios in the troposphere modelling lately. The ridge across the UK has a presence here too, which implies some strong staying power. Might make it harder for the high to gain much latitude though. If there was one on the Pacific side to pinch the polar vortex, the situation would be a lot more interesting! For UK snow seekers, the tropics are doing their part but the lower stratosphere has to open a door for the amplification to establish in the high latitudes. Otherwise, it seems it will be at best cold and dry with some sunshine, at worst mild and cloudy with light rain at times, depending on just how far north high pressure manages to set up shop in 8-10 days time. For mild seekers, there's a shot at another impressively mild spell if the high focuses over mainland western Europe. The ECM 00z is along those lines, though it appears the high would migrate north a bit after D10.

The EC46 is often grossly misrepresented in here. It’s an ensemble mean, so it suggests what could be more likely than usual, as opposed to predicting a particular pattern. It predicted raised odds of HLB well into Jan, but then that signal faded and the climate ‘norm’ dominated the week 3-5 predictions instead. Now we see a change from that for week 2 with signs of high pressure more to the west of the UK. Likely an extension of the current 8-10 day direction of travel... as it so often is. There I will say the model has a fault - it’s too fond of persisting what’s developing in the 8-10 day range through weeks 2-3, even 4 sometimes. Most striking when the 8-10’l day change is strong, like it was pre-Xmas.

Remember, these charts show the flow aloft, not the surface setup. There’s a big deceleration of the 500 mb flow over the UK there which corresponds to air ‘piling up’ (like traffic approaching some roadworks) and descending to drive a surface high development. Still a brisk zonal flow for at least the northern half of the UK, but not unsettled except perhaps in the far north.

Funnily enough, this Tue through next Sun or Mon, the Atlantic trough looks near-stationary overall, with lows swinging into it. Not what you’d technically call a zonal pattern, which is defined by a sequence of troughs advancing eastward, not how westerly the surface flow is across the UK. 8-10 days from now there looks to be a retrograde of the trough to the west or northwest, which opens the door to high pressure building in the vicinity of the UK. Question is whether we get that further north than seen in 2nd week Dec, so we can avoid the mild, widely overcast conditions.

I concur that a Scandi High looks a long shot. Polar jet likely to overrun - but the Canadian polar vortex imprint could well set up far enough west that high pressure holds across the UK for a time with a chance of a cold feed into the south. Nothing dramatic but at least it would feel seasonal. GFS’ insistence on dropping a broad trough down to our near-east for middle third Jan is starting to intrigue me.

Loosely, yes - allowing for variation in fortunes locally. One of those months where a well-timed stalling front could deliver substantial snowfall at short notice. Such a month usually features one brief lying snow spell for my area of southern England. Often only half a day but better than nothing I suppose! Models continue to hint that this month could be drier than 2019 & 2015 in at least southern UK. I for one will be glad of some breaks from the daily nuisance rainfall of late!

The tropospheric pattern is still highly amplified and that will make it difficult for the polar vortex to establish a sustainable imprint until further notice. Signs are, even if it does occur within the next fortnight, it will be in a divided manner, between Canada (mostly) and Asia (to a lesser extent). That means room for high pressure to build near or over the UK. Maybe even a little north/northeast the UK as per the ECM 12z. I believe this has much to do with the east-focused La Niña. Composites for such in Jan feature above-normal SLP around the UK with an anomalously cold near continent. UK largely neutral, likely due to varying precise position of high pressure areas. Temps most often near average, sometimes cold, occasionally mild. So it’s not an exciting, ‘big cold spell’ Jan that I’m seeing, but it’s not a run-of-the mill zonal style one either. With that comes scope for snow events here and there, mostly short-lived but still enough to provide some interest.

I believe GFS establishes an exceptionally strong low-level inflow of cold air that undercuts the front a bit, physically plausible but inherently very low probability - it requires a very strong pressure gradient.

Notable that even GFS isn’t developing a broad tropospheric polar vortex imprint in the 10-16 day range. Cold air continues to spill into Eastern Asia, keeping the overall polar mass on the small side.

I believe it’s troposphere-driven, a wave-2 pattern with Pacific and Atlantic side ridges. I’m not convinced the Atlantic side one will be west of the UK though, 00z GFS felt more realistic in that regard. The MJO isn’t everything... we saw that with the Xmas shortfall and might well see it in the other direction in Jan (Euro pattern less zonal than MJO alone would suggest).