stodge

Absolutely - I thought there was more divergence at T+120 than I've seen at any time in the past 2-3 weeks. Normally I like to think the pattern is set at T+120 and the model output is broadly similar but tonight there are some notable cross-model differences so the evolution at the end of the week is far from resolved. The original notion of the Icelandic LP dropping south toward the British isles has been abandoned tonight in favour of a more Atlantic-driven evolution but the message remains a sluggish Atlantic allowing for amplification - I think Scandinavia is where we should be looking and a more anticyclonic evolution generally - whether we end with a mid-latitude or a high-latitude block or move from the former to the latter I'm not sure.

Evening all I don't drop in here during the day but I've seen signs of much wailing and gnashing of teeth so presumably the earlier output has been less than optimal for the continuation or even intensification of the current cold/chilly spell. It's only the weather - the world won't end if we don't get any snow, honest Anyway, let's have a look at what's throwing toys out of perambulators: 12Z GEM: T+120 takes us to Friday and there's a residual NE'ly over much of southern and eastern Britain with a ridge from the Atlantic into northern and western parts. A new LP is forming to the north and west of Iceland with another LP over Iberia. 850s still at -4 to -8 over the British Isles at this time. From there, the Icelandic LP deepens sharply but doesn't go anywhere. The Atlantic HP sinks south and allows milder W'ly air to encroach with a shallow LP to the south of Iceland. From there, the Atlantic pattern is set with heights rising over or just to the south of the British Isles and the Atlantic LP (not vigorous features at this time) moving from south west to north east with any heights over Greenland. 850s of +4 to +8 by T+240 so mild or very mild conditions in place. 12Z GFS OP - GEM ends the cold spell with a whimper in about a week's time. Let's see what GFS OP has to offer. By T+120 there are weak LP over southern Britain with the Atlantic HP ridging further north and west than on GEM. Uppers of -4 to -8 across the British Isles at this time. From there, the divergence with GEM continues - the LP to the north and west of Iceland at T+120 deepens and moves east before starting to fill and move south. The HP has slipped south but a ridge now extends across southern Britain with a W'ly air flow now in place. This brings less cold air across the British Isles. Moving on, the LP comes further south before moving to eastern Scandinavia - a brief but I suspect quite potent N'ly is replaced by a ridge of HP as a new LP swings NE towards Iceland. 12Z GFS Parallel - again, more divergence than I would normally expect at T+120 suggesting more uncertainty in the synoptic evolution than usual. Parallel phases part of the European trough with a new LP off the coast of North Africa and has the Icelandic LP further east than other models by this time. Again we have -4 to -8 850s over the British Isles by this time. Moving on from T+120, the Icelandic LP meanders to the Norwegian Sea but a secondary feature develops, deepens and drops south to Denmark with a N'ly down the North Sea. The Atlantic HP is still holding to the north of the Azores while there's a messy upstream profile. Despite the synoptics, the air mass over the British Isles remains cold with -8 uppers at this time. The Atlantic HP moves closer and builds in across the British Isles but then re-orients more north-south throwing a ridge to Iceland. Meanwhile, the European LP has moved all the way to the Ukraine as a significant feature with a residual trough back north west but with signs of heights building across Scandinavia from Russia - a fascinating chart. Milder air is slowly working in from the west at this time. 12Z ECM - both GFS OP and Parallel less keen to end the cold spell as quickly as GEM but I suspect ECM will be leading the way in the "return to mild" stakes but I could be wrong. Once again, more divergence earlier - ECM looks very different from the other models as early as T+120 with the Atlantic HP ridging across the British Isles to heights over Scandinavia. The LP has formed to the west of Iceland - it looks clear and cold with -4 to -8 uppers in situ over the British Isles. The Icelandic LP runs east but by T+192 is turning SE toward Scandinavia. A ridge of HP covers most of the British Isles by this time with a SW'ly flow to northern and western parts bringing in less cold air but uppers are still zero to -4 at this time. From T+192 not much changes quickly - the HP to the south west remains the dominant feature. The LP has slid SE through Scandinavia to the Baltic States with weak heights both to the north of Iceland and the north of Scandinavia. Milder air has got across the British Isles by T+240 but much colder air remains close by to the north and east. Moving further into FI, as I tend to do now, the T+312 and T+384 charts from GFS OP and Parallel respectively: Just as a clue, one of the two restores a raging PV to Greenland and NE Canada and the other one doesn't. 12Z Control is also very anticyclonic into FI. Finally, the 10 HPA charts from GFS this evening - Control ends with a weakened but impact PV over Eastern and Northern Europe. OP is similar but yet another warming underway in far FI. Parallel is the same - perhaps the PV showing more resilience tonight then seemed likely a couple of nights ago. Conclusion: - the retreat from cold continues apace tonight. GFS continues to hold the line through next weekend while GEM especially and ECM to an extent call time on the cold spell (yes, I know, it's been miserable because we've not had -20 uppers and six feet of snow) but we will have had 14 days of below average temperatures which isn't bad. The sluggish Atlantic, in the absence of strong zonal winds, offers continued possibility for amplification but I think the trend is now to look at an anticyclonic evolution into and past mid month. If we can get the Scandinavian trough to exit cleanly to the south east and can develop heights over Scandinavia, the possibility of a mid-latitude block becoming a high-latitude block remains for later in the month. The signal currently is for HP to be sitting to the south or west which isn't optimal for cold fans but if we can develop heights to the north and east, there's every chance we can get a more favourable orientation later in the month.

Evening all I commented earlier on the vagaries of model discussion. Some would claim any discussion of model output beyond T+120 to be pointless and maybe that's true but we all like to spot a trend and for that reason I find the model output up to T+240 to be informative as to which way the wind(s) are blowing. Any road up, on to this evening's output and let's see if the projected break of the current pattern to something milder mid-month is on track and any indication as to whether that will be a significant change or a temporary blip. 12Z GEM: T+120 takes us to next Thursday and one or two interesting changes to previous evolutions at this time. The LP over Iceland is not moving south but moving away NE to the Norwegian Sea with LP more pronounced to the far south off the west coast of north Africa. The Atlantic HP is oriented north-south from the south-west of Iceland forming I believe it's called an Omega block. There's a narrow ridge of HP into Scandinavia and all this leaves the British Isles in a col under calm conditions. 850s generally at -4 but some colder air filtering down from the north-west. Moving into Phase 1 FI and a new LP forms to the east of Iceland and runs SE to the northern North Sea bringing a N'ly flow across the British isles by T+180. The Atlantic HP is giving ground to the south and signs of heights falling over Greenland but the complex European trough over or just to the east of the British Isles dominates the weather. Perhaps the briefest of mild interludes before much colder air encroaches from the north with -8 850s widely across the British Isles by T+180 and -12 uppers fringing Scotland. From there, the LP to the north-east of Scotland phases with the trough developing over Southern Greenland and deepens in situ to become a significant storm by T+240 due north of the British Isles with some secondary features in its circulation. Heights are rising further south as a more Atlantic-based pattern takes over. Milder air takes over briefly but a new push of colder air is coming back from the north-west. 12Z GFS OP: - a breakdown from the north - who'd have thought it? I'm not sure about the GEM evolution but let's see what GFS OP has to offer in the same timeframe. By T+120 perhaps more progressive than GEM with a more defined N'ly and clearer signs of the Atlantic HP starting to withdraw south. Plenty of -8 uppers across the British Isles by this time. Moving on to T+180 we see the new LP which developed to the east of Iceland more directly north of the British Isles with the trough extending south to Iberia. Heights remain in the Atlantic. As might be expected, there's a less cold section through the frontal boundary but behind the cold front there's a new push of cold air from the north or north-west. From there, the GFS OP evolution begins to diverge from GEM - the Icelandic LP continues to move slowly south and by T+240 is over NE Scotland with a secondary centre over Cornwall. Heights remain over Greenland. There has been some mixing out of the coldest uppers by T+240 and some milder air is close to the south west presumably in connection with the secondary LP. I'll cover Phase 2 FI further on. 12Z Parallel: On then to the Parallel and by T+120 some divergences already in place. The Atlantic HP is throwing a ridge east across western Britain keeping the troughs to north of Scandinavia, close to the Azores and over central and eastern Europe. Uppers at -4 to -8 but perhaps more issues with fog and frost than snow from this set up at this time. By T+180 a new LP has developed to the east of Iceland and deepened to be a significant feature to the north of Scotland with the whole of the British Isles in a NW'ly air flow. Heights remain over Greenland and the Atlantic at this time. As we see elsewhere, a milder section in the frontal boundary before a new push of colder air to the north-west. Moving toward the end of Phase 1 FI (T+240), the LP fills and moves south to be over northern Britain but it's a complex trough with a small but vigorous secondary feature approaching from the south-west bringing a SW'ly flow to eastern and southern areas while a cyclonic flow dominates elsewhere. Uppers of Zero to -4 by this time. 12Z ECM - at T+120 closer to the GFS OP than the Parallel to my eye. The trough perhaps less defined to the east and uppers of zero to -4 for most but with some -8 air close by. Moving on into Phase 1 FI and by T+192, the original trough is now over southern France with a residual NE'ly feed to southern and eastern parts but a ridge of HP us producing calmer and more settled conditions further north while the LP sits far to the NE at this time. Uppers still -4 to -8 but milder air approaching from the west. By T+140 the Atlantic is back - heights to the south and, it has to be said, a broad but weak trough extending from the north of Scotland west - we're not talking 950MB storms heading across from Canada by any stretch. Looking into Phase 2 of FI - from T+240 to T+384, I'll offer the equivalent charts from GFS OP and Parallel from T+312 and T+384. OP first which ends very mild and the Parallel which keeps a broadly cold evolution throughout - the Parallel, in particular, looks set for further amplification. Looking elsewhere, 12Z Control ends zonal-ish but not zonal. Perhaps we're seeing GFS charts looking for a downwelling of westerly winds to pick up the jet and reduce amplification - that's been my thought for a couple of days. We aren't talking a monster PV or massive storm systems racing west to east - it's all a bit meh, if I'm honest. It just seems to suggest there's a thought of some residual westerlies as the PV fragments - could also be other teleconnections at work of course? Looking at the 10 HPA and Control ends with the -60 zone of the PV over Scandinavia and Eastern Europe. OP is similar but with a slightly colder and larger residual PV (-64). Parallel fragments the vortex more or less completely with some residual -60 remnants over Siberia. Conclusion: tentative signs of an easing of the cold pattern are coming into view at T+240 but it's far from resolved. GEM and ECM are keenest tonight to see an Atlantic flow return while GFS keeps the cold pattern a little longer. There's no great battleground between competing air masses which I find curious - it seems as though the cold just runs out of cold and the Atlantic, weak as it is, can move in. There are signs in Parallel at the far end of Phase 2 FI of further amplification and, as other have said, the ongoing SSW is throwing an enormous SSW-shaped cat among the atmospheric modelling pigeons. ECM is alone in fading the Greenland heights completely while other models bring the HP south and then east into Europe allowing the westerlies back in. There's a lot to be resolved.

