snowking

Tomorrow will be a very long but hopefully very fruitful day! Could be the first snow falling since March 2013 for most of us. Every tenth of a degree and every inch of height will help in this one - Chilterns, higher ground in Essex/Herts (East Anglian heights I believe they are known as)...possibly even the downs a little later on. We do have to track this carefully because modelling has continued to decrease the depth of the low (hence the increasingly southerly track), which also happens to aid with potential evaporative cooling with winds slightly less strong than initially expected. But equally there is every chance it ends up 1 or 2 more mB deeper or shallower, which could be the difference between the snowline moving 50 miles further North (and reducing most of us to Turkey-concentrated tears) or 50 miles further South, allowing those down in Kent/Sussex/Surrey to join in the fun. Finally, I'll leave you with this tease - the Stansted airport TAF until midnight tomorrow - probability of rain-snow/snow after 9pm. SK

ECMWF has very similar track to this below, posted on Twitter from the hi-res NMM output: This maintains the general trend from today of advertising a generally Wales-Midlands-East Anglia track of the snow threat - once again to stress this does not mean this is the final track, but at present this seems the most likely scenario on the basis of forecast models. This met office visual firms up on this idea too: I wouldn't be paying too much attention to projected totals just yet as these situations tend to become pretty dynamic. I'm off to enjoy the rest of the day - hopefully an exciting day of watching to come tomorrow SK

12z Euro4 maintains a similar track to its 6z run, with similar temperature profiles, with the possible snowfall zone shown on the link below: http://www.weatheronline.co.uk/cgi-bin/expertcharts?LANG=en&MENU=0000000000&CONT=ukuk&MODELL=euro4&MODELLTYP=1&BASE=-&VAR=prty&HH=36&ZOOM=0&ARCHIV=0&RES=0&WMO=&PERIOD= Over to the ECMWF to see where it takes the track for tomorrow, but right now the GFS is much further north with this (in to the sort of zone that HC indicated further up) We have seen these scenarios many times over the years, and in all honesty by this stage modelling tends to go out of the window and it becomes a case of monitoring WV imagery, then later radar imagery to see whether the track and intensity of the precipitation match that of any particular model, or whether it is a nowcasting situation - the latter being the most common. FWIW Essex Weather Centre highlight the risk of snow in to North Essex by Saturday Morning - but they can only go from the model data and this offers no guarantee that the modelling of the situation turns out to be correct, it could well end up further North or South still at this stage, though I would say generally the margins for error tend to be around 50 miles or so at maximum - that does not preclude unexpected 'extra' developments surrounding frontal systems though. Over to the ECM SK

GFS at present is still the furthest North solution - not for the first time in these scenarios. Will be interesting to see the 12z ECM later...after a bucket load more food and wine SK

Even North of the M4 upland areas are more favoured obviously, but even South of the M4 (if the current track is correct) some of the heavier bursts could turn to snow overnight. Of course if we see the low even shallower than current projected it may yet still shift further South...in which case a total recalculation of snowfall potential would be required. Dew points generally look marginal, but as the precipitation becomes more intense the hope is that the temperature profiles will become more and more favourable. Nocturnal cooling should also play it's part so the more precipitation you see after dark, the better the chances. We will know more after the 12z suites but often these situations come down to nowcasting SK

A quick update before I enter a food induced coma: 6z Euro4 increases the snow signal slightly running to the North of the M4 coridoor. It also has the bulk of the precipitation significantly further south than the northern extent of the warning area. If the southward corrections continue across the 12z suites we could be looking at a Wales - S Mids - East Anglia event. 12z ECMWF in particular awaited with interest at the various met organisations I'm sure. SK

FYI Met Office warnings - seem to be based largely upon 0z ECMWF - paints a picture of generally 1-2 inches across a wide area of England and Wales, North of the M4 (as ever!), as much as 4-6 inches up across parts of The Midlands and Lincolnshire and Southern parts of Lancashire and Yorkshire from the Weatherbell liquid equivalent output. Euro4 far less keen away from higher ground: Though in these situations things like evaporative cooling are rarely picked up, but Dew Points do not look supportive at this juncture on the Euro4 to lower levels: Hi-Res NMM output (based upon GFS data) shows a similar picture to the ECMWF, but again marginal dew points so it is likely to come down to precipitation intensity tomorrow (as well as of course the final track of the system!). Anyway enough of that for today. Merry Christmas to all and lets hope for a nice late snowy present tomorrow. SK

Merry Xmas to all and I hope everyone asked Santa for a big Greenland warming! Have a great day and may Aberdeen see some snowfall so that the bet I've had on since late October comes in! SK

