Nick F

Busy at work but just seen 12z GFS and UKMO singing from the same hymn shèet with the easterlies getting far south - think we have slidergate Dec 2021 on our hands! Will EC join the party?

A rain band similarly reaches North Wales, N Midlands to E Anglia early Xmas Day, but no snow on it, the cold enough air to the north not connecting.

GIF below shows GFS operational over the last 4 runs has trended south with snow on northern flank of rain band moving in from the SW on Christmas Day. Will it keep trending south? No support from the other models yet.

Although it would be nice have snow on Christmas Day, for those seeing family before the inevitable lockdown, be nice not to have to worry about snow and ice travelling. Not been looking likely for a day or so now and unlikely to come back, but between Boxing Day and New Year there's still potential for us in the south IMO. Marginal of course.

Height rises over western/central Europe with amplifying trough over the Atlantic aren't such a bad thing if they eventually extend north over the UK as per 18z GFS F.I. - as this can often lead to a Scandi high - especially with the current tropospheric profile of fragmented 500mb vortices moving around to our NE and to our west. So long as we keep that deep cold over Russia and NE Europe.

There's always a strong surface high over Greenland plateau, even if 500mb heights are low, due to the dense cold air at the surface over the extensive ice sheet, though a chart showing mean sea level pressure is misleading as the plateau is high, so pressure is adjusted as if it were at the surface. But in the coming 10 days 500mb heights are forecast to be anomalously high over Greenland when usually they are low.

Worth noting that by the end of coming week, there is potentially a developmental baroclinic zone moving into Bay of Biscay, with models developing and deepening a secondary low to the NW of Iberia in the left exit of jet streak / forward side of shortwave trough. Where this low then goes is perhaps causing the differences in how far low pressure extends NE over the UK or remains supressed like 12z UKMO and now 18z GFS. GFS Christmas Eve UKMO Christmas Day 18z GFS doesn't really deepen the low that forms but it at least moves east across France, 12z UKMO did for a time before moving it east towards southern Germany and it dissipating , 12z GFS swung a developing secondary low NE towards and over the UK for Boxing Day, so low pressure was too far north. But if this low moves east like UKMO and now 18z GFS then we are seeing the colder air extend further south over the UK.

Patience is a good virtue that some seem to have lost on here and many in the modern world in general. We live in a culture of instant gratification, where you can click a few times and get up-to-date news or data immediately. Not like the pre internet days where you relied on the Countryfile forecast every Sunday to see what may be in store for the week ahead or the the after the new bulletin forecasts for more short range forecasts. When some sees something they like and may get it soon but then they don't get it, it seems some can't cope. So what if we have to wait until the New Year for snow. Still 2/3rds of the winter left. And it's looking like one of those winters where we could see more high latitude blocking episodes, particularly in the second half IMO. But, my advice too, wait a few days to see if the models settle down and agree over several runs on the way forward. Things will always crop up with new data on reach run to change the direction of travel in the models at range of more than 6 days ahead. The problem I see now that models have picked up on is the jet extension over the Atlantic in the week ahead being fuelled by a huge temperature contrast between a frigid Canada and warm USA, that low over Florida Singularity mentions being sucked in adding extra fuel too. This is driving a strong zonal jet and this extending out into the Atlantic pushing lows towards NW Europe, this in the absence of strong TPV over Greenland and a block instead. You couldn't make it up! We really need the jet to amplify again over the Atlantic to help get the cold south, but it remains rather flat upstream over N America for a while.

I think it would be prudent not to cast too many forgone conclusions this far out for Christmas onwards, even using the ensembles. There are still a few days for the cold to correct south. Though the trend seems to be a retreat north of the cold over recent runs, at least for the Christmas weekend. The EPS and GEFS T850 mean and anomaly trend gif below for Xmas Day shows the cold air has retreated north over the last 4 runs of both models: EPS last 4 runs for 00z 25th Dec GEFS last 4 runs for 12z 25th Dec Though interestingly EPS shows colder air further south over Europe to our east than GEFS, it seems that GEFS is keener to build ridging further north into SE Europe pumping mild air across the central/S Europe. Past Boxing Day, the spread suddenly becomes quite large as a result of the uncertainty of what happens with Scandinavian Vortex and also troughing extending east across the Atlantic. Do we keep the southerly tracking jet or does it try to turn NE and pump rising heights over Europe like 06z GFS does and we all get mild? A swing to cold pushing south/southwest between Boxing Day and New Year certainly still an option. The mean certainly turns colder after the 26th: Don’t think the models handle on the progression of the MJO is helping. This stalling in 7, perhaps in response to Super Typhoon Rai, is likely the cause, but once that has gone, the models should continue the MJO wave towards 8 and we may see more amplification over the Atlantic rather than the flatter solution the GFS ops show in extended range. The MJO likely to fade in 8, as the colder than average La Nina SSTs further east over the central/eastern tropical Pacific weaken the MJO convection progression east.