Afternoon all I can only assume the clearance of the cloud has introduced some colder and drier air. Pleased to see the back of the slate-grey overcast - suspect if we can keep clear skies there will be a sharp frost tonight.

Afternoon all I've come on here over the past couple of weeks every evening and done a thorough analysis of the post T+120 picture (FI is from then in my view currently for looking at the larger picture, not for the "is it going to snow in my street?" question which is for the hi-res models and there are plenty who swear by them and indeed swear at them). FI itself splits into two in my experience - stage 1 FI is T+120 - T+240. That's the important time when looking at the trends, the signals, the themes. The divergences across and within the models increase from then as each scenario plays out but within that range you can see the immediate consequences of different scenarios - some runs you get a smaller series of variable outputs, sometimes the differences are much greater. By T+240 therefore there are a small "cluster" of recognisably similar scenarios (ECM and GFS and indeed other models all have these). It's unlikely any of the clusters will verify exactly at T+240 as shown at T+0 but it's more about a direction of travel. The next couple of days will show if some of what we are seeing in Phase 2 of FI (of which more anon) manifests in Phase 1 - if it does, it will attract growing support across the Ensembles as we move down to T+144. Phase 2 of FI is beyond T+240 - over the past 10 days, I've seen all sorts from GFS - a breakdown from the south, a breakdown from the north, a breakdown from the west, the arrival of deep cold from the east, the arrival of deep cold from the north etc, etc. Between Parallel, OP and Control (let alone the GEFS) there are a huge number of evolutions to view. ECM has 50 and if you look on weather.us and go through them at T+360, you'll find any number of divergent evolutions. In short , looking beyond T+240 is, as Peter Snow used to say on election nights, "just a bit of fun". T+120 to T+240 is the period of interest and what you want to see is consistency (I only look at the 12Z output - if someone else concentrates on the 00Z output that's fine) both across and within the models. Sometimes we have it, sometimes we don't: Yesterday's 12Z from GEM, GFS OP, GFS Parallel and ECM at T+240

Evening all The first day of 2021 in lowland East London was bleak and still - maximum at 3c so below average but tonight we've had drizzle. That's what happens with slack airflows in winter. The output last evening was very strong for cold fans into FI with the notable exception of ECM which did suggest a milder evolution by T+240. Let's see where we are this evening. 12Z GEM: T+120 takes us to Wednesday next week. Heights over Scandinavia from a ridge from the Russian HP and heights extending south-east into the Atlantic from Greenland. The remants of the current trough over Poland and a new LP developing to the east of Iceland. The British Isles has a slack ENE'ly flow for southern areas and calm conditions further north with a light WNW'ly flow for northern areas. The lowest 850s across the south with a hint of something less cold further north and west. By T+180 the Icelandic LP has moved SE but has filled and phased with the trough over Poland while a new LP has formed and deepened over Iceland. Heights remain in the Atlantic with a broadly NW'ly flow over the British Isles. Uppers at -8 for most so still cold. From T+180 the new Icelandic LP slips SE down the North Sea and into Europe where it forms a complex trough through the central and eastern Mediterranean. Heights develop over Iceland and northern Scandinavia and the British Isles is covered by a slack E'ly flow by T+240. Perhaps a brief less cold interlude but uppers by T+240 are again at -8 so still cold and colder air out to the east. 12Z GFS OP - a solid enough evolution for cold fans from GEM. On then to the American OP and at T+120 the British Isles is in a col with LP over northern Germany and moving south from the east of Iceland while heights remain in the Atlantic and over Scandinavia. Uppers over the British Isles remain at minus 8 by T+120. The LP to the north is a fairly shallow feature and quickly phases with and fills into the European trough but by T+180 a new and more vigorous feature has moved down from Iceland and is close to northern Scotland with a WNW'ly flow over northern and western Britain while calm conditions prevail further south. Uppers remain at -4 to -8 with a new cold feed coming down with the new LP. The LP moves down through the British Isles and by T+240 is over southern France. Heights build from the Azores HP north east to the west of the British isles with perhaps a ridge to Scandinavia (we'll see) while small areas of LP move to the north of Iceland. Heights over the British Isles remain at -4 to -8, lowest in the south under a residual NE'ly flow. Moving further into FI, the Azores HP topples into France and Iberia and Atlantic air quickly moves round the top re-introducing milder air by mid-month. At the end of FOI, a new trough digs into northern Scandinavia but it's a more mobile pattern with a brief ridge of HP to the west of the British Isles and a new LP developing to the south of Greenland. It's milder through into FI and I note the main lobe of the PV is moving back west across Siberia to be over Scandinavia by T+384. 12Z Parallel - a disappointing evolution for cold fans from the OP going into FI (an outlier perhaps?) but let's see what the Parallel has to offer. No great surprises from the T+120 if I'm being honest. Uppers at -4 to -8 generally. Moving on, the LP to the east of Iceland sinks south through the west of the British Isles and phases with the Atlantic trough which has been shunted far to the south over Iberia and North Africa. By T+180 the complex trough has centres over Biscay and the Balearics with heights over Iceland and Greenland and this means an E'ly airflow for the British Isles. Perhaps a slight modification of the cold over the British Isles but colder air is again heading towards our shores from the east. From there, the European LP migrates east and phases with a new deep LP over northern Scandinavia. There's a new LP just to the east of the Azores and a weak ridge in the Atlantic. The airflow over the British Isles is a slack N'ly by T+240 and the uppers have moderated to -4 generally by this time. Moving further into FI and it's a tale of LP encroaching on the British Isles from both north and south. From the north it's a tale of the new Scandinavian LP moving west and spawning a new centre to the south-west of Iceland. The airflow is from a W'ly direction by T+312. This has brought less cold air into the south but it's still chilly further north at this time. By the end of FI the Atlantic is back in charge with strong heights over Iberia and a very mild SSW'ly flow. However, cold PM air isn't far away to the east by T+384 and overall the mildest air is confined to the far south. 12Z ECM - more evidence of the GFS trying to call time on the cold pattern in FI but it's still FI at this time and we'll need to see it come within at least T+240 which it hasn't yet. Now, for ECM which perhaps picked up on a change to the pattern last evening. T+120 contains few surprises as you might expect. The first LP to the east of Iceland is pushed out of the way by the Atlantic HP but by T+192 a new more vigorous LP has developed to the east of Iceland and is attempting to phase with the residual European trough which is centred over the southern French coast. Uppers over the British Isles remain at -4 to -8 with a new push of colder air from the north west. From there, we get a very different evolution with a small but vigorous LP over Cornwall by T+240 - I can only surmise this is the interaction of the two troughs and this LP is part of a large and complex trough over northern Europe with a new Atlantic LP negatively aligned to the west and residual heights extending from Greenland south. The air mass over the British Isles at T+216 is very cold but the new LP introduces significantly milder air into southern and western areas by T+240 but it's still cold for northern parts and I'd be surprised were plenty of snow not to come from that feature for the luckier northern and higher areas. Looking elsewhere, 12Z Control brings back the Atlantic by T+228 but by the end of FI the pattern is negatively aligned with colder air back over northern and eastern parts. As for the 10 HPA, 12Z Control has shredded the vortex by T+384, it's not much better on the 12Z OP by the same time. 12Z Parallel has raised the 10 HPA temperature to -56 at its lowest (unprecedented for mid January in my experience). Conclusion: there's a growing signal from GFS to end the cold pattern from the west just before mid month though the extent to which the ongoing destruction of the PV is impacting on medium range modelling is one for others to debate. Most of the models keep us cold or very cold up to T+240 and one of two of the proposed breakdowns look very snowy but I suspect the modelling into FI reflects some real issues as to how to simulate a rapid downwelling of an SSW this early in the winter.

