LomondSnowstorm

It has been pretty quiet today, a very nice crisp cold day with what cold charitably be called a dusting on the Lomonds. I'm still fairly hopefully for tonight, we can see the front starting to move east on the radar on the eastern edge of the low. Still a pretty amazing level of uncertainty about the placement and eastward extent, with recent runs placing it to the north of the central belt before sliding southwards, but I think it will really be a case of radar watching to see where it ends up.

Very intense squalls coming through there, some bright red on the radar. Briefly cleared again so hopefully a bit of drying out although the radar looks very active. A notable drop in dewpoints to the west already - Glen Ogle down to -1C dewpoint, the Hebrides appears to be uniformly 0C which is no mean feat! All seems broadly in line with model projections, if anything a tad slower but that's as much based on the low being a little slower moving based on some of the pressure observations than perhaps modelled rather than any major issues so far. Fingers crossed there'll be something wintry to wake up to for some of you...

That's a good point! Especially with the relatively high upper air temperatures compared to surface. I think the low surface pressure helps significantly though in the sense that you've got 50hPa less height than normal from the surface to the upper layers. In particular this will mean the convection over the sea should be very impressive given such a high lapse rate.

Good afternoon folks, very much the storm before the storm here - very high winds and rain which does seem to have the odd sleety component to it despite relatively high temperatures for that. We can see why on this forecast NMM skew-t graph for Freuchie - quite a steep lapse rate from the surface meaning the freezing level can be pushed quite low, especially with heavier precipitation: Tonight we'll see the first snowfall to relatively low levels as showers on the back end of the second cold front turn quickly to snow: With temperatures around 2-3C and dewpoints just about 0C it won't necessarily lie: But again - the intensity of precipitation might be very helpful here. On the latest UKV, for example, we see very significant CAPE values offshore, suggesting really quite intense convection possible (with thunder and lightning too as KW suggests): As the night progresses, there's cross model agreement that temperatures will, at least for a time, no longer be a problem, and any precipitation from around 3am onwards is very likely to be snow and to stick: Totals are difficult to pin down - the ICON mesocale model for example tends to have precipitation concentrated towards the west coast, with this being the furthest east extent after 2am: However, the UKV has showers coming through most of Scotland throughout the night and into the morning, with potentially some very hefty accumulations particularly just to the south of the central belt - when you bear in mind these are totals for 1 hour just, I wouldn't be that suprised to see 6-8 inches in some parts, and 1-2 inches more widely: For eastern Scotland it'll be bit more hit and miss, really reliant on individual streamers lining up for us, and ideally more of a westerly tilt to the flow - part of the reason the ICON has less of an eastward extent is the angle of approach locally has more of a northerly tilt through the night, with the low a little further east. My money is very much on higher precipitation totals though, and I don't think there's going to be much issue with showers making their way east. As the day progresses the uncertainty grows as the centre of the low approaches. It's been very difficult to pin this down, particularly because a few miles east of west makes an enormous difference, but there's still some incredibly snowy setups on the table - for example the ICON model, which makes up for its earlier relative lack of snowfall by taking the low into the Clyde, bringing frontal snowfall across central Scotland by evening: This is really the best setup we could ask for at this stage, with most areas seeing significant totals, no real danger away from the west coast of marginality and the occlusion stalling over us even as the low moves southwards: If this comes off, you're looking at easily 1 foot plus in a number of spots and 4 inches quite widely. How likely is this? Difficult to say - the GFS has been much more keen to keep the low out in the Atlantic, with significant snowfall for the north and west of Scotland but largely missing the eastern half In this setup there's still the possibility of secondary lows developing, and you can see that any smaller feature developing out in the North Sea has some serious potential to bring snowfall inland. However, something closer to the first scenario would be the ideal setup. The UKV, for what it's worth, follows along similar lines to the ICON, so I would tend to think the low is a bit more likely to end up doing serious damage for at least the western two thirds of Scotland (although again hard to say quite where the front will stall and therefore where we could be talking about something more exceptional): The following day, the snowfall risk moves to the east coast as a slack easterly follows to the north of the low: The risk of it becoming a sleety mess is diminished somewhat by the slackness of the flow and the surface wind direction being more northerly, but certainly suspect Hawesy at least may have some trouble seeing lying snow from this especially during the day. Note also the secondary low forming on the GFS - some worries about it bringing in an upper warm sector or lifting dewpoints a bit too much at the surface but certainly brings a significant snow risk with it as well for the northeast (does have a December 2009 look about it too...) In summary - I really think this has the potential to be a very special spell of snowfall for a large part of Scotland. The details even at this stage are very difficult to pin down, but from 2am onwards the vast majority of precipitation away from the immediate coast will be snow, and there are at least three events in a 48 hour period that could deliver 4+ inches to a large swathe of Scotland. Now we wait...