Yes, may well do. UKMO and GEM are taking the lows in across France and Iberia, but GFS is still further north on the 12Zs. One thing I've noticed as an improvement on yesterday's runs is higher heights over the GIN Corridor, meaning blocking is extending further east to our north - which prevents the low heights over Scandi spreading too far west and southwest and phasing with low heights over the Atlantic which forces lows too far northeast over the UK. Instead the blocking is keeping lows tracking east just to the south of the UK. All heading in the right direction ... for now. But we aren't out of the woods yet.

A raw day for Christmas Day from UKMO at T+168, with a bitter Ely or SEly off a cold near continent. Wintry showers for the northeast and rain showers for the southwest, but dry elsewhere. Quite like the broader synoptic situation too, deep cold not far away to the east and northeast. So many different options on the table this far out for Xmas Weekend, but overall it's looking cold, snow potential too far out to worry about, looks even colder still from Boxing Day to the New Year. The high latitude blocking doesn't look to relent and the jet stream looks well supressed to the south, keeping us locking in the cold airmass, which may turn much colder.

Blocking area of high pressure will be with us through into early next week, but it looks turn increasingly unsettled as we head into the Christmas Weekend. With cold air not far away though, we could see some snow, but low confidence for now. View the full blog here

Don't panic those in the south, although there's cross model agreement on the 00z runs, there's still time for the lows to correct south. And even as it stands, there is still a chance of snow in the south, albeit marginal. GEFS and EPS for London still show plenty of members colder than the high res run over Christmas. I don't think the models will have a confident handle this far out on the shape and position of the block around Greenland and Iceland later next week, blocking is a synoptic that they tend to verify less well with at range. The shape and position of the block will affect angle and latitude of approach of the upper trough and surface low pressure moving east over the Atlantic next week and also the track of the secondary low that develops off the east coast of Greenland early next week, which is modelled to drop south over Norway and towards Denmark - behind this the deep cold air. That's not to say the 00z runs aren't right though and I'd say that the north has the greatest chance of being colder with a chance of snow over the festive period, the south lower chance and rather more marginal, but not negligible for snow over Christmas.

Evening. Hopefully we've seen the back of the GFS's brief jaunt of blasting the block away with Atlantic westerlies taking over during Christmas. One has to wonder whether the model had been overreacting to Typhoon Rai by stalling or backtracking the MJO, as shown by its RMM plots, so in effect flattening too quickly the amplification and trying to revert to La Nina background forcing, when in reality the MJO is moving through 7 towards 8 - albeit slowly. The descending -eQBO, so I've read, has a positive effect on amplifying the MJO wave too, as the easterly winds descending into the lower tropical stratosphere can enhance convection in the MJO wave. Which in turn amplifies those Rossby Waves poleward from the Western Pacific. Still looks like MJO will slowly move through 7 and into 8 for the rest of the month and into early January, meanwhile, looks like a spike in AAM from GFS to compliment the forcing from MJO. All of which suggests a prolonging of the high latitude blocking signal to the end of the month IMO, but perhaps into early January too. EC weeklies mirrors this too. In the medium range, the broadscale pattern is heading in the right direction now as models begin to agree on the direction of travel over the festive period, but we are a good few days away from knowing how cold it will get and where might see snow. A fascinating spell of model watching in the run up the Christmas for sure.

Quite the turn around by GFS 6 hours later, most of UK under sub -10C T850s by Boxing Day in a bitter easterly or northeasterly. Lets hope 12z EC keeps the momentum going. We are a good few days off from being out of danger, i.e. the block over Greenland goes too far west allowing cold air / low heights to spill west from Scandinavia to our north meaning low pressure to our west and southwest with a west based -NAO. Or the blocking high collapses, which is less likely.