Evening all First, a Happy New Year to everyone on the thread and across NW especially the long-suffering Mods with whom, as you can see, I am shamelessly trying to curry favour. Cold and still here in lowland East London with a maximum of 2c and a number of locations in the south-east had an ice day. Last evening, there was a lot of uncertainty in the output especially from T+144 to T+240 so let's see if any of this has been resolved. 12Z GEM: T+120 tonight takes us to next Tuesday. Between a slack area of LP over Northern Italy and a ridge of HP from the Atlantic across northern Scotland to Scandinavia, an E''ly airflow covers much of the British Isles. As we are told these days, uppers mean nothing and we should be looking at dew points. Despite that, 850s are -4 to -8 generally. From there, a new LP develops to the north of Iceland and starts to move SSE toward the British isles as the HP splits with one lobe setting into the Atlantic. The new LP looks to be phasing with the residual trough over the Ukraine. Shortwaves over France but the Atlantic LP extends a ridge while the Atlantic LP is near the Canaries. Perhaps some slightly less cold air moving in from the north-west. Moving on and the LP moves south over western parts of Britain and Ireland and then more SE to be in the Channel by T+240. An E'ly flow re-asserts across northern parts as heights build back over Iceland and Greenland. Uppers remain -4 to -8 with some colder air still lurking in the North Sea to move across northern parts. I'm sure I'll be told it's all cold rain - I don't believe it (as Mr Meldrew would say). 12Z GFS OP - well, a cold or very cold evolution from GEM to start the evening. At T+120 no surprises from GFS OP. Uppers of -8 generally. From there, as you would expect, some differences emerge. The LP develops to the north and tracks south though well to the west of the British Isles. It phases with the continental trough which moves back over the south of England and across Ireland. The airmass over the British Isles remains cold at -4 to -8 850s with a SE'ly developing over northern Britain by T+180. Heights remain to the north west and north east. From there, the LP first shifts to the south of the British Isles and then moves ENE forming, by T+240, a complex trough over Central and Northern Europe. Heights in the Atlantic but a new LP forming to the west of Iceland. 850s remain at -4 to -8 at this time. After T+240, the LP over Iceland goes nowhere and the trough begins to fill and new heights build over Scandinavia extending a ridge SW across the British Isles which would suggest a drier evolution. The Atlantic airmass is trying to push in at T+312 but the Scandinavian block is holding firm and keeping the cold air over the British Isles. Further into FI, the HP moves SW to be over the British Isles as a deep LP and a cold lobe of air sink into eastern Scandinavia. There's a hint of ridging back toward Greenland by T+384. It's milder in the south but cold air is still over the north-east with some frigid air over Scandinavia. 12Z Parallel: another cold evolution from GFS but with that reversion to milder we've seen for the last few days albeit still at the far reaches of FI. Let's see what Parallel has to offer this evening - well, at T+120 no surprises. Uppers -4 to -8. The T+120 indicates instabilities to the north of Iceland and by T+180 a new trough has formed which is moving SE towards the North Sea. Heights remain to the west and a NW'ly airflow covers much of the British Isles. There's cold air to the east and moving down from the north and north west. The evolution to T=240 gets more complex - the trough effectively splits with one section moving quickly south into Europe and forming a new major LP in the Mediterranean which keeps an East or North-East flow across southern and eastern Britain. Part of the energy remains to the north west and forms a new LP to the west of Iceland but between them a small ridge of HP stretching from the Azores through central and northern Britain to Scandinavia keeps most settled and dry. Less cold is moving into Ireland but it remains cold or very cold over most of the British Isles. The milder air moves in from the west but with heights building across Scandinavia a battle ground develops and Atlantic LP disrupt against the cold block and shift SE across southern Britain into Europe. By T+312 the uppers are telling the story with -12 850s off the east coast and positive uppers over Ireland. As often happens with these battlegrounds, neither side wins - by T+384 there is a strong anticyclone over Scandinavia and a SE'ly flow over the British Isles with the Atlantic pressure systems driven back west. The 850s are turning much colder with -12 850s crossing into eastern Britain and -16 to -20 uppers in the flow over Europe so this could be dry and frigid air. 12Z ECM - I have to say the Parallel looks a real winter treat for some even if there's a temporary milder incursion. On then to ECM - the T+120 is as you would expect -the profile of the heights marginally different but -8 uppers widely across the British Isles. As with other models, new LP develop to the north of Iceland and start to move south but this phases early with the European trough and remains to the north and east so the airflow over the British Isles is a NW'ly but the uppers remain -8 so still cold. From there, the LP moves south r south west and phases with the Atlantic trough - with heights now rising to the south, the wind flow shifts round to a milder W'ly direction and I wonder if we're seeing a west-based negative NAO set up developing. Milder air coming into the south but still cold to the north. Looking elsewhere, the 12Z Control is mostly cold throughout with milder air encroaching at the very end of FI in quite an unsettled synoptic pattern. As for the 10 HPA charts, 12Z Control instigates an initial split at T+78 and while the PV regains some cohesion, it is split apart by a new warming in FI and by T+384 is in fragments. It's a similar evolution on the Parallel which begins the split at T+60 while the OP is much the same. Conclusion: with the notable exception of ECM (which could be an outlier), another very strong evening's output for cold fans. The Parallel offers a huge promise of some really frigid air for mid month if we can get the Scandinavian HP in place (always a better bet for longer term cold than a Greenland HP). There's a lot of uncertainty into FI as you would expect - the extent to which that is being driven by events in the strat I don't know but the split now looks on at 10 HPA and I leave those with greater expertise than me to determine the speed of downwelling and the impact. The west-based negative NAO remains a concern but the overall picture remains cold and we're seeing from GFS the possibility that might develop similar to other recent fast downwellings in terms of allowing frigid air to transfer west from Siberia towards Europe. The times they remain interesting.

I wish you'd told me this before I started doing my nightly analysis !! I do agree the degree of uncertainty from T+144 to T+240 in particular is very high tonight. A couple of days ago it was all about LP moving up from the south and interacting with the cold block but now it's become more complex with LP coming down from the north and phasing with the European trough over the British Isles. I really don't know tonight.

Evening all A little earlier than usual to take a canter through the evening output. Another chilly day in London and while the SE Regional Thread seems to be full of wailing and gnashing of teeth about the lack of snow for the moment, last night's output suggested we would see some of the white stuff in time. Onwards and sideways.. 12Z GEM: T+120 this evening takes us to next Monday, January 4th. Between a ridge of HP stretching from Scandinavia west and south-west to an HP in mid-Atlantic and an LP in the Gulf of Genoa (where else?), an E'ly airflow covers most of the British isles. Dry for the north you'd think but a possibility of precipitation further south and east. 850s of -4 to -8 widely at this time. On to T+180 and the HP has now established across the British Isles (or rather the ridge has moved a little south). Any residual E'ly influence is confined to the far south and the eye is drawn to a new LP forming to the north-west of Iceland in what looks some very cold air. The airmass over the British Isles remains cold at this time. The LP starts to move south but all that does is pass the HP to the south of the British Isles and allow milder air to move in round the top of the HP which by T+240 is over northern France. Actually, it's not that mild (it is less cold in the south) and there's some colder air filtering into Scotland. It's one of those instances where the 500s and 850s don't tell the same or even the whole story. 12Z GFS OP - an intriguing but slightly disappointing GEM for cold fans and the potential phasing of the Icelandic and Atlantic troughs doesn't augur well so let's hope the GFS OP keeps the faith (if there is such a thing). At T+120 no real surprise with a cold ENE'ly covering most of the British Isles. 850s of -4 to -8. Moving on 60 hours later and not much has changed. NO sign of cyclogenesis over eastern Greenland but a deep LP is moving to the north of Scandinavia. For the British Isles, a light E'ly flow is maintained. 850s not much different by this time. By T+240, the LP has dropped south into Scandinavia and a N'ly flow is passing over or just to the east of the British Isle. Strong heights from the west of Ireland north-west into Greenland by this time. The 850s show a slight easing of the cold but there's some really cold air in that northerly (-20 uppers for those who like that sort of thing). By T+312 the evolution has got messy - the LP has become stationery over the eastern Baltic with come very cold air in its circulation. A slack area of LP sits off the south-west of England with a ridge in mid-Atlantic. The airmass is becoming less cold over the British Isles by this time. By T+384 the cold spell is over and we have a more typical Atlantic profile. 12Z Parallel -as with last night, the OP lessens the cold spell in mid January and perhaps in tandem with other teleconnections we will see a brief easing of cold in the middle of the month. The T+120 has no surprises on the Parallel. From there, the divergences begin and by T+180 the British Isles are in a pincer between LP moving up from the south and another LP moving down from the north-west. The airmass over the British Isles at this time remains cold with -8 850s widely. As expected, the two LP systems phase over the British Isles with the LP from the north-west sinking south into France and keeping the air flow from the east. There's a bit of mixing out of the cold uppers by T+240 but I think you'd be looking at a lot of snow from this feature. From there, the trough remains a complex and dominant feature with a new LP developing to the south-west of Iceland and another feature approaching the south-west of Britain at the same time. Less cold air is steadily moving in from the south west but it remains cold over northern and eastern parts. By T+384, as with the OP, the Atlantic is back in charge. 12Z ECM - so more output suggesting an easing of cold in mid-month. On then to ECM and at T+120 no surprises but the 850s are uninspiring so for fans of cold sleet or rain great news - I'm NOT going there. By T+192, the HP has receded west and a complex trough covers the British Isles with centres to the north and south-east. Some very cold air in the vicinity but not so much over the British Isles - however, still a cold airmass to work with for fans of snow. From there, a dominant centre forms, deepens and runs south to be over Ireland by T+240. By then, the less cold sector is over southern Britain but the airmass remains on the cold side and the big question is where that LP will go from there. If it runs south into France, there's plenty of cold air to the east and north. Looking elsewhere, 12Z Control goes milder in FI for southern areas but cold remains close by and never really leaves the north before returning south but it's another messy evolution. As for the 10HPA charts - 12Z OP initially splits the PV at T+114 but Dr Who-like, it regenerates before coming under a new assault in FI which renders it weak and vulnerable by T+384. 12Z Control achieves the split at T+336. Parallel achieves the initial split at T+78 and a further split in FI. Conclusion - a greater degree of uncertainty than for many days in the models tonight. FI starts at T+144 and the evolutions from there are many and varied. What will happen to the north and north-west in terms of cyclogenesis seems to be the new question or theme of the night (and we seem to have a different "theme" each evening). GFS ends the cold spell by mid month from the west. ECM certainly keeps things interesting and GEM is uncertain. The evolution from T+144 to T+240 looks fraught with uncertainty tonight.