As BA posted earlier, I can confirm the Control at least is very much surface cold, with 850's anomalously high from days 10-15. The latter timeframes of the control though play out similarly to the GFS(P) with heights retrogressing and a strong tendency for the Atlantic cyclogenesis to dive SE in to Iberia, whilst ECM EPS mean keener to retain the heights to our ENE instead. SK

Dear Wave 2 our old friend, Welcome. Total split of the lower Strat by day 9: Less of a split by day 16: But with this coming down from above: And showing signs of downwelling pretty quickly: An intriguing start to 2015 to say the least. SK

I don't think I get enough opportunity to complement your continuously fair, unbiased and easy to understand overview of the models, and the time and effort you take every day to produce this is nothing short of phenomenal. A very merry christmas to you too and I am sure everyone is looking forward to your return in 2015. Before we even consider potential Greenland warmings, instant trop responses etc. we perhaps needs to keep a closer eye on this weekend too. For the cold-hearted among you, here is the GEFS ensemble mean snowfall projection: I suspect the GEFS is still a little too quick with this evolution vs the european suites, the ECMWF det certainly a little less bullish with totals when compared to what is shown above, but for anyone with travelling plans the combination of heavy rain, strong winds and potential for snow in places is really worth keeping a close eye on. SK

Somebody must have wished really hard this year and Santa has listened: Unfortunately it's been a little late in arriving because Santa chose to use Yodel... I suspect that Tony and Ed are responsible for this wish. If Ed has managed to see this from his Chalet I'm sure he's gone and got some champagne and ice, ready just in case...or possibly just fallen backwards off his chair. Of course this is just a few runs at the very latest timeframes showing it at the moment, but even on the ECMWF you can see the hints of something brewing over Greenland at day 10: Fits very well with Tony and Ed's forecast so if this comes to fruition then absolute full credit to the two of them...who needs GP anyway (NB - that is tongue in cheek - we miss you Stewart!) SK Edit: Probably my favourite chart of the morning: Upwelling, downwelling and...mid-welling

Not really sure I understand your final point there IDO... Sure the positive height bias to our west may result in stronger cyclogenesis....but more pertinently the height bias over the UK is a negative one, which would make the assertion of higher heights (in association with an MLB) from the ECMWF seem even more likely, especially at day 6. That is not to say this will be the outcome by any means, but the height bias charts shown there do not suggest to me any less tendency for the ECMWF to be correct here. SK

Rob you seem far too keen to see Barry naked! Just a couple of observations. The hints of height rises over or to the ENE of the UK remain - and this is still ultimately the direction I see things heading in. The killer detail will be deciphering how much energy goes above and below any potential block...and with the main thrust of the vortex stratospherically at least being pushed, via wave 1, over to our sector of the Northern Hemisphere that energy balance is something that NWP is likely to continue to struggle with over the coming days. This is then further complicated by both downwelling and upwelling wave 2 potentially in around 10-15 days time which could once again throw energy budgets and direction in to a little bit of a chaotic state for the poor old models. We have already seen plenty of over-suggesting of height rises by the GFS(p)...but equally we should remember the known westerly bias of the GFS, and more especially the tendency of the GEFS to reintroduce zonality, when faced with a block, far too quickly. So overall then expect plenty of chopping and changing of outputs to continue for the next few days and beyond...and I'm sure that will correlate to the mood in here too... SK

*standard on phone so no charts disclaimer* Well pretty encouraging output this morning on the Berlin charts. Reversal at 1mb by day 9 in response to the large wave 1 attack, though there is an uptick once again by day 10 as this relents a little. Wave 2 then increases from the top around days 8/9, before we start to see concurrent upwelling and downwelling temperature waves by day 10. Overall then a pretty inhospitable environment for a polar vortex. Tropospherically there are a few factors in current NWP leading to the strong vortice shown to set up across Canada, but this is mostly driven I would suspect by the large wave 1 attack stratospherically taking place from the Pacific side, pushing more energy towards our half of the Northern Hemisphere. What I would say though is that this is a particularly fluid situation and it is perhaps best not to take too much tropospheric output beyond day 5-6 as gospel right now until we see what the effect of all this activity above is. Support currently from the lower stratosphere for this but with the increase in wave 2 shown by day 10 the final effect could be something very different. You feel that something has to give sooner or later, with nothing particularly resembling a vortex showing for the foreseeable in the stratosphere. SK

Well BA first mentioned it yesterday morning as a feint signal in the longer term EPS spreads, I then gave it a subtle mention a little later in the day, and it's still there this morning for all to see - albeit a little more obviously: Forget looking NW for now, if we are to see anything meaningfully cold (potential short northerly and snow risk aside), we need to look NE. FWIW we retain a split vortex right out in to FI - possibly linked to the weak Wave 2 activity BA talks about over in the stratospheric thread earlier - but right now later in the run the split becomes unfavourable in terms of vortice placement to our NW. Of course should we manage to get some sort of block in place to our NE in the interim, there's nothing that should necessarily preclude an undercutting scenario under this setup...perhaps a la Jan 2013? (Caution: speculative ramp) SK

Yes sorry badly worded - tropospheric low

Come on then own up, who created an actual 'weatherbomb' and deployed it on 6th January?

