snowking

Still just plain old rain in this part of MK41, I suspect you might be further up into the clouds than me to the north of Bedford park. Good signs this early in the day though!

Firstly @Griff sorry to hear that and I hope the ECMWF has a little something in it to help cheer you up. Onto less important things (and I think it is worth everyone keeping that in mind in the context of the wider world right now, it's just a bit of weather!) - if you remember back to my post from yesterday (which referenced posts from before then - meta or narcissistic, you decide), there's a reason I had referenced the 15th as a check in date for hopefully clearer resolution of the medium term prognosis. This is the day, and has been roughly forecast to be the day for some time now, that the split in the stratospheric vortex reaches it's highest point, at around 5mb: And just to demonstrate this occurs all the way from the bottom up to this point, here's 70mb: @bluearmy referenced this earlier too. I suspect modelling is chopping and changing as it tries to cope with the very complex dynamics brought about by this split. As Nick said, the expectation would be that around this time, LP would be struggling to move past the longitude of the split - in this instance this lines up quite nicely with the meridian. Don't be surprised to see further swings back and forth until this split is within the initialisation data at all levels the split is going to occur at. The next watch date is then the 18th as everything tries to come back together - again expect some volatility between 15-18th as this occurs (though hopefully at least having the initial split into the initialisation data by this point should help with the volatility somewhat) as this occurs, shown below for those who prefer their picture books Admittedly these charts are now 24 hours out of date, so I could well be left with a lot of egg on my face in an hours time, but the progression from there onwards looks pretty favourable for us for keeping any westerly momentum away from the Atlantic, higher heights to our N and NW, and hopefully allowing colder air from Asia/Siberia to slowly trickle down towards us: The point here is the trough should track slowly Eastwards over time, and the hope is it does so far enough East in the first place to allow us to sit on the colder side of the trough. Unless things drastically change stratopsherically, it would be very surprising for us to end up tropospherically with anything that doesn't include higher heights to the N and NW, and a mean trough over us or to the East. That's not to say it can't happen - something like tropical interference could interfere with that into the medium term, but without any other extraneous influences, be very suspicious of any runs dramatically lowering heights to our North/West

It's an interesting point and some recent GFS suites have hinted at the possibility of a surface high developing in Scandinavia in the last few days, though nothing concrete has yet developed along these lines. The big Pacific ridge is really shaking things up into the medium term, and with heights transferring ~ Pacific - Atlantic/Eurasia, nothing is off the table in terms of possibilities in the next 10-15 days. Models hate handling these sorts of scenarios. For what it's worth on the longwave scale, I think the 0z suites were far too keen to remove heights from the Arctic region (apart from the ECMWF) and so was probably just an overreaction to something. That doesn't change the trials and tribulations in the shorter term for our tiny island, but given current model volatility, as @Nick F suggested earlier, probably best not to concern yourself with too much more than +144 for now.

Still a lot of trials and tribulations to go through yet during the next 8 days or so, but I set a target date a few days ago of the 15th for ensemble suites to start to resolve the situation into the final third of the month with a little more clarity (following the early leads hinted at by operational modelling). We're now starting to edge towards this clarity I hope at least, with the 6z ensembles posted above advertising the expected sharp decline in 850mb temperatures as we head into the final third of the month and, increasingly it seems, into the opening period of February. The ECMWF ensembles demonstrate this nicely too closer to the surface: We should still remain cautious - as is visible there, there's stiil a few stragglers in the suite advertising something much more mundane. For this reason, ensemble means will continue to be watered down somewhat, and of course being in the UK we cannot completely rule out the milder option, given the propensity for the atmosphere above our heads to lean towards a habit of clutching defeat from the jaws of victory. But the forecast is very much suggestive of the stratosphere driving things, rather than the troposphere as has been the case up until now, into the medium term. Stratospheric forecast still divergent between GFS and ECM operational runs at least but both continue to advertise further displacement of and disruption to the polar vortex for at least the next 10-15 days. Whilst none of this guarantees epic cold for the UK we do look, in the extended range, to be in the vicinity of a broad NW European trough right through into February - high risk, high reward at times as a result. But as time goes on, and we start to see an 850mb temperature profile like the one below to our North and East, any sort of marginality should become less...well, marginal, as time goes on. Spot the difference between the two profiles below: I'm not ruling out deeper cold reaching our shores towards the end of January (in fact I'm still fairly confident this will be the case), but even if the very coldest air doesn't quite make it to us, for as long as we're locked into a cold trough with the near continent freezing up and plenty of movement within the trough, there will be snow opportunities aplenty, just as, in fairness, there have been already this month unless you happen to live in the South-Eastern quadrant of the country (this should not be forgotten, pre 2009-2013 the spell we have already experienced would have been notable for many) I appreciate part of the fun in here is the ride, checking over the details of every operational run, for some finding that historic snow event at T+300, for others trying to find how the cold spell is going to end before it is even forecast to establish, never mind actually even being established in the first place. But if you appreciate your own sanity, the theme and the dates have not changed and at the moment look unlikely to change - final third of January, broad trough vaguely to our East, height rises to our North-West and the cold slowly creeping around the Arctic high from Asia into Europe. Even with some unfortunate phasing of lows, something would have to go really drastically wrong for that broad theme not to come to fruition. Here it is via both ensemble suites: Let's give it a few days and check in again, but I'm fairly confident it will be the same. Are we guaranteed a historic cold spell? Absolutely not. Does it look likely we will be using the S word fairly frequently for the final third of this month and beyond? I would say so.