Evening all A chill day here in lowland East London - just 3c and slate coloured skies all day. That's what can happen with a slack N or NE'ly flow - plaudits to those who saw some snow, even temporarily. The models last evening promised 7-10 days of cold and then a potential "battleground" with LP moving up from the south and of course lurking behind every FI evolution is the threat of the dreaded west-based negative NAO. Let's see what tonight has to offer. 12Z GEM: - T+120 takes us to Sunday which will be January 3rd. It remains cold over the British Isles with a light nE'ly flow. A ridge of HP extending out of Scandinavia to the south of Iceland holds an Atlantic trough back while LP to the south-west of the Azores seems to be going nowhere. To be honest, this doesn't feel as positive as the last few evenings - there are clear signs of more activity around NE Canada but let's see. For now, -4 to -8 uppers cover the British Isles. By T+180 not much has changed. HP dominates to the west and north-west with LP to the south centred over Morocco and the Russian HP appearing far to the east. The Atlantic has quietened. The air mass over the British Isles remains cold. Moving on and as the HP retrogresses slightly, the LP over Iberia deepens and moves NE into central and eastern Europe. By T+240, the HP is to the south-west of iceland and signs of a new LP forming off the east Greenland coast but for the British Isles the air flow remains between north and east. The swing to a more N'ly orientation keeps the air mass cold or very cold. 12Z GFS OP - a strong opening for fans of cold from GEM. On then to the GFS OP and at T+120 no great surprises. The HP seems better defined on GFS and perhaps a notch further north - I also like to see the weak heights over Scandinavia. 850s of -4 to -8 over the British Isles at this time. On to T+180 and again it's not too far removed from GEM with a slack ENE'ly flow over the British Isles and the HP ridging SE from Greenland. The airmass over the British Isles remains cold. From there, the Greenland HP continues to control the weather with its ridge SE to the west of the British Isles keeping the jet well to the south and leaving LP to affect the Azores but there's a hint of phasing with the trough over north-west Europe and a new LP has moved NE across Europe to be over the Baltic leaving the British Isles in a slack NNE'ly flow. Needless to say, the 850s suggest a continuation of cold or very cold surface conditions. Further into FI and a lobe of HP detaches from Greenland and slips SE just to the west of Ireland which would produce a more settled outlook for southern and western areas with a NW'ly flow by T+312 and a hint of a new trough forming over Iceland. Perhaps a notch less cold but you'd think there could be some sharp frosts. For those wanting milder conditions, far FI offers plenty of hope - the HP slips down into Europe and opens the door for milder air and an Atlantic flow to return by T+384. It looks a bit west-based negative NAO to my eyes. 12Z GFS Parallel - let's see if it follows the OP or goes its own way. by T+120 the HP is perhaps a little closer than on other models and signs of more Atlantic activity especially off Canada and close to Greenland. by T+180 the anticyclone dominates the north of the British Isles ridging into Scandinavia and across to the south of Iceland while a deepening LP over Portugal strengthen the E'ly flow in the far south. The uppers are generally -8 at this time so still cold across the British Isles. The LP over Portugal tries to come north, then north-west but is beaten back by the block and by T+240 there is an elongated trough from northern France to the Azores while to the north a ridge of HP extends from the southern tip of Greenland to northern Scotland. Between these, an E'ly flow continues. Any milder incursion is very brief and by T+240 uppers are again at -4 to -8 across the British Isles. Hints again of a new LP over Iceland. Further into FI, the evolution starts to get complex and a bit messy. The HP ridges south, splits the trough and leads to the formation of a new HP cell. One LP dives SE into the North Sea but another shortwave phases with the Atlantic trough. Despite all that, the airmass over the British Isles remains cold with the LP dragging down another push of very cold air. From there to T+384, it's a pretty rapid reversion to a more typical Atlantic-based pattern 12Z ECM - T+120 brings the Atlantic HP close to north-west Britain ensuring settled conditions while the south and east remain under the influence of the continental trough. -4 to -8 uppers at this time keeping it cold. Moving on, the HP extends back west in a classic retrogression pattern as new LP develops to the north of Iceland and the jet is held far to the south. It gets less cold in northern areas while the south remains cold. by T+240, one LP is over northern Scandinavia while another is over southern France and a slack but potent N'ly is spreading across the British Isles with HP remaining to the west and north-west. Progressively colder air is spreading south with -8 uppers coming into Scotland and some even colder air behind that. Looking elsewhere, 12Z Control has plenty of snow opportunities and ends with a battleground between air masses which, to be fair, the milder flow seems to be winning. Onto the 10 HPA charts and the 12Z OP ends with a split and diminished PV - the current warming does most of the damage and a smaller warming finishes off the PV in the second week of January. 12Z Control produces another very strong warming at the end of FI but the PV is still just about intact. 12Z Parallel initiates the split at T+108 and the PV looks broken and beaten throughout the rest of the run. We're used to a huge vortex with core temperatures of -88 - this season mid-January has a vortex with a core temperature of -64 which tells you all you need to know. Conclusion: - GFS toys with ending the cold spell in mid month, whether that's a signal (MJO?) or a reset to "normal" conditions in the absence of a signal remains to be seen. Within the immediate it's cold and as far out as T+240 it also remains cold. ECM toys with another round of retrogression to bring a new cold blast from the north and in the context of a split vortex, that would release some serious cold. GEM keeps the focus more to the north-east. The 10 HPA charts continue to show the PV weak and disorganised, if not completely split.

Evening all A chilly day in lowland East London rather than a real wintry one but there's an expression about getting the cold in first and then worrying about snow. Last night's models continued the very positive outlook for fans of cold and snow in the coming 7-10 days though with some small complications in far FI as you might expect. As we move into 2021, let's see how the year starts (meteorologically at least): 12Z GEM: T+120 takes us to January 2nd and the cold theme remains evident with the LP now in the Gulf of Genoa and heights building toward Iceland leaving a weak NE'ly flow over the British Isles. Uppers -4 to -8 by this time. The HP eases back west to the south of Iceland and re-orients rather than retrogresses and a new trough moves down into Scandinavia keeping a NNW'ly airflow over much of the British Isles but the emphasis on drier conditions especially for western areas. The briefest of milder incursions before colder air returns by T+180. I'm not quite sure what to make of the evolution beyond T+180 - the heights withdraw further west and LP starts to move down from the north within a lobe of very cold air moving through Iceland (uppers below -20). This pulls the wind direction back to a W'ly and draws in some milder air over the British Isles. Looking at the NH profile, I see a piece of the PV has detached and moved round the top of Greenland and started to come south. Is it heading our way? 12Z GFS OP - well, an interesting offering from GEM to start the evening and perhaps an example of what synoptics could develop with a disorganised PV. Let's see what the GFS OP makes of all this. At T+120, we have the NE'ly airflow over the British isles but the eye is drawn to the disturbance to the north of Iceland and how that might develop. -4 to -8 uppers over the British Isles at this time. From here, the Atlantic HP moves in briefly before starting to retrogress toward Greenland and this allows new areas of LP to form to the north of the British Isles and move south in the N'ly airflow. The 850 profile at T+180 is informative with a burst of very cold air on the western flank of the LP (is it a Polar Low - well, not really). The LP deepens and moves south through Ireland and by T+240 is just off the tip of Cornwall. Two other observations - there's a tiny LP to the south of Greenland which doesn't seem to belong and I'm wondering if we're heading toward a west-based negative NAO if the LP over south-west Britain phases with the Atlantic LP to the west of the Azores. Meanwhile, the E'ly over the British Isles is drawing in increasingly cold air. Looking on, I was wrong to worry about the west-based NAO - instead, the Atlantic LP phases with the small LP to the south of Greenland but northern blocking has thrown a ridge south over Iceland and down to the north of the British Isles and by T+312 we remain in a chilly if not cold NE'ly airflow with the trough over central and southern Europe and heights over Scandinavia. By T+312 -8 850s are in charge. By T+384 battle is joined as the Atlantic LP tries to move in against the block of cold air. The temperature gradient just to the south-west suggests snow close at hand. 12Z GFS Parallel - another stunning run for cold and snow fans from the OP. On then to the Parallel and by T+120 the main elements are in place but perhaps the parallel is making a bit more of events in east Greenland with more defined LP development. Uppers over the British Isles -4 to -8 but I can see the rationale for the LP development with a wedge of less cold air moving up from the south interacting with the frigid air over Greenland and initiating cyclogenesis. To be fair, and slightly surprisingly, this doesn't really have any impact - heights build first to the north of the British Isles and then ease to the north-west with the HP centred to the south of Iceland leaving the jet far to the south and a slack airflow over the British Isles. While the south remains cold, some milder air is approaching the north of Scotland having moved round the top of the Atlantic HP. From here, the evolution becomes more interesting - to the south, the LP system over the Azores starts to move NE towards the British Isles but is only over northern Spain by T+240. Meanwhile, cyclogenesis has occurred to the west of Iceland and a new LP is heading south toward the British Isles which, by T+240, is effectively in a col but with the two troughs threatening to phase together over the British isles. In short, colder air is coming south as milder air is coming north. The phasing occurs and by T+312 a complex and elongated trough covers the British Isles with heights negatively aligned to both NE and SW. There's a calm airflow for most but a NW'ly for southwestern areas and a SE'ly for the far northern isles. The coldest air is mixed out by the phasing of the troughs but the uppers remain at -4. By T+384 it's just messy with heights to the north-east and a deepening LP over Biscay. Milder air is slowly encroaching by this time. 12Z ECM - so Parallel ends messy with any number of potential battleground scenarios as we move toward the middle of the month. Finally, to ECM. At T+120 no great surprises. From there, the Atlantic HP builds a ridge to Scandinavia with the remnant of the trough over Europe and a new LP to the east of the Azores. An ENE'ly flow covers southern Britain with the uppers -4 to -8. From there, the LP slows and deepens just off Portugal while the HP re-orients in situ throwing the ridge south as it partly retrogresses. The E'ly flow strengthens over southern Britain with -8 uppers in the mix as milder air inches through France. Looking elsewhere, 12Z Control is cold until the very edge of FI when there's a big push of much milder air (including +20 uppers over North Africa). The 10 HPA charts from GFS continue to tantalise - OP splits the strat at the very end of FI (January 13th) as does Control. Parallel achieves an initial split by T+180 but the PV re-forms and just about survives FI though it is much reduced. Conclusion: - more interest for cold fans though another night brings another theme and that's the attack from the south and/or from the north. All the models suggest an attack from the south into the cold block but this will be a real struggle if successful and obviously the opportunity for snowfall is there. GEM and GFS OP tease with a simultaneous attack from the north caused by cyclogenesis to the west of Iceland and that obviously also has interest. For now, 10 days of chilly if not cold conditions look assured - beyond that battle may be joined to the south and as we know such battles between competing airmasses are the perfect breeding ground for snow events. Someone will hopefully get lucky.

Afternoon all Seems to be plenty of weather about all over the region. In lowland East London, we had a foggy start but that's lifted and we now have cloud. Very little wind - indeed, calm conditions so nothing's happening quickly if at all. Perhaps the lightest of N'ly breezes. Temperature has topped off at 4c and, going out for my socially-distanced lunchtime constitutional (I find snarling at people very good at maintaining social distancing - it worked for me long before Covid), I thought it was a reasonable winter's day. It doesn't look as though the precipitation to the south will come this way - the LP looks to be centred in the Solent for now though I expect it to start moving NE.