GEFS mean panels show nicely the continued nature of the warmings at 10mb: With an ensemble mean producing this sort of outlook anybody even hinting at writing off an interesting January stratospherically is rather foolish at this stage. We can see that at this particular height much of the warming is Wave 1 led - largely owing to the huge Aleutian low which has re-rstablished itself: We can also add the NAEFS ensemble mean in to this arena too: Of course whilst we can roll the dice and pray for something decent following a Wave 1 attack, ultimately the favoured solution is always going to be a Wave 2(+) attack, and we see some subtle hints at this from ECMWF ensemble means posted via WSI in recent days, and indeed from yesterdays 12z det: Even without it, notice the huge drop in mean zonal winds at 1mb, and how this is slowly downwelling to 10mb by this stage too: All of this makes for one very unhappy vortex, and we are starting to see some effects from this a bit lower down too as this destabilising takes place above concurrently: I guess if I were to sum all of this up in one meme: SK

Perhaps we should be looking NE in to the 10-15 day period.... Split vortex and ridging tendency heading more and more towards Scandinavia over the last couple of runs SK

As always another wonderful post Tamara, and the point above is a key one in to the medium term The GWO forecast linked is a GFS-derived forecast....and that sort of GWO projection ties in pretty well the strongest day 15 ECM EPS cluster upstream with around 20 solutions (by my own manual counting) re-developing some form of North Atlantic ridge, and most members agreeing on a continued amplified trend, courtesy of our -EPO ridge (in turn driven by the well advertised Aleutian low - which of course is also helping to drive the next strong phase of Wave 1 activity). ECMWF, which led the way with the last major Kelvin wave, keen on driving us back around the MJO phases too: Ultimately we would like to end up somewhere towards phase 7: Whilst I can understand why those who may have built up shorter term prospects a little prematurely are a little disappointed there is still very little reason to be despondent in to the mid-range SK

It's not so much a case of it hitting low resolution, so much as it's a case of, once again, as we move the timeframe closer to T+0, it seems that the operational models are leaving more and more energy around the Greenland locale (especially to the North) - or to put it more accurately, it's starting to look as though once again (for about the 10th time in the last 3 years or so) the models may have overestimated heights in this area. We can see this from the last few GFS and ECMWF runs (the ECMWF has been particularly guilty of this over the past few years as has been pointed out numerous times recently). ECMWF: This has been such a consistent occurrence in the last few years that it has made me strongly distrustful of the ECMWF in particular (as this seems most prone to incorrectly evacuating low heights from Greenland in the extended timeframe). This does not entirely preclude a colder spell in the shorter term, but any such spell is likely to be a little more short lived in nature than perhaps was initially suggested (assuming of course that this trend continues to keep a larger area of lower heights close to Greenland). Of course what happens beyond the xmas - new year period is still very much open to debate, but right now the model trends seem a little too familiar for me in the shorter term. Let's hope for something a little more positive the other side of the weekend. SK

And just to add to the drama for the weekend... @RyanMaue: ...and GFS parallel will be unavailable this weekend due to NCEP computer issues.

I think we perhaps need to clarify thisFrom what I can make out of that tweet, he is using intuition. The ensembles are not out in their entirety yet, it's the fact that the det is looking completely different from other det solutions this evening, and of course it has been consistently a mild outlier over the last 48 hours, that leads to that tweet I would imagine. I'm sure Matt will clarify this if he pops in to this thread, and of course we will find out for ourselves within the hour SK

Couldn't have put it any better Nick There is always a concern when the higher-res operational run sticks to a consistent theme for several runs, as you have to make an assumption that its superiority is picking out the key detail and may have the right answer.... However, the crumb of comfort for me is that the control is not following the op - if you have the two high res runs vs the ensemble consensus in these kinds of delicately balanced situations then you start to suspect that the lower resolution of the ensemble members is the cause (and is likely to be incorrect). But conversely in this particular situation they have been fairly steadfast in a colder scenario (be that from MLB or HLB) alongside the control. One key thing to remember is that low angular momentum situations throw the model suites in to chaos - and that is where we find ourselves right now. For your own sanity, wait until at least the other side of the weekend before making too many judgments on the upcoming spell SK