Agreed - Sunday morning has a real chance of disruptive snowfall for a good chunk of Scotland, and that's just for starters. On the other hand though, it's not exactly unusual for them to be slow to put warnings out for Scotland...

Fascinating to see that occlusion, certainly suggests the potential for the models to underestimate the precipitation (although again it seems difficult to pin down details even 48 hours out!). The sub 528 zone is east of the occlusion so good news for both of us But we'll see - I'm planning to do a summary tomorrow since the first snow will be within the 'semi reliable' 24 hour period

Merry Christmas folks! I've been on various overly sweet Christmassy drinks (and beer) so this may not be my best summary but a few points on how the charts have changed since yesterday: a) the low is holding a bit further north on Sunday than the GFS had yesterday, suggesting some model agreement (yesterday the ECM had it further north and west but the GFS had it bringing in a 'too warm' easterly early doors) : Is this a good development? Broadly yes - as the cold air is being advected down the west and then south of the low, the worry I had was a scenario where the low sunk before any of the cold air could reach us, leaving us with some surface cold where skies cleared but without too much chance of either snow from the west (often a good setup for a lot of us) or enough of a localised upper cold pool building up to withstand any warm sectors. b) there seems to be less chance of that 'too warm' easterly, with the low pressure generally more slack and the appearance of some secondary lows to the north: Is this a positive development? Absolutely - what is really not wanted is any sort of surface level mixing in this setup. As with our brief but heavy snowfall a few weeks ago, the ideal setup is convection building offshore, along the line of a front or secondary low/trough of some sort, moving inland, but slowly/ with winds from the land at surface level so the cold pool that should develop isn't immediately blown away and any warmer sector is unable to move in behind it. this works with either the upper level easterly/lower level northerly (a la December 23rd 2009/a few weeks ago) or one of your classic frontal snowfalls with southerly surface winds (March 12th 2006, or February 5th 1996), although these are a bit different in some ways. Generally the models have a bit less precipitation around but significantly more surface cold, and at this stage that's a tradeoff I'd definitely take. The one negative I guess is the tendency of the models to take the main low a bit further west, and in general sometimes build put additional lows to the west of us - these can end up alright but a 'west based negative NAO' is one of the few setups from this broad synoptic pattern that could cause things to go wrong. None of the runs quite show that though, some of the mesoscale models have had higher snowfall totals and there's still potential in almost every major run well out into the realms of Fantasy Island (300+ hours). Another point to flag up is perhaps an extension of the snow risk north and westwards too - the low being further north and more secondary features is a bonus although I would expect the mountains to take a pasting in almost all possible scenarios. All in all, as good a Christmas output as I've seen in a few years for snowfall

Well remembered! I was actually away in Austria at the time, ironically in one of the worst winters for snow on record there Other examples would be, I think November 25th 2005 (I remember it came on just as I was heading to school): and, I think, December 19th 2009, although that one was maybe a cm at most:

Almost certainly, it looked alright as far as intensity goes but it'll take more than a fairly heavy shower to actually land a hit in Fife from a northerly It has happened (maybe 5 times that I can remember) but I'm struggling to think of when other than those times it's associated with a proper cold front or some sort of mini-low.

Oddly enough the original one (in my signature since at least 2013) still works, although I'm assuming some of the links are dead or outdated.

The radar, as ever in northerly set ups, is doing just enough to tease with a fairly hefty looking band of showers (I believe basically along the lines of this funky shaped trough below) making it just past Birnam: I doubt it'll amount to more than a dusting at absolute most, especially since the temperature here is relatively high (although the dewpoint is comfortably below freezing so I'd imagine it would be snow), but I did sleep through most of the earlier snowfall (somehow, despite the thundersnow) so I'll probably give it 15 minutes at least...

Cheers! Those were the ones I was meaning Given the UKMO itself isn't that far from the other main models, it's at least a hint that it wouldn't take too much breaking right to get some proper snowfall from Sunday onwards...