Evening all A day of two distinct halves here in lowland East London - early heavy rain and strong winds led to a dramatic clearance to fine skies and an increasingly chilly breeze during the afternoon and into the evening. Will we get snow overnight and into the morning? A question for others on another thread... The cold theme persisted on last night's output with the end of 2020 and the beginning of 2021 seeing well below average temperatures for most but let's see some of the peregrinations on offer this evening: 12Z GEM: T+120 takes us into 2021 and New Year's Day. The new year starts cold and as heights build through Iceland, the airflow over the British Isles shifts NNE'ly with a shallow but complex trough over Scandinavia and the Low Countries and stretching down into France and Northern Spain. The airmass is cold with 85-s of zero to -4 widely and some colder air moving in. Not much changes by T+180 - winds are slack or calm with the LP now over Italy and a ridge extending WSW from heights to the far NE of Scandinavia. This is keeping the jet well to the south and 850s are widely at -8 as the UK seems to develop its on cold pool and I can imagine any snow cover leading to some very low minima under this synoptic set-up. On then to T+240 and again only slight changes - the trough extends from the Gulf of Genoa up over southern Britain with heights to the north building from Scandinavia through Iceland into the mid-Atlantic. Just a hint of a marginal easing of the cold airmass. 12Z GFS OP - a strong start for fans of cold from GEM this evening. As for GFS OP the T+120 picture tells a slightly different story. It's cold and we see successive LP areas within the trough develop over the Norwegian coast and swing south - not Polar Lows of course but they would accentuate any instability and trigger a new round of shower activity. Uppers of -4 to -8 don't tell the whole story of course but you'd think snow for anywhere with altitude and probability for lower ground as well. On to T+180 (January 4th) and the overall pattern looks very similar to GEM. A strong wedge of heights extends SW through Iceland and this is forcing the Atlantic to the south-east well away from the British Isles. A shallow but complex trough remains over Europe with centres over Italy and southern Britain. Uppers of -4 to -8 suggesting very old nights with moderate to severe frosts. On then to T+240 and a chart that reminds me of the Beast from The East of March 2018. In the interim, the core of heights has transferred to Greenland but the Atlantic is fighting hard to come back in from the south and the wind direction has moved from East round to a more ESE'ly point. Milder air is coming up into southern Britain but it remains very cold further north. The evolution from there reminds me of the discussion about west-based negative NAO synoptics the other day and this is one of those. The LP tries to move north and draw up a SW'ly flow but by T+312 the LP are crossing southern Britain with very cold NE'ly sourced air for many. There's a steep temperature gradient and all the ingredients for someone to get a right blizzard but who and where I'm not certain and no one could be at this time. These evolutions don't happen often and what's even more unusual is for the cold airmass to prevail but that'swhat happens and by T+384 there is a NNW'ly flow covering most of the British Isles. For northern Britain, the cold never leaves. 12Z GFS Parallel - the OP is a dream run for cold fans in the north and snow fans in many other areas. The FI evolution offers a real chance of some serious snowfall. On the Parallel, T+120 holds few surprises though perhaps not quite as pronounced heights over Iceland. T+180 keeps things on track with the centre of HP to the north-west of the British Isles and between that and the European trough, there's an ENE'ly flow. Uppers at this point are generally -4 so cold. From there the HP thinks about retrogression but doesn't quite go through with it staying around or just to the south of Iceland. The ENE'ly tightens to an E'ly as LP approaches from the south into BIscay by T+240. Cold and bitter for the south by this time. However, the LP can't push through the block which builds down over Scotland by T+312 leaving the LP as a cut-off feature but the Azores HP is building into North Africa and Iberia threatening to push the jet back north. Less cold air is now infiltrating the British Isles even as frigid air builds to the north and east. By T+384, battle is joined between the warmer and colder airmasses and there's likely to be some heavy snow somewhere from that conflict. 12Z ECM - both GFS OP and Parallel have hints of big snow events as warmer and colder airmasses do battle as we approach mid-January. That can be a result of retrogression and a west-based negative NAO or simply of the jet fighting to come back north. Back to the more immediate and at T+120 ECM follows the trend with heights over Iceland and a NE'ly flow for most of the British Isles with the LP centred over Belgium. Uppers of -4 in the middle of the day suggest snow for many but not all. By T+192, heights have built to the north with a broad ridge from Iceland to northern Scandinavia and between that and a broad but shallow trough over Europe there's an E'ly for the British Isles. 850s still not overwhelming at this time. By T+240 not much has changed - the HP is to the south of Iceland with the LP over Biscay and an ESE'ly flow for the British Isles. This is importing even colder air with -8 uppers in charge. Looking elsewhere, 12Z Control is cold throughout but with clear signs of a reversion to a more Atlantic-based type at the very end of FI. As for the 10HPA, the Parallel initiates a split by the end of next week but the PV recovers in FI though nowhere as strong or as cold as usual. Both OP and Control postpone the split to far FI. Conclusion: - another excellent evening of output for fans of snow and cold and GFS tantalises with some really snowy possibilities in far FI if we see a clash of warmer and colder airmasses. It's quite possible we'll see a brief incursion of milder air to the far south before the cold re-asserts and the milder air never gets that far north. The theme tonight is for HP to the north and north-west and the cold air to come from the east rather than the north which works for many. The pattern through to T+240 is starting to look set on GFS and ECM but the devil will as always be in the detail for those hoping for snow.

No, and the other thing is nobody remembers January or February 2011. We "could" be looking at a protracted period of colder conditions (7-14 days) with occasional bursts of severity but the real prize would be repeats of that into January and February. I wonder if it will be more like the winter months of the mid 1980s with prolonged periods of cold conditions and the threat of snow and the absence of mild zonality but without record-breaking numbers or snow depths.

Evening all With Storm Bella showing her hand across Ireland and western Britain, the immediate wea6ther concerns for rain and strong winds will take precedence. The active debate over possible wintry precipitation will no doubt be continuing but I'll take my usual nightly look at the medium to longer term trends and leave the short-term and nowcasting to others. Very strong output for cold weather fans yesterday with a prolonged cold spell likely though never as cold or as snowy as some would like (or hope or expect). The 850s were, in the initial stages, not seemingly supportive of the excellent synoptics but with time the air gets colder and that's the direction of travel for which snow fans will be looking tonight. 12Z GEM: T+120 takes us to New Year's Eve and a slack N'ly covers the British Isles with the remnants of Bella over Scandinavia as a weak but general trough and heights extending from Greenland south and south-east into the Atlantic with the next trough barely off the Canadian coast. Uppers zero to -4 with -8 850s just to the north of Scotland. By T+180 little has changed - the trough lies to the north east and east ut extends through the British isles to north-west Europe so we keep a slack N'ly flow. The Atlantic HP is ridging NE toward new heights emerging out of NW Russia. An LP is off the Azores. Uppers have cooled further to -4 to -8 generally. From T+180, the northern part of the trough phases with a new LP forming off the SE Greenland coast but that flow is cut off by heights building through Iceland. However, the infusion of energy causes the LP to re-strengthen as it moves south while heights build to the north and east. The British Isles remains under a cold airmass. 12Z GFS - you'd have to say GEM looks very good for fans of snow especially into FI. I'd be astonished if GFS OP was better at the same timescale. No great surprises at T+120 with the N'ly flow in place and shortwaves within the general circulation of the trough. The coldest air staying to the west of the British Isles at this time. Well, a very different evolution beyond there to T+180 as the Atlantic heights collapse back south and a new trough develops over or to the west of Iceland with a milder airflow heading toward the north of the British Isles. The remnants of the trough are over Europe but the airmass remains cold at this time though I don't like where this evolution is going. Need I have worried? It seems not - there's more than an element of rinse-and-repeat as the LP sinks south across the British Isles but, unlike Bella, this goes cleanly through and heights rise behind to the north and north-west drawing in an NE'ly flow by T+240. Cold air over the British Isles but a steep temperature gradient (which snow fans will like) and some much milder air on the eastern side of the LP pushing up into Denmark. By T+312 the weather is settling as HP edges in very slowly from the west. The trough is now over Scandinavia ad milder air is trying to edge in. By T+384, a more normal pattern is re-establishing though HP dominates over southern Britain. 12Z ECM - it's not worth stressing over the return of a milder weather more than a fortnight away. The divergence with GEM begins at T+180 and that's more of an issue. Let's see what ECM has to offer. T+120 is no real surprise - obvious similarities to GEM but weaker heights over Greenland more akin to GFS OP but we'll see. The evolution is again different to the other models - by T+192 the Atlantic HP has ridged NE through northern British to heights building over Scandinavia while the LP is in the classic "Gulf of Genoa" position leaving a slack NE'ly over much of southern Britain. Uppers of -4 to -8 widely by this time. By T+240 the HP is centred off the Hebrides and a NE'ly covers most of southern Britain with calm conditions elsewhere. Plenty of cold air in evidence and a cold feed on which to draw but the onus perhaps more on frost by this time. 12Z GFS Parallel - ECM decent for cold but perhaps less so for snow in FI. On then to my new mate, the GFS Parallel and by T+120 no great surprises. The T+180 evolution is quite messy with the new Atlantic trough extending SE well to the west of the British isles forcing the HP north to Iceland while the residual trough remains over southern Britain. Cold air remains over the British Isles at this time. The evolution continues messy to T+240 but clear signs of the Atlantic pattern re-establishing and moving toward the British Isles which remains in a slack cold flow. From there we see a pattern we sometimes see - without a strong PV to fire up the jet and drive the systems east, the Atlantic LP goes nowhere and that enables downstream heights over Scandinavia and by T+312 we have a strong HP over central and northern Scandinavia and the British Isles in the middle between the colder and milder airmasses. BY the end of FI, neither the cold not the mild "wins" - the stalemate allows new heights to develop over the British Isles and the run ends anticyclonic. However, the colder airmass is in charge with the milder air held to the west. Looking elsewhere, 12Z Control ultimately ends mild but messy. The trend is toward some form of anticyclonic evolution as we head toward mid-January but the placement and orientation of said HP varies as you would expect at this stage. I gave to say very different evolutions on the 10HPA profile this evening - Parallel goes for a split, OP has the strat weakened and coming under renewed pressure in the second week of January while Control keeps the strat much stronger. Conclusion - slight changes but nothing substantial this evening. An extended colder period looks on the cards - the interesting sign from GFS this evening is the evolution toward an anticyclonic solution which can be very cold if set up in the right place (Scandinavia). This is witness to the weakness of the PV and the absence of a strong zonal jet which gives more scope for amplification. Plenty of weather to comment on in the coming days but let's hope everyone stays safe from the initial assault of Bella overnight.

