kold weather

Haha I'm basically like "next slide please!" Yeah I'm comparing whats going on upstream. Nothing yet to suggest we won't get a very pretty FI, of course there is a big difference between a pretty looking FI and one that will actually give us the goods in real terms! There have been stunning ensemble runs synoptically, until you see the 0c isotherm well north of the Midlands.

Worth noting the ICON typically is overly conservative when it comes to snowfall in these types of situations (it showed nothing till the day back in early December) so the fact we have the ICON on board showing snowfall is hugely telling to me that we will be on the right side of marginal Don't get too hung up on the exact positon of the lows/fronts, they will shift around probably quite drastically from run to run, these secondary low features are nightmares to forecast. But from an IMBY point of view, I give the ICON a thumbs up, its close enough to my area to provide interest. I said a while back that the 27-29th period interested me, and the GFS ensembles have done a half way decent job in the super extended range flagging up the possiblity, though it was way overboard with the blocking in the end.

I think the 00z GFS did that to an extent, though it wasn't a true west based block, it had a huge stonking upper high that was big enough that it surpressed the jet enough to prevent us ending up on the mild side even.

Looking at the ECM ensembles plenty of the suite have some sort of snow between 12z 28th and 12z 29th. Most aren't 'that' exciting but most also do have at least a dusting. These types of events tend to get upgraded as we move closer to the time so its a positive sign to see reasonable agreement for there to be snow around. Whether it sticks all that well remains to be seen of course.

We are probably early enough and the set-u good enough aloft that we can afford to roll the dice on this one. If it fails and we end up in a SW atlantic wave train I suspect we will get another bite of the cherry and things will rapidly switch back to cold. Just a gut feeling if it does end up too far west. We've come mighty close to that on both the 06z and the 12z deep in FI on the GFS.

GFS ensembles are very unsure exactly where and how strong any upper blocking develops, so plenty of options out there for what may happen. The one thing there is some decent agreement with is the general trend of southerly track tracking jet with LPs either over us or just to our south, and also for an upper high pressure somewhere to our north. Put the two together and you have a decent chance at a snow event or a sustained cold event, with a bit of luck both.

Cold run overall, with a sustained below average spell coming up still. The HP again just sets itself up a little too far NW, though its closer than the 06z run. Would be a huge flood risk in the south if such a pattern were to establish, though its not really all that far removed from a big snow event, a slightly flatter initial low at 264hrs would bring home the goods. Its a great pattern bu one that is high risk - high reward. The failure for not making the cut is perhaps very wet weather and flooding.

Some really snowy members out to 240hrs on the GFS ensembles, though no real confidence onb where any upper high sets itself up, which is a little concerning but that should sort itself out in the wash. Pretty close to all runs are still cold at 240hrs, and a considerable number have snow events after the 30th December. @mountain shadowoh it will be marginal I agree, to be fair in this country it rarely isn't. However I do disagree with you when you say there will be little snow away from Scottish mountains. I think there actually will be quite a considerable amount on the proviso we can get a decent active front across the country. If we do the odds for a 10-20cms type fall is fairly high for somewhere.

I think I'm being very realistic personally, and I see nothing yet to swing me from my position that there will be a fairly decent area of snow between the 28-29th. It may well be marignal in some places and by more wintry in nature and obviously how well it sticks might well be a different question, but snow itself, I'm fairly confident it will be around. Especially on the cold flank of any secondary low. I think your being pessimistic and simply looking at 850hpa to heavily. As I said before a similar but milder set-up gave Kent and EA snow a few weeks back. also, far too early for any met office warnings, alot is going to depend on exactly where any secondary depressions set-up, which probably won't be known with any real confidence till the 26th I'd guess.

Yes the upper high though weakening does actually drift back SE towards a more favourable position by 324hrs which does leave the doors open again. However the west based -ve NAO is a real risk, looking through the archive charts from the past I've seen some utterly stunning upper blocking and strong southerly jet get ruined because the upper high set-up too far NW and we end up with a very wet LP train with SW airflow.

The other thing is the actual block itself is one of those small ones on this run, so effectively we need the core much closer to our shores. The 00z GFS for example is much larger so the core placement doesn't matter as much. It may end up eventually opening up again from the NE down the lin,e but we'd have wasted 4-7 days of prime time blocking to get to that point if the 06z GFS was right.

Its worth keeping a watchful eye on it is the best way to describe it. 06z GFS is it came off might just about be ok for N.Egland and Scotland, the rest of the country ends up in milder air by 300hrs, though with a surface flow from the SE it won't feel that way at the surface.

Yes my ears were twitching from as early as 192hrs on this run (my first post on the 06z run said I thought it might be a touch too fr west) Its a risk for sure, and as you say there were some ensembles that went down this path and it is certainly possible if everything does setup too far west... Which would be typical right!

