Nick F

Not a great deal of changes so far from the 18z in the reliable timeframe, other than to say the initial resistance from the block to the east means fronts making really slow progress across the UK next Tuesday/Weds, parts of England and Wales could get rather wet, though I guess rainfall is in deficit this winter - so every little helps. It's not until after mid-week next week that we see the big Atlantic storms modelled to roll in towards the NW, as we see a strong jet streak push across the Atlantic and fragments of the trop vortex engulfing N Canada and Greenland break off and head across the Atlantic as shortwaves that engage and deepen intense lows.

Friday morning's weak disturbance and hopefully wintry precipitation heading north still to the south of Paris atm. Falling mostly as rain away from high ground, but is moving into colder air over northern France and SE UK.

Could be right that it won't turn out as mild as forecast by GFS/MetO, afterall, they were predicting 5C for today in London a few days ago, then 3C today, but it has barely got above freezing. Having said that, as the wind veers southerly tomorrow, the source area of the wind coming up is southern France / northern Spain - though I question how quickly this milder air will get here. Anyway, an ice day for many in England today, not often we get those, certainly not since 2010 anyway. Some wintry showers for some in the east tonight could give a dusting too. But maybe this more to talk about in the regionals!

Need a planetary wavenumber 2 to split the polar vortex, the current warming of the strat and displacement of the sPV is associated with wavenumber 1 activity. So in essence, need two planetary waves for the split. No signs of a sufficient wave 2 amplification to do this for now.

Looks like we could see a proper ice day across many parts of the S, temps still widely below freezing in many parts, -2.6C just south of here at Kenley, clearer skies with sunshine over N France heading this way, but may be too late to stop an ice day 500mb cold pool moving north out of France this evening/tonight forecast to bring a wintry mix of precipitation, so some in SE England and E Anglia could wake up to a dusting on Friday morning, before turning milder during the day as wind veers southerly

500mb cold pool moving north out of France this evening/tonight forecast to bring a wintry mix of precipitation, so some in E England could wake up to a dusting on Friday morning, before turning milder during the day as wind veers southerly

^^^ Yep, certainly close but no cigar for a SSW on yesterday's Berlin charts, if anything less likely then previous days, still could happen by end of month, but PV surely weakened and primed for another SSW in Feb perhaps when wave 1 warming and favourable phase of MJO propagation combine. But I'd much rather see a split rather than displaced sPV - as split more likely to encourage HLB. But as Matt says, we need wave 2 activity to do the split

IMO, the downwelling of the strat warming/zonal reversal will be a slow process based on current zonal wind forecasts, so don't expect a quick response in the troposphere if the downwelling occurs. I would be looking at the MJO wave propagation to bring a change to the upper flow pattern that may favour blocking in the meantime, could be in a more favourable phase (7/8) for blocking by 2nd week of Feb.

@Argyle1980 certainly the jet is tracking further south across the Atlantic into SW Europe next week and beyond, but it turns north over or to east of UK, which is not very conducive to any ridge to our N or NE having any influence at keeping the Atlantic lows at bay, as the ridges look to be surface based. Also, there is no SSW at the moment, perhaps close to one end of the month, but may not come off and certainly we are way off the warming working down to the troposphere. So don't think we'll see any drastic changes until sometime in Feb.

Depends which part of the stratosphere you look at, higher up at 10 hPa the core of the sPV is over Norway at day 10, whilst lower down at 150 hPa the core of the vortex is over Greenland with a trough in the geopotential height field digging SW into NE Canada - so the lowest part of the strat is broadly reflective of what's going on in the troposphere - with a zonal flow across the Atlantic. Yes, we are seeing a warming of the stratosphere which is displacing the vortex off the pole, but because the warming/reversal of the zonal winds is above 10 hPa still at day 10, winds remain zonal below all the way down into the troposphere at 50-60N, if anything, the latest EC strat charts from Berlin suggest downwelling of the zonal reversal less likely at day 10. The much anticipated SSW to change the pattern is still not happening from EC projections, so maybe close but no cigar, but we may see further attempt in February. The problem is the strat warming and wind reversal in the top layer of the strat is working against a background of strong westerly QBO and with low AAM and weakening MJO propagation, for now. There are signs that further wave 1 warming impacts on the strat combining with MJO emerging out of COD into more favourable phases for wave amplification around 2nd week of Feb could have another go at bringing a SSW and also changing the trop pattern in the meantime while we await the effects of the warming on the strat working down to the troposphere. But, for now, although the Atlantic trough struggles to get east past the UK in the means out to day 15 on the 00z EPS z500, the UK will be too far west to benefit from the block to the east, so we've got an increasingly unsettled period of weather to endure from the weekend, with no time limit on it for now, some deep lows look like arriving from mid week too. But the way I see, it's been very dry this winter so far, particularly this month, so perhaps we need some rain to top up the water reserves before winter perhaps returns later in February.

