Nick F

Certainly chilly in the easterly flow on GFS day 9-10, though dry Still some uncertainty re: a few potentially troublesome lows arriving from the SW towards southern UK Friday and Saturday, GFS and ECM both have a low centre moving across England/Wales Friday evening, then another low moves through the English Channel/ N France on Saturday. T+90 GFS v EC T+108 GFS v EC 00z ECM tracks the 1st low further N than GFS on Friday - it is at its deepest phase to the SW before it fills across the UK, 00z GFS doesn't really develop it much. Saturday's low a more similar track and depth though, despite 24hrs later, and looks to be more of a problem from the heavy rain so soon after Friday's low Certainly need watching ...

Gets there in the end, that's 12z and 18z with undercut by mid Feb, yes I know FI, but a TREND if 00z carries on with it too 12z v 18z for day 15

Good evening 18z GFS struggling to get a decent undercut to advect the deep cold over W Russia west out to T+240 - which is as far as worth looking for surface. Looks like a long drawn out stalemate over UK between Atlantic trough and block to east EC weekly change of tact again, losing the low height anomaly to our west by week 3, strong height anomaly over Svalbard week 2 and generally +ve heights to our NW/N/NE week 4 and low height anomaly over SW Europe. Seems to be smelling the weakening PV and MJO colder phases later in Feb? We were lead up the garden path just before the New Year (though not all of us perhaps) by the models, for a time, into believing that a cold easterly would come off in early January. But this was against background signal that was less than favourable for this outcome, namely muted MJO / -AAM / strengthening sPV But looking ahead, we have probably the best chance that we’ve had all winter, given more encouraging background signals, these are: - The sPV shows signs of weakening as we have signs of wave 2 activity increasing in early February (via Scandi/Svalbard ridge) as well as the wave 1 attack that is displacing the PV. - The GFS this morning has shown the first proper split of the PV this season too. - The combination of the weakened PV via increase wave2 as we head through early February and the models recently trending the MJO propagation more amplified into phase 8 around the second week of February, which with weakened PV, will IMO increase the likelihood of a –NAO as well as –AO. But we may not see the transition to a colder pattern until 2nd week of February at earliest and more likely mid-month given the lag of the wave2 effects on the strat and MJO propagation. Both GEFS and EPS take MJO coherently through 5/6/7/8 which is better signal than all winter and suggests less interference with the propagation this time. Before this potential, Atlantic still in control, and ramping up a gear later this week – worth keeping an eye on the low arriving across the south Friday evening into early Saturday shown on both 18z GFS and 12z EC, doesn’t look to pose too much of a problem wind-wise other than gales through E Channel, but may be deeper if it phases better with shortwave trough and left exit of jet streak. Some heavy rain too. Another deepening low follows on its heels, though for now heads into France Sunday. A potentially stormy period into the weekend though. EDIT - the 18z GFS gets there in the end with the undercut - FI, but as it keeps churning this promising evolution out, we can call it a TREND

I believe it's only an official SSW when there is a reversal of the zonal winds (+ to -) at 10 hPa / 60 N. As you can see from the wind chart below, don't even get to 0, but as Chino said yesterday, we don't necessarily need a SSW to get HLB - also we may see a SSW attempt gain in Feb plus wave 2 attack to split the PV

Nacreous clouds over Britain now http://www.telegraph.co.uk/science/2017/01/27/polar-vortex-brings-rare-multi-coloured-clouds-britain/?utm_content=buffer9e08e&utm_medium=social&utm_source=twitter.com&utm_campaign=buffer

Interesting how the surface lows coming off the Atlantic across NW Europe never really get far past the meridian and certainly not over past the North Sea throughout the GFS run and also EC, instead they dumb-bell round the upper trough to the west because of the block to the east. The mean trough to the west and NW retreats west by day 10 on both the 00z GFS and ECM high res which allows the block to the east extend a little further west and EC looks like it wants to undercut past day 10, GFS certainly does. Can see the effects of the wave 2 on the strat PV by day 11 on GEFS (charts courtesy of weatherbell) though the strat PV does seem to bounce back to pole post day 10 after being displaced towards Scandi, but appears weaker Not only do we have the increase in wave 2 activity, we will also get some help from the MJO into 2nd week of Feb, as we see it forecasted to go through phase 7/8 - GEFS perhaps a little too quick. So all-in-all, looking pretty encouraging for HLB to the NE and maybe eventually N as we head through to mid-February.

Just need to the Azores ridge to do one, what interests me about the appearance of the Scandi, even if it doesn't bring an easterly initially, is the increase in tropospheric wave number 2 forcing it will bring - which will help towards perhaps splitting the strat PV. Split of the PV much better than displacement for long term prospects of HLB and cold/snow prospects as we head through February. Anyway, only one run and we'll see if the EPS z500 mean supports the idea later too.

Had some hail while I was on the bus heading into Croydon this afternoon

I am a bit more encouraged by the 12z EPS z500 mean this evening, with a ridge west from Scandi toward Greenland at day 10 and it's still there and still prominent days 11-15 too. Which could indicate tentative signs that the displaced sPV may be altering the height fields to our N/NE with time, though the displacement is initially responsible for ramping up the Atlantic for at least a week and probably more, though at the same time shifting the jet south. More runs needed for a trend, but the ridging to the NE, although too far away to keep the Atlantic at bay form us, will help create wave propagation up towards the strat, perhaps an increase in wave 2 activity.

A fellow UK storm chaser, Pete Scott, who's in northern Norway, posted this photo he took of nacreous clouds today on fb:

Even the City of London has stayed below 0C all day, though at 22z not far off creeping above.

Minor surface pressure differences will make little difference to the likely outcome of deeper lows rolling in close to NW UK eventually, need to look at changes at the 500mb level and see if there is a trend over following runs. Those 2 charts aren't a great deal different from the 18z and 12z ops at the 500mb for the same times. The broad scale 500mb pattern has changed little between the 4 GFS ops today up until day 10, though positions of surface lows moving around the Atlantic trough will change the further one goes out, as will surface ridge shapes.

Can't be bothered to post all of them, but here's weeks 1-4, not exactly pretty ...

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.

High pressure has dominated the weather for over a week, but why does it increase the chance of frost, fog and air pollution? View the full blog here

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.