Get the feeling it may just be a touch too far west and we may end up in the hands of a SW (at least the south) in deeper FI on this run. Its quite a restricted upper high in terms of size and so could probably do with being a couple of hundreds miles SE. Could be decent snow further north from it mind you.

I don't agree MS, there is a heap of potential for snow, especially from the 28-29th. You've got cold uppers aloft (500mbs looks more than good enough), low thicknesses, a wind flow that as any secondary lows that form swings the winds from Europe and any evaporative cooling IMO males a really compelling argument for plenty of snow if there are active fronts in the area. A near carbon copy set-up gave a fairly decent chunk of EA and Kent snow from an milder version of this a few weeks ago. This time nearly every metric looks colder.

Yeah its increasingly cold, and even if the Greenland high does set-up that far west it is still definately going to bring cold down. Looks like we might just about get away with it on the 06z OP, but thats the utter limit to how far west we wan everything to set-up.

Greenland high forms on the 06z again but this time looks just a little too far west for comfort, which runs the risk of the LP getting stuck over the UK and the good stuff never quite really making it for the south of the country. It does show the risk of the west based -ve NAO though again. However the basic background synoptics are great and no point getting too overly hung up on the exact placements yet!

As you were from yesterday with regards to 28-29th period, if anything the models are starting to firm up on a snow event somewhere as the secondary low forms and swings down. These set-ups are capable of really big dumpings if they perform a small tight loop around the old circulation center (the ECM kinda shows this) and so we need to keep a close eye on this, but also be aware it may take a few more days to get more confidence of this. Beyond that and there are still strong suggestions of a Greenland high trying to form, whether it makes it remains to b seen and frankly with the jet running to our south we may not even need it as strong as a slightly less surpressed pattern may allow a wave of lows with cold air aloft to butt towards the UK. Odds of a prolonged cold spell only heading up at the moment...perhaps at the one time we could do without it...

The 18z suite of ensembles does look a little warmer at 850hpa after the initial LP on the 27-29th...but as per Snowking earlier, that is masking what is actually happening. That is we seem to rotate in LP's towards our general area. It prevents the uppers getting quite as cold but with a decent undercut from Europe looking very possible if we can get the right angle, it won't really matter much providing we don't see any meaningfully deep depressions to our SW. In the later parts of the runs the ensembles move more towards a high pressure being a little further SE than previous runs, however it is but one suite and means little on its own. Before that and I'm still moderately confident we could see a fairly decent spell of snow between the 28-29th, though details probably won't firm up this side of Xmas on the chances.

But it doesn't show 5C? Most of those are showing 2-4c during the time period where any frontal system would come through. AND I'd never use the ensembles for an accurate representation of the temperatures in this sort of set-up as the resolution of the model ensembles won't accurately resolve the temperatures under evaporative cooling, something the models struggle with on both sides of the divide.

Its basically a re-run of a few weeks ago, except about 1c colder on most metrics at the surface, with a colder upper flow and cooler SSTs. I'm still fairly confident that snowfall will be around during that period, though as you suggests, its real marginal and so the more active a front the better, also the better the fetch from the south/SE the better as well. The big point is, will it reach that 24hrs mark!

Very good ECM, again the set-up *is* marginal ,we shouldn't pretend it isn't. However the ECM run is cold enough for a solid snow event and I suspect the GFS 12z would be as well. Beyond that the risk is clearly there for a west based -ve NAO on the 12z ECM, though the upper set-up is good enough for the time being to keep the cold coming and stagnating nearby our shores which as it gets more entrenched means any diving LP's into our shores will have an ever greater chance of being snow filled events. So plenty to watch out for everyone!

Broadly the same pattern as the 06z GFS BUT the upper high sets up in a somewhat more favourable angle/position and it makes the world of difference for most of the country. As you say, thats a cold set-up, and one that could well sustain for some time as well. Probably a snow breakdown attempt by 384hrs.

My best guess based on this run and that explicit chart on the 29th of December would be showers at night falling as snow, maybe struggling to settle close to coast. During the day I'd guess you'd have a classic wintry mix with rain in light showers turning to sleet/'snow in heavy bursts. From 16+ years of watching, thats what that sort of flow typically produces. -5/6C IS very marginal at 850hpa with an onshore flow as the boundary layer is moderated by the warmer sea. The upper cold isn;'t quite as deep either by that point, so whilst we are pulling in slightly colder air in at 850hpa its being counterbalanced by other factors which are less favourable. However, I think there will be plenty of interest before that point anyways!

Subtle shift past 180hrs makes the world of difference, the 12z GFS FI at least to 264hrs is looking very good for most of the country. This is going to be a humbdinger of a FI. PS - still a more than reasonable chance of a snbow event between 28-29th, though much depends on placement and strength of any secondary depressions. Too early to have any confidence beyond 27th.