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The current strat warming is not forecast to break up the strat PV, merely displace it over towards northern Europe. A displaced PV, particularly over to our side of the N Hemisphere, doesn't particularly favour HLB to our NW/N/NE for now. As you can see from the Berlin geopotential and temperature charts for the strat. quite a zonal gradient feeding down in the lower reaches over the Atlantic which is surely driving Atlantic lows towards us, but a slow process thanks to the block to our east. However, if the wave 1 warming keeps up, we could see a SSW as we head through Feb, particularly with help from the MJO which is propagating over western hemisphere now, so we may see enough reversal downwelling eventually to affect the trop to increase the chances of HLB. But don't expect a quick trop response from the current strat warming IMHO. With regards to the current trop modelling, still a weak ridge signal extending NW from Scandi day 10 on the 00z EPS z500 mean, though this morning's ECM high res has backed away from showing a decent ridge to our north, GFS never really keen on the idea as still not on the 06z. Looks increasingly stormy period from day 7, as the jet digs south. One last chilly day tomorrow, could be one of the coldest days of the winter too, with temps struggling much above freezing as we import colder and drier air from the SE off a cold continent, then turning less cold and increasingly unsettled over the weekend.

I think generally the MJO forecasts have been pretty poor this winter, with the RMM plots really struggling to pick up the 'correct' propagation. We've seen the NWP adrift from each other in the RMM on what the MJO does, with the MJO amplifying out of COD into phase 1 before quickly fading back into COD again without the a proper orbit through the phases. This incoherent signal maybe related to a rather strong and unprecedented persistent westerly QBO which is interfering with the MJO propagation. An easterly QBO seems to favour a more robust MJO propagation. Back to the current model output, 12z EPS z500 mean shows a clear 500mb ridge to our N from Scandi across Iceland into Greenland out to day 12, however, we are the mercy of the Atlantic trough sliding under this bringing unsettled and milder weather. 12z EPS control rather eye-catching though, sending low(s) on a southerly track into France/Iberia around day 10, though pressure rises into Iberia after then.

It's hard to find a great deal of literature on the downwelling effects on the trop of a displaced sPV on particular geographical domains in relation to where the PV core is displaced. Plenty of literature over troposphere upward mechanics. Though there are appears to be some consistency from recent NWP output that, like you say, points toward the PFJ being pushed further south as the zonal winds increase southward in the upper troposphere/lower stratosphere as the PV pushes over towards our side of the N Hemisphere.

Not great to get a weaker polar vortex and greater chance of high latitude blocking which would greatly increase chances of deep cold and snowy spell.

Not convinced yet that we'll see an official SSW (i.e. zonal wind reversal at 10 hPa / 60N)'looking at the latest ECMWF strat forecasts, and that's past day 10 as far it goes too. Zonal wind reduces to around 0 at 60N and doesn't show signs of a strong 'downwelling' of wind reversal. Any even if we do it's a displacement of the sPV rather than a split - a displacement is no guarantee of HLB, but a split is much better for high latitude blocking. So, the troposphere will carry on doing what she wants to do for now, chances are that further wave 1 propagation upwards toward the strat into Feb then a MJO signal perhaps re-emerging from COD into 7/8/1 (as per recent GEFS) will have another go at bringing a strong warming in the strat in a few weeks, which could trigger a SSW. Meanwhile, closer to home and in time, it'll probably change next run, but 00z ECM high res shows a front end snowfall event days 7-8 as the Atlantic system on the charts below moves in from the west, intiitally some heavy snow for Wales Tuesday evening, then further east into England Tuesday night before the front end snow moves up across Scotland before turning to rain as milder air moves in from the SW. Euros better at handling blocking and surface cold over Europe, so be interesting to see where we go over next few runs and if GFS backs off introducing a zonal and milder Altantic flow. 00z EPS control rather interesting too days 11-15, with southern Greenland/Iceland ridge building in from the east from Scandi, as lows undercut across the Atlantic and then UK/NW Europe. Certainly looking more interesting now the Atlantic struggles to break through initially, then as the ENS mean has hinted at, perhaps ridging over Scandi trying to build NW or even west longer term as the jet stream drops south.