The broader pattern still seems very well on track - almost all roads lead to a Greenland/Iceland high, low pressure in the vicinity or to the south east and and the flow generally from the north to east. The devil is in the detail, and we've seen a few more changes today (some not so grand as far as the marginality goes but more disturbances showing up on the mesoscale models that could bring significant snowfall too). On my phone at the moment but I would recommend the UKV (Netweather Extra plug) and the ICON model (meteociel) to see some of the potential for Sunday through to Tuesday. However, I would say that, if we see further warm sectors or North Sea driven mush instead (I think a middle ground with some decent snow inland is most likely at the moment), we're still looking very well placed for likely further cold incursions tapping into the proper cold pool - it's not always clear as it unfolds what the main event actually is, and sometimes what seems like a disappointment is the critical trigger flow that opens the floodgates. December 2009 does spring to mind on this - it was a long build up, and while an initial possible easterly ended up with just high pressure in place, and the trigger low was initially not especially cold, what followed, despite things seemingly being delayed from the original timescale, was an amazing winter.

Not too much to add based on the overnight runs, with the details still somewhat up in the air, but the ECMWF seems to me to be the pick of the bunch - the later stages in particular bring back the spectre of the vaunted 'polar low': Is it a true polar low? DIfficult to say but it's as close as you'll see to one at that (albeit speculative) range, with 500hPa temperatures below -40C: The centre is tantalisingly close by the end of the run, with significant precipitation alongside it: In terms of the bigger picture, the Met Office modelling seems to point towards a Sudden Stratospheric Warming, seemingly led by/in tandem with the exceptional polar vortex destruction you can see on the chart above at the lower levels. The basic pattern above is also seen with the GFS, which while again a bit less pretty in the early reaches comes out with more secondary troughs coming through from the northwest before an eventual full blow easterly/'reverse zonality' under the high (this is shameless ramping by the way, not worth taking seriously at this range but illustrative of some of the possibilities if the early part pans out): All in all, some encouraging output again. We have some issues with warm sectors on the low from the 27th which may cause difficulties but I wouldn't worry quite yet given the placement is very far from nailed down, and I've seen more runs that create secondary lows or even just split the main low anyway, which would certainly lower that risk.

So - the next 48 hours: Not too much of note for the central belt (other than some lovely frost tomorrow night into Christmas Eve) but, quietly, I do wonder if there's some significant snowfall potential for Inverness, Aberdeen and a lot of places in between (the precipitation for the Borders looks generally to be through the day tomorrow when it's a bit on the mild side for snow aside from maybe some of the higher hills): The key time looks to be between midnight and 6am, when the freezing level looks pretty good for everywhere affected by the showers: Although some suggestion that this will be sleet around the Aberdeenshire coast initially with slightly higher dewpoints. The GFS has this arriving a bit later and extending a bit further south, with some potential more widely: Again there's hints of slightly higher dewpoints near the coast but I think you wouldn't need to be very far inland for snowfall: Totals? Well I could see a couple of inches quite widely, and up to 4 inches where the streamers hit. A possibility of a dusting further south (maybe Midlothian+Renfrewshire and East Ayrshire with a shot of seeing something). It isn't really a full on cold front, otherwise I would suggest the risk would extend to more of us, but it seems like a classic disappearing trough which reappears once it runs into higher ground just south of the central belt, albeit weakly. Beyond that, the 27th is where the risk reappears, as the crucial Greenland High builds to the west of the next low, which sinks over us: There's a reasonable degree of certainty that this will happen (despite my worries), but the real question is where exactly is this trough going to end up, because with the centre being so close to Scotland even small changes will make a huge difference to what happens. The generally agreement really ends around 6am on the 27th (although I suspect that's overegging the certainty a bit), as the rainfall from the initally westerly turns to snow on the back edge, with higher precipitation totals towards the centre of the low: While these charts may look relatively similar between the NMM (12Z) and the ECMWF, the low was positioned significantly further south and weaker on the 00Z runs, so the timing and positioning is very unclear. In addition, we could easily see secondary troughs forming and convective precipitation, particularly around the west and south west. I'd also say that, as per the UKV, the UKMO shows the low holding further north through the day, bringing a significantly snowfall risk from the west through the central belt: By Sunday evening, we have three very different scenarios based on tiny tweaks to the synoptic pattern - the ECMWF with the low pretty well in situ, some precipitation associated with it but quite dry and very cold at the surface away from any features or convective precipitation (although bear in mind the caveat that secondary features will likely show up somewhere with such low pressure nearby): An UKMO pattern which would seem to bring a continued snow risk across a large chunk of western and central Scotland into the evening, maybe tailing off or shifting temporarily as the low sinks south more gradually and a bit further east: And a GFS which sinks the low quickly and bring in a very premature easterly: Now, as we saw earlier this month, these upper air temperatures don't completely prohibit snowfall even near the east coast, but what they do mean is that any snowfall from this setup would be transient as the winds shift round to a snow killing direct easterly and the initial snowfall turns to rain: Now this detail is relatively unique to the GFS 'pub run' so far - the earlier GFS had a similar feature but with the low staying a bit closer meaning we didn't see a snow to rain event quite so quickly. But this does illustrate a possible pitfall of this setup - you really don't want to be to the north or northeast of this trough because things will very quickly turn to mush. I should note, though, that even in this relative worst case scenario, the easterly comes good in the end because the Greenland high is very likely to win out, and the cold upper air temperatures eventually arrive from the east to save the day: And as KW says, the second half of that output is really quite something, could still be a very special spell of cold and snow longer term (which tells you that there are quite a few paths to the 'holy grail' here). The ECMWF keeps the low closer by throughout, initially drifting south then back into the North Sea near the east coast, with occasional hints at this sort of mushy easterly (this chart for the 29th) but with perhaps a better angle of attack, with surface winds more northerly and temperatures closer to 0C( the palest yellow is 0-4): The UKMO is probably the best shot longer term too - with the low sitting out ,there's the potential for some really significant snowfall as surface winds would be either relatively light or northerly, but with the trough close enough to aid instability: In short, the 27th onwards is a period of very significant potential. At this stage it's difficult to pin details down, but what I would say is to look out for the bullseye - if it settles anywhere in the vicinity of Fair Isle to Dogger Bank I would be getting the sledges out. Also a note on why we should be ok if any 'shallow' easterly is short lived - take a look at the soundings for Edinburgh Airport for the approach of the 18Z mushy easterly: Still pretty good! it takes until midday on the 28th in Edinburgh and 6pm in Glasgow to see temperatures rise enough to clearly see precipitation turning to rain. It's good to be back!