Was just about the comment the same - the Arctic high coming down into Svalbard is a thing of legend and very rare to see - only if this happens will the extreme 850mb temperatures reach our shores. No point stressing about any output at this range yet as it will be many days before we know exactly how the Arctic high is going to manifest itself

Yeah if anyone is looking for the exact synoptic pattern 10 days out you’re going to be disappointed. Keep on following the overall longwave themes: Heights rising to our NW Asian vortex segment retrogressing into Scandinavia (and beyond....) Big artic high (which is always poorly modelled beyond a few days) Continued warming episodes in the stratosphere For as long as the models continue to show these themes, we should all be happy. That’s as much as we can ask for right now.

Exactly this. With the chaos aloft at the moment, it's only really worth looking at the overall trends beyond a few days. The main things we're seeing: - An Arctic high which increasingly wants to come and play towards our side of the hemisphere - The jet stream running out of steam across the Atlantic - The temporarily Asian based vortex segment wanting to retrogress around the southern side of the Arctic high The detail will continue to vary hugely in between all of that (including whether the UKMO solution has any legs to it in the shorter term), but looking ahead to the latter part of January, all those ingredients keep us well and truly in the game.

And just in time to try and contradict the above post (entirely unintentional!) GEFS mean 10mb zonal wind forecast from the last 4 0z runs - note the emerging trend for a much slower recovery of any zonal flow. Still a forecast so we’ll see

This for me goes back to what I was saying last night - it's anecdotal in fairness, but time after time over the years I've seen the Op and Control lead the way, so they look for quite a few funs like the statistical outliers we're seeing now, before the ensembles with their lower resolution finally catch up. In normal circumstances I am all for ensembles being followed closely for trends, but I just have a suspicion (perhaps naively!) that we are not within the usual bounds for more accurate ensemble forecasting. We shall see!

Just as a quick note here - IF, and it’s a huge if, we are going to see continuing upgrades in the short-medium term, then the ensemble suites and means are going to be next to useless beyond T+144 due to their lower resolution - if we are indeed headed to shorter term upgrades then the higher resolution Operational and Control runs will lead the way and drag the ensembles behind them. Im not yet convinced that these shorter term (day 5/6) upgrades will amount to anything, but it wouldn’t be the first time. I have a vague memory of this happening in either 2012 or 2013 with an Icelandic wedge becoming more and more influential in modelling the nearer we got to T+0 (I think it might have been February 2012 where we saw a week or so cold snap with some frontal snowfall, but it might have been one of the 2013 cold spells instead) and @Steve Murr called it very early on. Whichever event that was produced the snow you can see in my profile picture. Of course since then many such potential upgrades have come and gone without producing much, but something feels a bit different this year.