Evening all First, thanks to @phil nw. for this help yesterday evening with my contribution. Second, I hope everyone has had or is having (if further west) a wonderful Christmas. The weather of course doesn't tale a break - it's been a quiet, calm and mainly fine day here in lowland East London but with Bella on the horizon, all that is going to change as we consider 850s, 925s, dew points and the like while waiting for the snow to fall. Moving on toward year end and earliest 2021, what can we expect? 12Z GEM: T+120 takes us to Wednesday and Bella remains as an LP in the North Sea just off the Danish west coast with a NNW'ly flow for much of the British Isles. The coldest air is, as you might expect, over southern and western parts at this time. By T+180 or the early hours of Saturday 2nd, not much has changed - the LP has moved slightly west back toward the Scottish coast. Heights remain over Greenland and ridging south into the mid-Atlantic. A new Atlantic LP is to the west of the Azores and I wonder if we will see some phasing with the trough over the British Isles and North-West Europe. -4 850s over most of the British Isles by this time. Well, not quite - by T+240, the LP has moved ENE and deepened toward Biscay while the residual trough remains over the British Isles. Milder air is approaching from the south but the cold airmass remains over the British isles for now. 12Z GFS OP - a good start for cold fans from GEM with at least 7 days of cold conditions setting in. On then to GFS which broke the cold spell down in far FI last evening. By T+120, a different orientation than on GEM but the net effect for the British Isles is much the same with a cold NNW'ly airflow and perhaps a secondary feature forming to the east coast of Iceland. 850s show colder air encroaching from the NW. By T+180, the secondary feature has developed and moved south across the British Isles leaving a shallow but complex trough across the British Isles. The 850s continue to look cold and it's secondary features like that which can bring heavier snowfall at quite short notice. From there, the evolution becomes more complex. The main LP heads south and deepens as it engages warmer air to the south and forms a new LP off Portugal. A residual LP remains in the southern North Sea with a new Atlantic feature developing to the west of Iceland. I'm not sure where this is going at T+240 although it stays cold over the British Isles. The Icelandic LP heads SE into north-west Europe though the residual energy over Iberia prevents a more southerly track. By T+312, there's an ENE'ly flow over the British Isles but throughout the airmass stays cold. From there. the evolution becomes messy but essentially it's a stand off between a very cold airmass to the east and milder conditions to the west. An LP slides down the North Sea but the onus looks to be on milder conditions. 12Z ECM - well, GFS was a confusing and messy offering tonight. It ended with -12 850s over the North Sea and positive uppers over Ireland and an old-fashioned stand off. ECM looked to be investigating the west-based negative NAO at T+240 last evening. The T+120 looks very similar to the GEM as you might expect but makes more of the developing LP to the west of Iceland. However, that comes to nothing and the emphasis is on rising pressure in the Atlantic but well north supported by an LP to the south-west of the Azores. This leaves a weak NE'ly flow over the British Isles. -4 50s over much of the British Isles by T+192 and -8 850s moving into Scotland. The evolution from there to T+240 is complex - a new LP forms around Iceland in a lobe of very cold air with LP moving SE across to the north of Scotland into Scandinavia. Pressure remains low over southern Europe with HP dominant in the Atlantic. Signs of something a little milder by the end of the run. 12Z GFS Parallel - as it's now a 4-run-a -day merchant, I'll add a couple of charts from this offering as well. The T+120 has the core of the LP further east into Scandinavia than other models but keeps a N'ly airflow over the British Isles. By T+240 heights jhve developed to the north and the jet is pushed north leaving a complex trough straddling southern Britain and an E'ly airflow for northern parts. Moving on, the cold theme continues until one LP slows and deepens in mid-Atlantic throwing the ridge in front of it up over north-west Europe and dragging the jet back north. It's only at the far end of FI the milder airmass moves in after 10-12 days of cold conditions. Looking elsewhere, 12Z Control is cold for much of the run. Both the OP and Control keep the PV under pressure at 10 HPA - Control has a split in far FI but OP doesn't quite manage it though the PV remains distressed. Conclusion: - another strong evening of output for cold weather fans. The initial passage of Bella over the weekend will usher in a chill N'ly early next week which remains in situ (or veers NE) for a few day into 2021. GFS tries to break down the cold spell in far FI but little or no evidence of west-based negative NAO tonight with the onus moving from Greenland or mid-Atlantic heights to polar or Scandinavian heights as we move into January. Nothing terribly mild, nothing zonal and the PV a bit player at best and coming under further pressure. All in all, plenty of interesting weather and synoptics to keep the discussion going in the coming days.

Evening all First, Merry Christmas to everyone on NW. After the year we've had, perhaps we deserve some Christmas cheer and last evening the 12Z output was in helpful mood with several days of chilly if not cold weather forecast and the hardy perennial of 850s, 925s and dewpoints getting a healthy airing. As Santa continues his miraculous circumnavigation, what are the models suggesting for the coming days and into 2021? 12Z GEM: T+120 takes us to next Tuesday - GEM takes the LP as a significant feature over the British Isles and it's still a large but shallower trough over the North Sea by this time. Uppers generally at -4 with the coldest air displaced further west. By T+180 not much has changed - the LP is centred off the Norwegian coast but the trough extends south through the British isles and a cold airmass remains in charge with heights over Greenland and a new LP well to the south at this time. Again, the coldest air sits out to the west. By T+240, the LP still covers the British Isles as a complex but shallow feature. There's an LP near the Azores and you'd wonder if it will phase with the existing trough and pull some milder air up over us from the south-west but that's quite uncertain with pressure rising to the north and north-west. 12Z GFS OP - a strong start from GEM keeping the weather on the cold side into 2021. GFS OP broke down the cold spell in far FI last evening and let's see where it takes us. By T+120. the storm, as a significant feature, has moved down over the British isles and remains the controlling influence on our weather. It's centred in the southern North Sea and while there are heights over Greenland I'm looking at the instability off the east Greenland coast to see if we get another LP forming. Again, the really cold air is off to the west. By T+180, some notable differences star to emerge. The trough is now shallower but elongated from west to east across the British Isles and across southern Scandinavia and to the Baltic. Strong heights to the north-west and an LP in lower mid-Atlantic latitudes. The re-orientation of the LP finally allows the colder air to spread down over the British Isles by New Year with -8 850s. On then to T+240 and significant differences from GEM. The Greenland heights have gone though there's still a strong HP ridging north-east to the Faeroes. The LP has sunk south over Europe and a NE'ly flow covers much of the British Isles. The air mass remains cold with -8 850s in the mix. However, moving further into FI and by T+312, the Atlantic HP has sunk further south and milder air has come round the top of the HP anad over the British isles while a new LP has deepened sharply and moved SE into Southern Scandinavia. There's a brief but quite potent N'ly before the Atlantic HP starts to re-exert its influence and by T+384 milder air is back on a WNW'ly with LP moving round the top of the HP centred just off the west coast of Ireland. 12Z ECM - GFS OP didn't end the cold spell as dramatically tonight and no sign of the much-discussed west-based negative NAO. It's more of a whimper than a bang if I'm being honest but we would still get several days of cold weather from that evolution. On then to ECM and differences with the other models as early as T+120. As with GEM, the LP has filled slightly and eased back NE to be over southern Scandinavia but there's already a new LP forming to the west of Iceland (not sighted on GFS or GEM). The HP remains in mid-Atlantic. Again, the coldest air just to the west of the British Isles. By New Year's Day (T+192), it's a messier evolution. The new LP phases in with the original LP and the complex feature first extends over the British Isles with a new centre forming over southern England. By the same time, a new P is developing to the west of Iceland while HP remains in situ supported by a new lower-latitude LP far to the south. -4 to -8 850s widely by this time so still cold. The evolution from there is interesting - first, heights build strongly over Greenland and the LP starts to move south but it phases with the mid-Atlantic feature and that in turn allows a small push of HP into Iberia which, you'd think, would stop the LP sinking any further south. This time we are looking a west-based negative NAO. The 850s confirm the push of milder air towards southern Britain and the steep temperature gradient in mid-Atlantic characteristic of this set-up. All very knife-edge. Looking elsewhere, 12Z Control in FI is a good example of "cold zonality" with short-lived milder incursions and longer-lasting periods of colder air. 12Z JMA looks cold for New Year but the longevity of any cold spell looks uncertain. 12Z Parallel is now back to a 4-run-per-day format and tonight's offering is very interesting with a brief milder incursion just after New Year swept away by a much colder wave from the east as we go into the first full week of January. Looking further up, the 12Z Parallel maps a split of sorts in the 10 HPA before the PV briefly recovers and then a new warming begins at the end of FI (January 9th). OP keeps the strat intact throughout which is the second night it hasn't shown the split. Control is also much less bullish on a split. Conclusion: - there's a lot to take in tonight. The signs for cold weather fans are still very favourable - GFS Parallel is very good. ECM shows how a west-based negative NAO can spoil the party. The GEM evolution is a bit messier. Those looking for a significant SSW at the 10 HPA level may be disappointed at this time but there are plenty of other straws in that breeze to discuss. In the shorter-term Storm Bella is going to be a significant feature with wind and rain for many and snow for some (altitude always favourable for snowfall). Several days of cold weather into the New Year look likely but the evolution, as you'd expect, from there is far from resolved.

Evening all Certainly some interesting output yesterday evening to get the discussion moving ranging from an underwhelming GEM to a much more "interesting" ECM and GFS. After a wet but mild day in downtown East London, we await the coming of the first cold snap for Christmas. The intriguing point was less whether we could get a second cold spell but for how long could it be sustained into 2021? 12Z GEM: T+120 takes us to Monday next week and the LP is right over the British Isles, @snowkingput up a superb post on this yesterday which is recommended reading explaining all the factors at work when trying to assess potential for snowfall. The 850s are -4 but other factors look favourable especially for higher ground. By T+180 (New Year's Eve), the LP has filled significantly with residual centres to the north-east and south and the British Isles remains in the cold trough. The uppers remain reasonable even if the LP has filled. The upstream profile looks messy but what we aren't seeing is a zonal Atlantic firing storms towards us. On to T+240 and a lot changes as little changes. AC strong HP over Greenland has forced the jet to the south but the next LP has sunk SE from west of Iceland to be just to the north-west of Scotland. The unsettled and cold pattern continues with a new push of cold air from the west and north-west. 12Z GFS OP - very solid output from GEM to start the evening. Now on to the American offering - will it be a fine turkey or a burnt offering? By T+120, GFS OP has the LP further south than GEM with some -4 850s but broadly zero to minus four but with some other factors possibly helping snowfall to higher ground. GFS OP then pulls the LP first to the south and then round to the east keeping the British Isles on the northern or western flank so it's a NE'ly backing to a N'ly by T+180. The airmass, as reflected by the 850s, only gets colder as we approach New Year. Elsewhere, there's a new LP forming around Iceland while pressure rises in mid-Atlantic. As 2021 arrives and get started, the new LP is phased into the trough which sinks south as pressure builds strongly over Greenland and the T+240 chart shows an E'ly for much of northern Britain. With 850s from -4 to -8, I suspect there's more snow on offer especially for higher ground. The problem with this evolution for cold fans is twofold - first, a strong Greenland HP often produces a west-based negative NAO whereby the ridge extends not south or south-east but south-west and LP comes around the southern end and can push NE towards western Europe and re-introduce milder air. The T+312 evolution is a knife-edge. The LP coming out of North America is initially steered SE but phases with the residual trough over the British isles and deepens considerably near the Azores as the warmer ad colder airmasses interact and then swings towards the British Isles but the block to the north-west is strong enough to forces the LP to disrupt and elongate along the Channel. Milder air gets into southern Britain but the cold hangs on in the north. FO ends with a complex trough o the north-west extending back into the Atlantic and heights over Greenland. The milder airmass is being chased away by a new PM airflow off the Atlantic. It looks very mild to the east over Europe. 12Z ECM - GFS OP is very interesting for cold weather fans tonight with plenty of synoptic interest throughout. Finally, the Europeans - will they complete a Deal for cold fans tonight or will we have to wait another day? The T+120 chart has the LP right over the British Isles with the mid-Atlantic HP perhaps more influential. 850s aren't exciting but as has been observed, perhaps not the only factor at work. By T+192, as we've seen elsewhere, the LP is just to the east and there's a weak N'ly flow over the British Isles. The 850s continue to struggle but it's not going to be mild. Upstream, a long ridge extends from Greenland with the next trough moving out from North America. On then to T+240 and something a bit different. A mid-Atlantic LP is helping to prop up heights over southern Greenland. Further north, a new LP looks to be developing off the east Greenland coast. Closer to home, a shallow LP over Norfolk and a weak NE'ly flow. The airmass remains cold over the British Isles. Touching on other things, 12Z JMA brings the Atlantic HP closer and cuts off the bulk of the cold flow so a worrying sign from a model always to be considered. 12Z Control is cold for most of the run and would be well favoured by cold weather fans. Another worrying sign is 12Z GFS OP backing away sharply from splitting the PV at 10 HPA. The strat is extended and weakened but there's no split tonight. Conclusion - more exciting output for fans of cold tonight especially as we head toward the end of the year and the beginning of 2021. I'll leave others to comment on where and when snow will fall but I can't see why anywhere with higher ground won't do well in the next week to ten days. Beyond that, the main models seem keen on prolonging the cold spell but one or two warning signs tonight. JMA is less favourable and while other signals may show something different, the 10 HPA charts don't look as exciting as they did yesterday. We'll see where we are tomorrow but it's something to think about.