Wooft! Very lucky - enjoy it! Glad to see so many familiar faces (I guess avatars/usernames really ) on here - hard to believe we've been on here for over a decade now! Something I've just noticed - (for future reference and I'll keep it separately in this post) - the netweather skew-ts appear to have the surface pressure basically constant, at around 1000hPa (or possibly standard pressure adjusted for altitude), which I think gives a distorted picture (either way really- too cold in high pressure setups and too warm in low). I may be wrong on this but there's a clear discrepancy between these and the weatheronline values for the same locations. So I will use the latter for the soundings, although reluctantly because the netweather charts are really exceptional and the UKV being available is a gamechanger for snow forecasting.

I have to say - I'm allowing myself to get a little excited about the output (unfortunately I've been busy with various non-weather chart related stuff over the last couple of days so haven't had time to put my thoughts together). There's a level of consistency to the output, although obviously I still worry about a low popping out of Newfoundland and nuking the Greenland high before it gets sorted. Probably need to wait at least 24 hours to rule that out entirely (although again, no basis for that other than experience of previous snowfalls gone wrong). I would say that the 850hPas aren't a great guide with slack low pressure setups - the surface pressure is a good 50-60hPa lower than your average easterly and you're not looking at the same levels of modification from the surface wind as normal. Most of the soundings I've seen, aside from the odd warm sector around the 28th or so, show the temperature sitting at or just below 0C at the upper levels, with the dewpoints generally below (that's for Edinburgh Airport for example, so nearer the coast it may be higher be equally the GFS sometimes overeggs the extent to which dewpoints increase at the coast). I've also noticed Christmas Eve has a little potential, at least in the sense the cold is very much there, everything north of the Grampians should see some snow and further south has a fighting chance of at least a flurry in the early hours... I'll try to post something more comprehensive tonight or tomorrow.

I'd broadly agree with this (also, amazing how that model thread is exactly the same as it was, what, 11 years ago? Utterly tedious in many ways!) I'll take a closer look tomorrow but there's certainly some December 2009 looking charts in the GFS output, albeit you can't have much confidence in the output at this stage. A lot of the signs are decent. I'm not sure quite how robust the blocking is but I do think there's at least some cause for optimism, especially since the kinds of setups we're looking at are more favourable to snow in Scotland. And given the restrictions, we might as well be cut off by snow for three weeks after Christmas anyway! Stay safe folks and I'll check in properly soon.

