Nick F

Seems the operational runs this morning have come to some broad agreement for mid-to-late next week over how far west the cold pool advecting west out of western Russia reaches and also its southern extent too. with the coldest T850s toward NE UK, though a look at the 06z GEFS, fair few members advect deep cold further south too (see below at T+144 / next Thursday), so by no means is this area resolved. This uncertainty re: advection west of deep cold initially is related to rather active cyclogenesis around the positively-tilted deep upper trough extending down across the N Atlantic from Labrador/Greenland early-mid next week, with a few deep dartboard lows swinging NE then N around the upper trough toward Iceland - which limits the westward extension of the of Scandi high. But the cyclogenesis to the west eases later next week, as a chunk of the 500mb vortex/low height over N Canada/Labrador/Greenland breaks off and moves SE toward SW Europe, with a more negatively-tilted upper trough extending SE on the SW flank of the block over Scandi - which will aid CAA westward again. And it's at this juncture going forward will decide the destiny for whether we: a) BEST SCENARIO - see the Scandi high continue strong and in situ with enough low pressure extending east through the Med from Iberia to prevent the high sinking and to continue the deep cold air advecting west from Russia toward the UK --> b) WORST SCENARIO (like 00z EC high res) - we see an upper trough cut-off and isolate over SW Europe/NW Africa - which will allow the Scandi ridge to weaken and extend/drop south while merging with an Azores ridge extension across the UK from the west while Atlantic low pressure pushes east over the top toward Scandi OR SOMEWHERE IN BETWEEN THE ABOVE OF COURSE However, those worried about the likelihood of 00z ECMWF high res (above) evolution coming off, look at the EPS z500 and mslp mean and spread shows a fairly high standard deviation for the upper low over Greenland to push east toward Scandi, so could the high res could be too progressive in the area Also, we saw a flip earlier in the week, think it was between the 00z and 12z operationals on Tuesday, which advertised the cold pool coming out of Russia still way to the east for 12z Weds next week, then the 12z brought the deep cold pool with -10C T850 line pretty much knocking on the east coast. Which goes to show that there may well be more twists and turns on the way from the model output as they come to terms with how to evolve the Atlantic trough disruption against the block over Scandi and how much energy to send underneath across southern Europe - which in turn determines whether the Scandi high keep in situ or sinks and how much deep cold gets advected west. Still not a done deal this morning's operationals, though I suppose the high res should be ahead of the ensembles for z500 pattern up to day 7, beyond I would revert to ENS.

Ouch, I heard rumours there may be a sting jet on the back of Saturday's low that could bring gusts to 100mph along coast of western France. But hopefully you, Nick Sussex and any other expats in France on here will be spared such violent winds! Back to the 18z, pretty reasonable GEFS support that we will be in a deep cold and perhaps snowy easterly flow come next Thursday

Also worth noting general model agreement that the deep very cold upper air pouring southeast out over the Atlantic from northern Canada/Labrador/Greenland will actually help force the strong jet stream on a SE trajectory towards SW Europe rather all the jet energy sliding N/NE up the western side of the block over Scandi. A crucial part of the jigsaw, as well as the deep cold pool advecting west under the block, to get us on the road to cold and snowy nirvana. However, the detail at the surface is far from resolved and will make all the difference to peoples hopes.

A pretty decent run from GFS operational for cold and snow up to day 10, my only criticism is that it doesn't push low heights/pressure far enough east across the Mediterranean in later part of medium range to stop the Scandi ridge from extending south across eastern Europe - which cuts off the deep cold advecting west from Russia. But that will quite possibly change. But shorter term certainly some upgrades, with the -10C T850 line crossing the N Sea as early as Tuesday morning. The difference between 18z GFS and the 12z EC high res is the depth of the cold advecting west, much colder/deep pool on the 18z GFS moving west under the Scandi block. This is going to be painful for those who wish to micro-analyse each GFS, EC and UKMO run. For me, I'll sit on the fence and not expect any run to be more likely, as quite frankly the models are notoriously bad at dealing with Atlantic trough disruption against blocks to the NE and subsequent advection west of deep cold.

A look at a low arriving across the UK on Friday bringing heavy rain and gales, along with some intense storms set to batter France this weekend. View the full blog here

This time round the atmospheric drivers for change are a lot more favourable, namely the MJO coherently moving into colder phase 7/8, the best MJO propagation we've seen this winter, with enough amplification to induce HLBs rather than MLBs. The MJO forcing concurrent with planetary wave number 2 over Scandi/Svalbard propagating up from the trop will mean the stratosphere PV will be in an increasingly weakened state as we head through early Feb too. Could go wrong if the HLB doesn't position right, but more background support this time.

Strong EPS z500 mean support for Scandi high at day 10, trop PV well and truly retreated to N Canada by then Extended EPS z500 mean features negative height anomaly over SW Europe and western Mediterranean and positive anomaly over Barents and Scandi days 10-15, what could possibly go wrong? chart courtesy of weather bell

The QBO is not a make or break factor in whether we see high latitude blocking. There are occasions in the past where the easterly QBO values were not a factor in some of our really cold and blocked winter spells. For example, the blocked and very cold months of November and December 2010 had a (westerly) QBO value of +12.16 and +10.96 respectively. February 1986 was extremely cold, 2nd coldest Feb of the 20th century, yet had a QBO value of +8.73. QBO Index: http://www.cpc.ncep.noaa.gov/data/indices/qbo.u30.index A westerly QBO regime, a major stratospheric warming is less likely but not impossible. Though a SSW is close but no cigar for now, probably because of the wQBO. As such, overall, a positive QBO regime would suggest a stronger stratospheric Polar Vortex. However, we know that last autumn that the stratospheric and tropospheric Polar Vortex was actually weak and the QBO had been westerly for some time, November was strongly positive at +14.16. As such, one has to question just how much of an influence the QBO is having on the stratospheric environment. As some have pointed out – the blocking showing from the models recently is being trop up (wave 2) lead rather than strat down – MJO moving coherently through 5/6/7/8 will help amplify the flow further as we head towards 2nd week of February. This is the best MJO propagation we've seen all winter. So the westerly QBO may not be heavily in the equation in terms of getting blocking, we’ve seen a lot of blocking this winter, despite the strongly wQBO, just the blocking thus far has not placed itself favourably to advect deep cold across the UK for more than a day or two.

Be nice if people could comment as well on the tweets they post! Anyway, will have to see if there is growing support from the ENS for cold, as the cold members still more in the minority camp for now which suggests uncertainty over whether we can get the block to retrograde to get the UK in on the cold air developing to the east. For now, all we have is a stalemate playing out over the UK between Atlantic trough and block to the east, with UK more under trough influence, but perhaps easing with time.

Well it looks like the sPV will bounce back towards the pole after day 10 after being displaced over toward Scandinavia this week looking at GFS and EC 10 hPa charts. Get close to 0 m/s 10 hPa / 60N at T+48 on yesterday's EC on Berlin - but by day 10 its zonal again. So no technical SSW for now So we'll have to see if trop up wave 2 activity combined with MJO moving into 7/8 around 2nd week of Feb will be enough to retrograde the building block to he east over Scandi and Norwegian Sea to give us an easterly, certainly looks like the trop PV to our NW will retrograde with time.

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.