Evening all On and on and on we go, where it stops, no one knows - well, presumably the destruction of the planet might cause the PV some disruption but that's a shade drastic. As a work colleague reminded me today , midsummer is only 6 months away so that's winter done with (apparently). Back to the here and now and a very mild day in downtown East London but the output last night suggested a colder evolution for the post-Christmas period through to New Year after which confusion reigned supreme. Let's see if tonight's output provides any clarity - I suspect not. 12Z GEM: - the LP which develops over Iceland starts sinking toward the British Isles and by T+120 is a significant feature just off the north west of Scotland. The last of any less cold air is being chased out of south-east England as the colder uppers move down. By T+180, the LP has slipped further south and east to be centred over northern France and on its northern flank a NNE'ly flow has set up dragging in -4 850s widely across the British Isles. The mid-Atlantic HP remains in situ with a new LP developing to the north of Iceland. By T+240 this has developed into a complex feature and moved a little south but the Atlantic HP has thrown a ridge across southern Britain and the airflow is now coming off the Atlantic but heights remain over Greenland and LP dominates the Mediterranean so the longer-term evolution not clear. It's become less cold by this time for most but whether this is just a temporary state remains to be seen. 12Z GFS OP - a slightly underwhelming GEM with a 4 day cold spell. Let's see where GFS OP takes us. As it has been the last few days, it's more progressive than GEM at T+120 and makes more of the polar heights to the far NE. The LP is directly over the British Isles with -4 850s spreading in though as nothing compared to the -20 850s over northern Finland. By T+180 it's not too far removed from GEM with the LP in the North Sea, heights to the NW and a cold NNE'ly across the British Isles. The new trough developing off east Greenland is more pronounced. The cold airmass over the British Isles is more pronounced. The LP develops further and it's almost a rinse-and-repeat as it sinks south and by T+240 it's right over the British Isles with strong heights to the north-west. For those who follow 850s, still below zero though I'd expect a warm sector of sorts with the second LP as well. The Greenland heights help push the jet well to the south of the British Isles. The next Atlantic troughs phases with the trough over the British isles and the new LP runs to the south along the Channel at T+312 (yes, I know). Heights are -4 in southern counties at that time so you'd be thinking snow, wouldn't you off a Channel low in January? At the far edge of FI, a lobe of the PV sets up directly to the north of the British isles and that does enough to re-align the trough positively and bring back milder air but it continues very unsettled with a lot of rain. 12Z ECM - well, the GFS run was wonderful for the most part for cold fans and there's certainly plenty going on. As for ECM, will it be the belle of the ball or a party pooper? Let's see. By T+120, the LP is, if anything, slightly deeper than either GEM or GFS OP so plenty of rain and snow in the circulation you'd think by Sunday. 850s not terrible inspiring in contrast to the cold air for Christmas Eve and Christmas Day. By T+192 it's all looking a bit messy - the LP came south and has returned north to sit over NE Scotland but with the British Isles still very much in its circulation but not as cold an airmass as both GEM and GFS at this time. By T+240, a new LP from Greenland has started to move SE having phased into the initial trough. It is pulling in a colder airmass and if it slips down across the British isles you'd think it would keep things cold. Looking elsewhere, as I like to, 12Z Control ends with the PV over the Pole and a weak ridge across southern Britain after an extended colder period. The GFS 12Z OP splits the PV at 10 HPA once again which continues to be a positive even though not all the GEFS achieve quite as dramatic an outcome. Conclusion: - lots to be positive for cold fans tonight. The GEM evolution is a shade underwhelming but both GFS and ECM extend the cold out to at least 5 days if not more. The question now is longevity as much as severity. We aren't as yet looking at -20 uppers and a storm force E'ly but the longer we stay on the cold side of the trough the greater the prospect for a colder airmass and there was an excellent contribution from @Kentspur yesterday evening as to how 850s are a long way from the whole story. I'd be surprised if the GFS and ECM evolutions didn't produce widespread snow for many to greet 2021. As always, it's going to be one for the lamp-post watchers and those with altitude are going to have the best prospects but the initial cool-down on Christmas Eve and Christmas Day is going to help in the medium term.

Evening all Let me start this evening's ramblings with an apology and a rant. I did a full analysis of the 12Z models for the medium to longer term and started having problems attaching charts. I pressed on and finished and then found the thread was closed and my entire contribution lost so all I will say to the Mods, who, to be fair, did apologise for what happened is this: Aaaaaaaaaaaaaaaaaaaaarrrrrrrrrrrrrrrrrrrrrrrrrrrrggggggggggggggggggggggggggggghhhhhhhhhhhhhhhhhhhhh!!!!!!!!!!!!!!!! That's better . On then to this evening's model mutterings. Last evening's output, while not as stellar as Friday, was a marked improvement on Sunday and promised a colder spell of some sort albeit with a very uncertain evolution as we go into 2021, all against the background of a split PV. It seems, on any number of fronts, we are cursed to live in interesting times. 12Z GEM: - a better evolution for cold fans last evening than on Saturday but worth stressing none of this a done deal yet. T+120 is St Stephen's Day or Boxing Day if you prefer. N great surprise at this time - the LP is starting to head south or south-=east and the HP is slowly easing back to the west or north-west while maintaining a ridge to the south of the British Isles so a reasonable day for many. That all changes rapidly and by T+180 the trough is over or just to the east of the British Isles with a cold NNE'ly over Scotland and Ireland. 850s -4 to -8 so you'd think snow from any precipitation at night. The evolution to T+240 is similar to that postulated by the ECM 12Z OP yesterday evening. A new LP forms over eastern Greenland and the trough aligns NW-SE across the British Isles with the HP similarly aligned in mid-Atlantic and hints of heights rising to the far north-east. Minus 4 850s to end the year but the really cold air is digging further west and that worries me is it would create cyclogenesis to the west and pull milder air back up over the British Isles in the early New Year (suggested by GFS in far FI over the past couple of days). Also worth noting is a more noticeable PV formation in the usual place. 12Z GFS OP - another run of ebbs and flows last night switching from very cold to very mild very quickly in far FI (see above) so where are we this evening? As I commented yesterday evening, GFS is more progressive than GEM even at this time with the LP already showing its head to the NW on Saturday (T+120). By T+180, the LP has moved south - the profile isn't too different from GEM with the main centre over England and a second centre just to the north of Scandinavia. A cold N'ly wind down the western flank but -4 850s over much of the British Isles. By T+240, much of the British Isles remains cold with a N or NE'ly wind with a complex LP to the east but unlike GEM a small ridge of HP has split the trough . The Azores HP is in position and a new LP coming off the eastern seaboard. On into 2012 and by T+312 signs of a return to a more usual pattern but it's slow and pressure is high to the nE but with the Azores HP edging into North Africa the Atlantic trough is slowly aligning more positive. T+384 looks much milder with a SW'ly flow despite heights over Greenland and to the east. It looks like a west-based negative NAO pattern but I could be wrong. 12Z ECM - an interesting if messy conclusion yesterday evening but tonight will doubtless be completely different. T+120 is a recognisable blend of GFS OP and GEM. By T+192, the LP has moved south and now sits over to just to the east of the British Isles but the Greenland heights have rapidly collapsed and a new Atlantic trough is moving off Eastern Canada. By T+240, however, that LP has swing SE to be just off the north coast of Ireland with heights appearing to the east and the Azores HP ridging in to mid-Atlantic. It's a logical progression from the previous evening's scenario and hints at a return to the negative alignment pattern we saw recently. 850s of -4 for many during the day on New Year's Eve so chilly and you'd think any precipitation might well be snow especially to higher ground. Looking elsewhere, the 12Z GFS OP 10 HPA forecast doesn't quite split the PV tonight but does leave it severely diminished and weakened. the 12Z Control is largely cold or very cold until far FI and ends with a stand off between milder Atlantic systems to the south and west and colder anticyclonic conditions to the north and east. Conclusion - it looks as though we will get a colder interlude for 3-5 days minimum between Christmas and New Year with opportunities for snow for plenty. Hopes for a Greenland HP appear to be fading and as might be expected a weaker PV and diminished zonal winds are helping the Russian HP to become a player in the New Year with the options of negative trough alignment or a stand-off between competing air-masses. Plenty to resolve after New Year as you'd expect and with the PV still under pressure we just need the cards to drop right for a January cold spell. All in all, an encouraging start to the week.

Any idea why? My first thought is the response to the first slight warming is to push the PV back to Canada where it can fire up the Atlantic jet. The second much more substantial warming begins after Boxing Day and peaks on the 30th effectively splitting the strat and far FI has a hint of warming over America which would be fascinating. Perhaps the EAMT signal isn't as strong as some envisaged.