Well glad my scepticism wasn't that well founded! A decent covering here although it's starting to melt (which does seem sadly inevitable today). A great way to kick off the winter. Taking a closer look at the upper air temperatures they were, if anything, warmer than forecast - some -1Cs in the mix as the main front passed over. It was really the intensity of the precipitation that kept it mostly as snow - such a deep low and associated front enhanced by North Sea convection really produced, and without a surface easterly it was cold enough for it to stick.

Cheers! I was - never seen a storm like it! Glenrothes Airport ended up with over 100mm in 12 hours! I think the level of intensity we're seeing with convection now makes it pretty tough to forecast - even last night seemed more intense than equivalent sorts of set ups, although this is an unusually deep low. I did have a wee look at doing a winter forecast but didn't properly have time, provisionally I think it's shaping up quite well though - certainly some promising signs for the next few weeks with the signals for blocking and vortex weakening (at least last time I looked at the charts properly at that range).

Hi folks, it's been a while since I've dropped in! I was away again finishing my Master's this time in Innsbruck, and while I'm not quite done yet I'm back living in Scotland. Fairly interesting setup, although I suspect probably not one that'll be much use for me personally, aside from maybe a few flakes in the next hour or so. As Blitzen says, the basic problem is it's just not a cold enough airmass coming in - we're already looking at 850 temperatures of around -3C at the coast: If this were a standard easterly that alone would really rule out any chance of snow. Fortunately for some, it's very much not a standard easterly - this is the sort of angle of attack you'd really be looking for to maximise snowfall chances if the upper air temperature profile was just a bit better. You've got surface winds from the north with precipitation coming in from the east, which really minimises the dreaded North Sea modification: The problem, longer term, for most of us who don't live up a mountain (and actually for some who do), is that, it's still relatively windy, and once the precipitation hits you will still see enough mixing to drag temperature above freezing, even if there is a bit of snow to start with. The further up, and further west, you go, the less likely you are to see this sort of mixing and the more likely you are to see some snowfall. One possible saving grace is the precipitation does look pretty heavy - ultimately what's needed to avoid the 'meh' upper air temperatures killing the snow potential is our old friend evaporative cooling. And it's certainly not a lost cause - the EURO4 18Z is pretty bullish on keeping just enough cold air in place to turn the heavier precipitation to snow: The problem is, the relatively light stuff at the beginning. I think too many of us will see temperatures and dewpoints rise pretty rapidly when the light precipitation hits, and even a bit of back end evaporative cooling probably won't be quite enough to get anything to stick, leading to a sleety mess. But, if you hang onto the cold air for a bit, and catch some of the heaviest stuff like we saw last night, you might be in for a fun night. It's been a while since I've really had a go at forecasting on here (mostly because of lack of opportunities since March 2018) so I'm interested to see how it pans out. I'm in two minds about it, because a lot of things are really quite positive, but I just think widespread snowfall at low levels, with those uppers, without a *really* deep cold pool, is pushing it a bit. Anyway, good to be back and I'll hopefully be on a bit more often this winter! edit: I was also going to add that I was doing daily forecasting as part of the course this week and Innsbruck suffered both a non-event that was forecast to be a good snowfall and an agonising near miss, with over a metre of snow forecast in Eastern Tyrol while they're likely to only see sleet. Reassuring in a way to see it's not just us that has these struggles...

Looks like I should've stayed at work if I was wanting to get the full easterly experience Amazing totals in the central belt, really superb stuff. It's been a fairly decent evening, nothing lasting too long yet but fairly heavy when it has fallen and relatively frequent. This next line looks very impressive from here and I did see the thundersnow earlier so here's hoping the snowless pockets get filled in tonight

I'd describe the intensity of the snow here as 'it's thinking about it'. Slightly worried that we're somehow going to end up slightly too far north for this streamer but we'll see...

Me too - there's been a streamer no more than 2 miles to the north of here yet no more than a few flakes have reached the ground since 12.30pm. There's a lot of bright echoes in that stream and it does look very promising so here's hoping!

The max here so far was -2.2C (reached at 9am, it's been below -3C since 10am) which I'm fairly certain is our lowest, certainly the lowest since I changed weather stations a few years back. There was maybe a day in 2009/10 which matched it but it's incredibly impressive given the hours of daylight. The radar is starting to pep up just to our north so we might be in business soon...