It's hide behind the sofa as the 12z EC rolls out later then! One of the model's high res runs this morning is very wrong with the way it handles 1.) the potential trough disruption (or not) over the Atlantic as upper troughing moving east encounters the Griceland/NE Atlantic blocking, and 2.) movement and breakdown of blocking high, and 3.) the movement of deep cold pooling and lows dropping south over Svalbard, Scandinavia and NW Russia. Think GFS op may be too progressive in breaking down the blocking ridge moving north next week, the GEFS look more robust with the ridging over Iceland/Greenland. But we'll see over the next few days who's on the money ...

GFS is not all that bad for Xmas Day, feeling seasonal with temperatures 2-5C at best generally with a cold SE flow off the near continent. Of course, not as cold as EC, which has a strong bitterly cold easterly with snow showers spreading west across many areas on the 25th. Obviously the models will change for the big day, but both showing cold over the festive period. GFS green for most though.

All goes west-based -NAO by day 10, but least it's that far out, not worth worrying about anything that range when there's so much disagreement between GFS and EC high res runs after 6 days out. The path ahead still looks cold and increasingly wintry around Xmas weekend and beyond to me.

If only we had more faith in GEM than GFS, looks stonking to me Slack and cold easterly.

GEM to calm the nerves a bit? Premature to write-off the resilience of the block just yet, 12z GFS looks way to progressive to me...

Overall, the trends are in the right direction from the 00z EC, GFS and ensembles for coldies. Though there is still risks involved when we introduce the Atlantic into the equation when trying to get cold air in form the east under the Griceland block that seems to be agreed on by 10 and beyond. The trend GIF of H500 anomaly for Christmas Day below, using the last 6 GEFS runs, shows the shift north of the blocking anomaly up to the most recent run but also the introduction into the equation of Atlantic troughing from the SW for NW Europe: The risk is the Griceland block being too far north with mild air making too much inroads across the south. The north may not care though if they see a cold enough flow for snow! Still think the models still haven't got a handle after day 7, with the block more resistant and extensive over the NE Atlantic sector than they suggest. More runs needed to see where we are headed by the end of next week and into the Christmas weekend.

Morning. Not a huge amount of difference between 00z EC det from yesterday's 12z run, there seems to be a signal for a lobe of the TPV over northern Canada to surge across the Atlantic next week, but encountering blocking high over Griceland and NE Atlantic. Think the models this far out won't resolve well how the jet and lows moving in from the west across the Atlantic evolve when they encounter the block. Models are all too often too hasty to break down blocking, 00z EC more of a negative tilt than yesterday's 12z, with the approaching Atlantic trough as it pushes up against the block, meaning lows will disrupt and go under the block. How far south the lows disrupts makes all the difference on our small islands to the rain/snow boundary - given cold air will be trying to advect west to the north of the lows approaching the SW. But no point getting hung up on this from this far out, the models unlikely to have a handle how the block evolves against the pressure exerted from the west over the Atlantic. Blocking often ends up more resistant than the models predict at range.

Slow burner EC, but huge deep cold pool over NE Europe edging west and southwest at day 10. This anticyclone is going to wobble around a lot in the coming model runs, sometimes irritating us, sometimes teasing us as it lets in deep cold from the east. Far from resolved yet, but I remain positive that the centre of the high will drift far enough north at some point Xmas day onwards to allow deep cold to visit our shores. If not a day or two before.

I just had an ooh matron moment looking 12z GFS operational for after Christmas. With most models dropping a deep cold pool into western Russia from the arctic before Xmas, no surprise that GFS FI will start including charts showing the deep cold backing west over Europe towards the UK.

I mentioned yesterday about the formation of a tropical cyclone over the Western Pacific, today 96W looks highly likely to form a TC in next 24hrs https://www.metoc.navy.mil/jtwc/jtwc.html models developing a TC and move it west toward the Philippines perhaps eventually deepening into a typhoon, might be why the RMMs are doing cartwheels in phase 7 so slowing the MJO eastward progression into and through the western Pacific. and why the models seem uncertain with what to so with high pressure over NW Europe towards the Xmas period. Still optimistic it will orientate favourably eventually, ignoring the 06z GFS op, perhaps more emphasis on building into Scandi initially before perhaps retograding as the MJO wave moves towards P8 late December - which there is good indications it will Patience needed, as frustrating as it is seeing the operationals churn out uninspiring positions for the high to move into.