Evening all Last evening's output was remarkable on a number of levels and probably some of the best for cold fans for a long time. It's hard to see how things could improve for cold fans from that and I imagine the slightest hint of a downgrade will bring out the "I told you so" merchants (trolls mostly) and the ritual toys being ejected from the permabulator. It's only the weather for crying out loud... Let's see what mood the models are putting us in this evening: 12Z GEM: - t'was Christmas Eve on the models (well, T+120 at any rate). GEM has downplayed the LP coming up from lower latitudes and it's now a secondary feature so we don't get a pull of PM air as suggested earlier in the week leaving the Atlantic HP to build toward us as the next trough emerges from Eastern Canada. We get a fine Christmas Day and Boxing Day too for most but by T+180 the HP is an elongated feature through north-west Europe and into the Atlantic. Meanwhile, the trough and the PV lobe is gathering to the north of Iceland so we'll see. By T+240 the LP has moved down into Scandinavia and extended into eastern Europe but the British Isles remains too close to the HP to pick up any real cold airflow and indeed a new LP is coming past Greenland so a disappointing end to this run for cold weather fans. The PV looks to be rebuilding over NE Canada so I don't see a quick route to cold. 12Z GFS OP - a stellar run yesterday at this time but with GEM disappointing badly for cold fans, we really need GFS to stay on track. By T+120 only slight differences with the GEM - perhaps the HP ridging slightly more strongly to the north though I suspect it won't make any difference. The evolution from there goes much more according to plan than it did with GEM - the LP develops strongly to the north forcing the HP to retrogress as warmer air moves into Greenland and builds the HP. By T+240, the LP has sunk down over the British Isles but has become a complex feature - the 850s are zero to -4 so that should mean snow to higher ground and to lower ground in the north but I'm no expert. By New Year's Day, the trough is still over us - I suspect a slight build of heights to the south is preventing the trough sinking away but heights persist over Greenland and the jet looks well to the south. 850s generally -4 by this time. Unfortunately from there it all goes a bit wrong - the LP drops to the west of the British Isles in the pool of very cold air and re-invigorates but in a position that draws up a mild or very mild SW'ly airflow with heights building in Europe. For a second night running, the 10 HPA shows a split in the PV at the very end of FI. Fine margins but it doesn't end quite as well as the medium term might have led us to hope. 12Z ECM - the end notwithstanding, GFS would provide several days of cold and potentially wintry weather so not to be sniffed at. Will ECM make it 2-1 for the coldies or will it follow GEM into the mild camp? At T+120, I think I see the problem. There is a shortwave in mid-Atlantic and that pushes the HP closer to the British Isles and quickly cuts off the PM airflow. After the LP has cleared through, a clear and cold Christmas beckons. The LP develops but not as cleanly as GFS but better than GEM and the HP is giving ground. At T+192, the core of the LP looks to be heading for Norway rather than the British Isles. That's pretty much what happens with the HP close to but not oriented over the British Isles. There's a cold col over Germany with LP to the north and south and HP to the east and west and that's where we should be. Upstream looks messy - there's an HP coming out of America and a weak LP near Iceland which might, I suppose, run SE towards the British Isles (or it might not). The 850s are rising slowly with some warmer air in the Atlantic but it may not be coming our way. Looking elsewhere, 12Z JMA looks okay, 12Z UKMO less convincing while 12Z GFS Control is not that different to the OP. Conclusion: - the value of caution was never more evident. The stellar medium term charts from last evening haven't disappeared (GFS OP is certainly good) but there's still plenty to resolve with FI perhaps as early as T+120 this evening. The major LP coming out of Canada and moving round the top of the Atlantic HP is the key player - GEM keeps it far to the north with the HP close to the British Isles, GFS OP drops it over the British Isles but then brings back milder air through the back door while ECM is messy. I do wonder if the LP over the Canaries is propping the HP up at mid-latitude and stopping the LP dropping cleanly through the British Isles but we'll see. Much still to be resolved but encouraging to see the possibility of a split PV to start 2021 very much in evidence.

Evening all Christmas looms on the horizon but looking (unexpectedly) at a longer-range forecast on the BBC News Channel this afternoon, what interested me was the post-Christmas evolution and the LP dropping through the British Isles (perhaps) rather than Scandinavia. We'll see - a cold, crisp Christmas Day looks the form horse but perhaps the evening output now has other ideas. 12Z GEM - T+120 takes us to midday on Wednesday, the day before Christmas Eve and it looks bleak with an LP over the British isles and rain for many with some strong winds to the west and north of the storm centre. A ridge waits upstream. Oddly enough, the small LP exits more to the south east but a calmer Christmas Day is followed by the onset of a new Atlantic LP which deepens close to Iceland by T+180. The large Atlantic HP has eased slightly and the possibility of negative trough alignment is very much there. Indeed, the trough drops SE over to just to the east of the British isles and on its western flank a potent N'ly sets up by T+240. The evolution from there is open to question - I don't expect full amplification to the stage of an omega block - there are signs of shortwaves developing but it's a fair push of warmer air and heights into Greenland so we'll see. 12Z GFS OP - an exciting opening from GEM but they've been an outlier before so feet very much on the ground for now. Indeed, some differences as early as T+120 on the GFS which, instead of having the LP over the British Isles, has already introduced a PM airflow. The LP never makes it to the country fizzling in the south west approaches. The ridge is building in but the Atlantic is starting to gather. The T+180 GFS chart is more progressive - I suspect because we've not had the LP sit over us and then exit delaying the ridge moving in. GFS sends the deepening LP toward Norway with heights building back north west toward Greenland from the HP. While I wouldn't call it a mild NW'ly, the truth is the 850s aren't impressive at this time for cold fans but as we know that can quickly change. The two HP cells join over Greenland and with the trough dropping into Scandinavia, the T+240 GFS chart is a cold weather fan's delight, albeit 72 hours late for Christmas. Minus 8 850s for the British isles wit the really cold air sitting out to the west. From there, the Greenland HP quickly breaks down but the LP core moves into Europe and the British isles stays on the very cold side of the trough with the cold air staying in situ to end the year. From there, it becomes a nirvana for southern snow fans as we get what I could call a "reverse Channel Low" - that's to say ab LP forms over the Netherlands and runs south west into the Channel. Needless to say, that would be a real snow storm for the south - we don't argue margins in early January with -4 to -8 uppers. The run ends cold with heights building to the north but it remains to be seen if we can squeeze much more out of the spell. 12Z ECM - a phenomenal GFS OP for cold fans, likely an outlier. Were it to verify, serious snow fall for London and the south-east. Enough drooling and dribbling - it's all far FI and won't verify. On then to ECM - will it be a hell raiser or a damp squib? ECM goes the GEM route to T+120 (as you might expect and it's what the BBC forecast suggested earlier). By Boxing Day (T+192), the deepening LP is bearing down on us from the north-west (again as per the BBC forecast) but looks to be heading straight for us rather than for Norway. That's pretty much what happens - the LP drops pretty cleanly through the British isles to France and draws in an ESE'ly flow. Colder air on the northern flank of the storm and with -8 uppers crossing a relatively warm North Sea, are we looking at our version of "Lake Effect", which is what has happened in Japan where cold air has crossed the Sea of Japan (relatively warm) and dumped feet of snow on Japan. Even more drama as the 12Z GFS OP splits the PV at 10 HPA which would be in itself extraordinary. To dampen the mood very slightly, 12Z Control ends anticyclonic but still chilly. Conclusion - what a way to end the week !! Spectacular output for cold fans with cold blasts forecast from either the north (GEM and GFS), the North East (GFS OP FI) or the East (ECM). That being said, we've been here before and we've been let down so often before caution remains very much the watch word. The most exciting chart for me tonight is the 10 HPA on GFS OP which shows a split in January which would offer the possibility (but NOT the certainty) of prolonged cold. For now, the behaviour of the post-Christmas storm looks the key - does it head into Scandinavia and pull down a N'ly or drop through the British Isles and draw in an E'ly? Heads we win, tails we don't lose looks the answer - the BBC forecast was tending to ECM this evening but it's all still to be resolved. 12Z Control shows it could go less well so caution, caution, caution would be the three watch words this evening (well, it's the same watchword but you know what I mean).

Oddly enough, it was being floated as far back as Monday by a couple of the models and then dropped as it was phased in to the circulation of the current LP. It's an interesting evolution but as I recall it didn't end well in terms of cold (surprisingly).

Evening all It was all hanging in the balance last evening and I suspect it will be tonight as well. GEM and ECM suggested something milder in the run-up to Christmas while GFS went for a much colder outcome especially into FI. Let's see what tonight's entrees have to offer for our meteorological taste buds: 12Z GEM: by T+120 (Tuesday), the weekend's rain has passed to the north east and a brief ridge of HP covers much of the British Isles ahead of the next feature coming up from the south-west. It's mild or very mild with a TM flow. The LP deepens further but runs more east than north-east and by Boxing Day is over southern Scandinavia and we've had a brief but chilly PM airflow with a ridge arriving to provide a fine if frosty Christmas Day for many. The HP is already under pressure from a new Atlantic LP coming out of eastern Canada. However, heights rise over the British isles and extend to form a mid-latitude block to the east so it stays predominantly dry with a hint of cooler air advecting from the south east so frosty and perhaps foggy for some. 12Z GFS OP - produced an excellent OP run for cold fans yesterday after a shocker on Tuesday so consistency is not its middle name currently (it's not mine either but that's irrelevant). At T+120. some slight differences with GEM. The main LP is shallower and the secondary LP from lower latitudes further away from the British Isles. Let's see if these small differences become big differences. By T+180 (Christmas Day), there's a keen N'ly over East Anglia and the far south-east but elsewhere winds are dropping light as the ridge builds in so a frosty start for the bog day you'd think. The Atlantic profile also looks different with the trough less extensive and modelled further north so my money (and very nice money it is too) would be on a mid-latitude block. Not for the first time, I'd be wrong - the HP settles back in the Atlantic to the north of the Azores as a strong storm moves round the top and down into Scandinavia - it's a real brute and brings strong winds and rain to the British Isles and a real blizzard for Norway. It heads further south as a major feature dragging the trough far down into eastern Europe and we get a brief but potent N'ly before the next system flattens the flow once again by T+300. It's a rinse-and-repeat pattern through the end of FI with one trough moving over Scandinavia introducing a PM airflow before a brief ridge followed by the next upstream LP coming out of Canada. 12Z ECM - will this be the top dog or a wet fish? GFS was nothing if not dramatic in its latter stages. By T+120, no major differences with GEM or GFS and after a brief chilly PM airflow for Christmas Eve, it's all flattened out by Christmas Day with the HP already retreating as the next LP develops and moves across Greenland. The evolution then follows the GFS as the LP develops as a major feature before slipping SE into Scandinavia (not quite as potent as GFS) and the British isles ends in a PM airflow with strong winds and wintry showers but there's no obvious amplification upstream. That said, I've seen stronger PV modelled and a more fired-up Atlantic so I'm not wholly convinced. Looking elsewhere, the 10 HPA forecast on the 12Z GFS OP shows a further strengthening of the warming and quite a disturbed PV by the beginning of the New Year. 12Z Control ends interesting to say the least with signs of rising pressure to the east. while 00Z Parallel would start the year with 850s of -8 to -12 across much of the British Isles. Conclusion - GEM sits on its own this evening while both GFS and ECM build a pattern we've often seen of a strong mid-latitude block in the Atlantic and LP moving round its northern periphery and down into Scandinavia. It's good news as it cools down Scandinavia and if we can get that break for some amplification the chance to advect some colder air is very much there. Christmas Day looks fine and frosty at this stage and we need to see if the pronounced warming around Christmas has enough impact to make a difference into January.