Although deep cold and snow looks like a distant dream, looks like it could be a rather raw day on Thursday as we drag in a strengthening SEly flow with quite low dew points for daytime, temperatures struggling:

As the days get longer the westerly QBO gets stronger, just hope the QBO turns easterly for winter 2017/18 (well overdue anyway) - as the strong QBO state now surely is working against the wave driving/warming that's trying to weaken the PV and zonal winds.

In a nutshell, you're fairly near the mark re: 500mb green contours - ties in with the 12z GEFS 10-15 day 500mb mean chart - which is not too dissimilar to what the 12z EPS mean days 11-15 shows either - i.e. the Atlantic trough drop further south (indicative of a more southerly tracking jet) while ridging over Scandi expands W/NW over Norwegian Sea toward Greenland in the means. I'm always skeptical of the tendency for models to push for the jet further south in the medium/extended range, as often it they retract the jet/trough further the north eventually. So we'll see.

Always had a suspicion that this first wave 1 warming into the strat wouldn't do the job in late January. Still chance that another round of wave 1 warming plus more favourable MJO propagation in Feb could have another go at weakening the PV. But it maybe too late to save winter from the Atlantic or Atlantic/Euro block stalemate.

I wouldn't pin hopes too much on a displaced strat PV encouraging high latitude blocking, certainly not for a few weeks at least. If anything, IMO the displaced PV over towards our part of the northern hemisphere may encourage an enhancement of cyclonic activity over the N Atlantic for a bit - perhaps why the models are keen on the Atlantic ramping up in the medium to extended range. Also it is still uncertain whether we'll see an official SSW and displacement (i.e. zonal wind reversal 60N/10 hPa). I would much prefer to see the polar vortex split rather than displaced, as this would more likely encourage high latitude blocking over the Atlantic / N Europe sector. However, a split looks unlikely for now, as would need an anomalously high wave number-2 hit on the stratosphere as well. Currently we have anomalously high wave-1 activity concurrent with MJO wave propagation warming the stratosphere and nudging the PV off the pole. But the jury's out whether it'll be an official SSW with PV displacement. We may need to wait until mid-Feb for another rapid warming, as wave 1 activity will still perhaps be favoured with an Alaskan stratospheric high and MJO propagation looks to enhance again as it emerges back out in 7/8/1 after we lose the signal to COD for a bit. Given the westerly QBO this year and weak ENSO, we are really reliant on poleward vertical trop wave activity/MJO wave propagation putting pressure on the strat PV, but will it be enough to save our winter? Well, it will probably take a few attempts to obliterate the PV, it if does, but it may be too late by the time it does to save winter, but could bring another cold start to spring!

A rare winter HIGH RISK of severe weather from severe thunderstorms across parts of SE U.S.A today. 30% tornado risk box.

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I think many have been put off by the growing advance of the Atlantic trough into western Europe over recent runs, which is fair enough. But what I see is an increasing trend for Atlantic trough disruption against the blocking to the east over Scandi/E Europe and western Russia. Certainly evident from the ECM high res and EPS and also hints in the 18z GFS too. Longer term, as we head into beginning of February, as well as the mean Atlantic trough not really advancing much past the meridian, there are hints from GEFS/EPS on recent runs of the block to the east building north over Scandi and further toward the pole, even a EPO/Scandi 'ridge bridge' - which would force the trop PV south from the pole as the -AO develops. Hence a more southerly track jet advertised by EPS in the extended. This is without knowing the potential impacts of the wave 1 warming on the strat and the PV displacement forecast. Should the PV get displaced to E Europe/W Russia, it is not quite certain where the trop PV will be displaced. I have read today that it may get displaced to eastern Canada - which is not all bad - as it may re-inforce the Scandi block.

Although there has been a lot of anticipation that we will soon see a SSW, there is decent probability that the current wave 1 induced warming may not be enough to produce an official SSW, i.e. a reversal of zonal winds, with the potential that the displaced PV will bounce back toward the pole with an uptick in zonal winds. However, strat charts show stratospheric high over Alaska which may re-in force wave 1 forcing which may keep the PV off the pole and a second warming in Feb could I guess be better placed to seal the fate of the PV. Often it takes a few attempts before we see an official SSW. That's not to say the current warming won't have an effect on the troposphere pattern, but it remains to be seen, that this displacement in conjunction with the lagged tropospheric forcing from amplified MJO phase 1/2, will favour a pattern change for the UK that will bring deep cold and snow. Certainly no positive direction from NWP high res runs yet, other that the odd hint of a Scandinavia high in FI.