If anyone wants to check out model performance you can do here: NCEP EMC Global Model Verification WWW.EMC.NCEP.NOAA.GOV Currently GFS para outperforms standard GFS out to day 6, thereafter the GFS is the better performer (but we currently have only 25 days of data for the para, so not an entirely fair comparison just yet). At day 10 on the 12z runs at least, the old GFS is king: However the most accurate run of the day at day 10 is the ECM 0z:

Apologies for delayed reply Zak. Well From a local perspective it’s been incredibly frustrating so far, and tonight is currently looking like another miss for us based on the majority of modelling - though we shall see if the radar later throws up any surprises. I’ve never known a spell like this where every potential snow event tracks west of this region. But despite the limited opportunities here, the majority of the geographical UK have seen a 2 week period of below average temperatures and some accompanying snowfall - and in the days before 2010 that would have been seen as pretty significant. So great to see for fellow snow nuts elsewhere!

The two 0z runs either side of the run above shown by @sebastiaan1973 also showing at least a partial split around day 10 - hopefully we continue to see continuity in this regard right down to T+0

I said in my post a couple of days ago that the period which most interested me was the final third of January and into early February, thoughts have not changed much during that time - the only downer of course is that it looks like we'll go through a transitionary period through the middle third of the month which initially longer term modelling had not suggested. As quite a few posters have alluded to above, a general rise of heights on the axis running between the Pacific and the Atlantic is what we would be expecting should the stratosphere imprint directly on to the troposphere (far from a guarantee, but plenty of hints that it might), with a subsequent injection of cold to our east. The concern will be whether we see too much energy left over in the Atlantic, with some extended model runs suggesting as much in terms of the stratospheric pattern. However as we can see from the image below the main story remains the majority of the vortex being shunted over to the Eurasian sector: As demonstrated on this afternoon's runs just because we have heights rising somewhere towards our North/North-West, doesn't guarantee that our tiny island will end up on the right side of the pattern, but we're in the draw for the cold lottery at least. With successive runs now starting to show an ever deepening cold 850mb pool to our East: All the ingredients are there for later in January and into Early Feb for a more substantial cold spell than the one we are currently in the midst of, can we get them all together? Expect some volatile but crazy FI charts over the next few days and then let's see whether we can start to get the longer term suites settling down into next week for what may lay ahead.

It's certainly been frustrating so far. Even the anaprop circles completely around my location

The answer really is, much as with tropospheric modelling at the moment, anything beyond T+120 should be taken with a huge great big snow shovel's worth of salt. The next two weeks, stratospherically, will involve models trying to resolve dynamics in an area of the atmosphere in which they don't necessarily have as good coverage as they would do in the troposphere (and we know how fragile tropospheric modelling can be at the best of times), trying to handle one of the more complex stratospheric events that I can recall seeing. We're already seeing that at less than 24 hours out even the model with the best stratospheric coverage, the ECMWF, has been underestimating the scale of the reversal at the top of the stratosphere and has been doing so consistently for some days now. So we are likely to see many twists and turns in the stratospheric modelling of our soon-to-be-declared SSW for many days yet. Ditto (and consequently) in the troposphere - we're likely to start seeing some pretty stark arctic heights modelled in the coming days and weeks and tropospheric modelling does not handle this particularly well either.

There will be some hangovers in this thread in the morning. These are the sorts of charts I think a lot of us were expecting to see around the day 15-20 period and here they are for all to see. Almost certainly a bit extreme and changed again with the next run - but then again, a 12 day+ reversal likely peaking at over -20m/s, this is not out of the question...it’s just probably on the very edge of the envelope

Agree with this - there’s three explanations for any sort of uptick around mid month. The first is the model dynamics not handling what is a pretty major reversal all the way from 1mb down towards 100mb (and beyond). The second, as Crewe has mentioned, is that actually the models are handling the dynamics perfectly and we’re seeing the flushing down of the zonal winds aloft into the troposphere as the zonal wind reversal works its way down. This is by no means guaranteed, and should this be the case should be fairly short-lived. The final is, as @Uncertainy mentioned earlier, the MJO phase 2 could be either driving the troposphere, or driving models towards an incorrect solution. These thoughts have been mused over the last few days by various posters, and it will be the middle of next week before we know which (if any) of these explanations might be the cause of the current day 16 modelling. Either way it is of little concern to me really, as even if we do see a relaxation of the pattern, I’ve still got my eyes on the final third of Jan.

@sebastiaan1973 synoptically it might not be too far away from our expected set up on Jan 6th at a very broad brush scale (Siberian high, Aleutian low and heights in the vicinity of the N Atlantic) However I think Feb 2010 was a straight split SSW, where as what we have coming up is a displacement followed, hopefully, by a split. My expectation would therefore be that we would see entirely different dynamics to this warming vs the Feb 2010 warming. Not to mention that with this one taking place over a month earlier, there’s less risk of any consequences of the SSW being prone to seasonal wavelength changes. Thats all just theoretical from me though, and I guess we will just have to wait and see what happens through the second half of Jan. It’s a fascinating watch regardless of how it unfolds

Great stuff as always. In terms of the split, the last set of postage stamps we’ve had sight of was 12z yesterday for day 15 - I’m more inclined to think that any split at 10mb will be out towards day 15 now. At this point on the 12z EPS, 33 of the 51 members had a split, with 4 or 5 others very close to a split at that stage - which makes sense given, as you said, the continued Ural high/Aleutian low setup sending more wave activity into an already severely bruised vortex. Given the current favourable trop synoptics, and assuming of course that we don’t see too much interference of the short term pattern in the meantime via any SSW effects, this would hold a lot of promise for something even more substantial through the final third of January and into early February - that could well just be the cherry on top by that point.

Indeed. As much as I would continue to advise not concerning yourself with anything beyond day 5 right now on operational (because solutions after this time will continue to flip wildly from solution to solution I suspect) that certainly seems to be the direction of travel by around day 15. Who knows where it will end up (NWP certainly won’t for at least another 10 days), but if we are seeing a direct stratospheric/tropospheric imprint, then that leaves us in a pretty good situation in terms of Greenland heights in the extended period - we may, as a result, have to see a slight relaxation of the prevailing pattern between days 10-15 to get there so don’t be surprised to see models toying with this idea (though whether it happens or not who knows - with an Arctic high in the mix everything is up for grabs) General idea is increasingly cold and zonality nowhere to be seen. When was the last time we could say that at the start of January

Cheers to that. Happy new year to one and all. And for anyone who’s mad enough to be up for the 0z’s, just remember that we might as well bin anything beyond day 5 right now (though I hope it’s still fun to watch), so don’t let 2021 get you down this early on.

I’m really liking this pretty picture: Just to add even further weight to @Catacol’s post above, even if we do end up with part of the vortex in the Atlantic, is it really going to have much influence? I would think not. Note the big void around Greenland and the North Atlantic....I wonder what could fill that gap...

To answer my own question from earlier, the ECM ensembles fairly keen on the GFS suggestion of a split in the extended range, and even without a full split the displaced vortex is so stretched that it will have very little effect on anything. We manage to get a sneaky view of the 0z ensembles courtesy of Twitter: I make that 26 members with a split, and several more right on the verge of one.

Thanks as always @SqueakheartLW Still varying shapes to the vortex across model suites but the one constant is that the pacific wave does not look like going anywhere in the next 15 days or so at 10mb, and this means that any plans the vortex has of trying to return to its default position are pure fantasy, meaning we will see suppressed zonal winds for some time to come through January, with at the very least a displaced stratospheric vortex over towards Scandinavia/Siberia. As you point out the GFS is becoming quite consistent with another significant warming episode towards day 12, serving only to weaken what remains of the stratospheric vortex further - indeed the last 2 GFS runs have gone on to show this further warming coupled with the continued forcing from the pacific wave and just some very minor pressure from the Atlantic leading to a stronger split in the latter stages. Will be interesting to see as the next warming episode comes in to the day 10 range whether we see this on the ECMWF too. All of this is suggestive that we will likely continue to see a suppressed zonal wind profile through the stratosphere over the next 3 weeks or so (and increasingly likely perhaps beyond that too). So it’s over to the troposphere to set up whatever it is going to set up pattern wise - whatever pattern is set up is unlikely to shift around too much without some significant forcing from the tropics I would suggest. If we did end up with a stratospheric imprint on the troposphere, the weird looking, almost summer-like (in their longwave set up rather than temperature profile I should clarify!) charts with the atlantic side of the hemisphere almost entirely devoid of low heights might not be too far off the mark. The general set up I would anticipate in such a scenario remains the same - higher heights to our north/west, and lower heights to our east.