LomondSnowstorm

Good to see some areas getting a bit of a top up today - while this was sort of on the radar (to pardon the pun, especially given the amount of false echoes I saw nearby to start with) it wasn't really widely flagged up, especially given the wind direction, south of the Grampians, to the point I'd not really tuned in properly to it being a real possibility until I saw a few flakes when I was out walking today! It was in fact a wrap around occlusion, not an especially strong one away from the northwest Highlands but definitely fits with the 'get the snow in and cold will follow' (even if the cold pool is a little on the shallow side today): And I see the radar is still pretty active even now! Tomorrow's feature is another of these relatively rare beasts - the warm/occluded front from the north! The triple point of the fronts and the occlusions are the key to the snowfall tomorrow - if you're under this, or to the west of it, you're likely to see the heaviest precipitation and you're most likely to see it as snow. As per the FAX chart we can see this has moved a bit to the west over the last few runs, with it now sitting around Glasgow by midday. The warm sector also shows up quite nicely on the theta-e charts - a pretty marked warm sector by midday: Prior to the arrival of the front, though, we're looking at some low temperatures, with skies clearing overnight: There's a bit of a difference between the NMM and UKV on this though, I think partly to do with the timing of the front - the latter has the front coming through a bit earlier, meaning the lows are higher as cloud moves in earlier to Fife and Stirling: and ICON has something in between: This does make quite a big difference even aside from the placement of the front, as it means precipitation starts off as either snow or freezing rain as it runs into the cold air. While this, sadly, probably won't be enough for coastal parts of the northeast, or even many places below 100m more generally with a wind off the Moray Firth in addition to the warm sector, it does mean that the front is quite likely to bring wintry precipitation to start with as it reaches central Scotland: It's also notable how much difference there is on the placement of the main front, and how this impacts on the main snow risk - the UKV is the furthest west of the three, to the point where I think it's really limiting snow potential to higher ground and doesn't really have much frontal precipitation for Fife or the Lothians. The ICON keeps the triple point much further east but sliding southwest as it moves through, meaning the snow risk is much more widespread, and continues through to tea time: Unsurprisingly this is quite a snowy run, with a covering for almost all of mainland Scotland at some point during the day: The UKV, on the other hand, really doesn't bring very much snow, with it mostly confined to high ground and areas where there's currently a notable amount of snow cover (this is the maximum extent): And the NMM is, again, somewhere in the middle (slightly higher accumulations north of the central belt during the morning on this that melt later on): We are really dealing with very fine margins again, and while it's likely we'll see some snow melt with temperatures getting up to 4-5C in places behind the front some places could still end up gaining on the day if something closer to the ICON setup pans out. The margins are emphasised, again, with this sounding for when the front is passing over on the NMM for central Fife: Both the dewpoint and temperature hug the 0C isotherm up to 900hPa. Further west it's pretty similar, but with the dewpoint at the surface lower: This does create a risk for freezing rain, particularly further east as the warmer upper air moves over the cold surface, so I would certainly watch out for that as well. Which setup is more likely? I think the most recent satellite image does suggest it's a bit further north than the UKV suggests, so I'd tend to think they're a bit fast with the arrival of the front, although we're already seeing temperatures of around 5-6C on Orkney and 4C on the north coast. I think you'd do well to get a couple of inches out of this if you're below, say 150m, but the further west and higher up you are the better shot you've got, and there's certainly a lot to look forward to beyond Hogmanay (but more on that either tomorrow or when I'm suitably hungover on the 1st!)

We've also been pretty unlucky with it - seemed to largely miss us entirely other than a few flakes at various points! Good luck to everyone else this evening (always nice in these setups when the cold air is in place and you get precipitation cropping up at short notice!). Going to do another summary this evening I think, with some considerable potential tomorrow and for the foreseeable...

I reckon you might just squeeze out enough of the last of the easterly flow to get a dusting in Glasgow before the northerly kicks in and the convection dies. I think the best of it is over here, all seems to have shifted a bit to the west. Still a pretty decent covering so can't complain, and probably a couple of showers still to come judging by the radar, maybe some heavy. I'd really just take it not melting at this stage to be honest - the UKV looks reasonable enough for bringing a proper clearance of the low cloud around 3am and bringing an air frost.

Cheers! It's incredibly marginal isn't it! During the lighter periods it's looked a bit sleety here too, with a bit of melt on the front of cars, but just the intensity and extra tiny bit of altitude/distance from the coast makes a huge difference.

We seem to have scored a direct hit with this one, amazing to see! If form is anything to go by with these setups there should certainly be some left for areas south and west of here too...

I think the bottom of it *should* hit Edinburgh pretty well...

Incoming... (and I wonder if the intensity might fix some of the marginality problems...)

Huge flakes here now: VID_20201228_212012.mp4 For reference this is green on the netweather radar - incoming is yellow/orange over a large swathe....

Snow back on at the moment here, firmly as snow but not quite heavy enough to do much yet. The next band looks quite impressive though, and looks like it could end up a bit further south than this one... Okay now it's coming on pretty heavily! Still just a dusting but it's at least visibly accumulating again

Ooft - haven't seen an outlook like that in a very long time. At some stage with synoptics like that you'd inevitably end up tapping into some serious cold pools...

The wet bulb itself is fine, like -0.5C (if the temperature and dewpoint are fine the wet bulb will be too) but must be that pesky upper level warm sector pushing in from the north east, melting the snow too high up. Very difficult to say exactly where is the wrong side of marginal will be with this since even the models aren't clear. The Euro4 suggests it's a bawhair between snow and rain everywhere from Angus to the Borders:

I think the heavier stuff is still likely to move in in a couple of hours, the UKV at least has a lull then some more later on Better for the Lothians and Dundee too (moving into Perth as well) but certainly suggests we're not out of it just yet!

Thanks! Sorry to see it missed you, the oddities of these events is they become so localised. Nothing more here for a wee while either. It's basically the air temperature minus 1/3 of the difference between it and the DP (so like in your case around -0.5) but you can calculate it here: Psychrometric Calculator WWW.KWANGU.COM At the moment it seems fine enough, and the Freuchie station is approx 1/-1 so wet bulb temperature around 0C

Happy Hogmanay from the ECMWF ?️

Seems to be pepping up pretty quickly on the south side of the Tay judging by the radar! Euro4 just out, certainly doesn't show the 950hPa temperatures getting much worse than it is now over the next few hours for Fife, although definitely looks like lowland Aberdeenshire is going to struggle to maintain much snow sadly

Yeah, it did get a bit more intense after I took the video (looks like it'll become a bit lighter although more seems to be heading in) but with the frost not lifting it started settling immediately.

It is indeed! I believe if you go back a couple of hours you can see some convection offshore heading towards Fife, but dying down, but I believe this is it intensifying again as it runs into slightly higher ground. Starting to stick quite quickly here!

Unfortunately not, unless he's on and kind enough to share What I do have is this: VID_20201228_180030.mp4

It's a difficult one to forecast certainly - even the 2km WRF doesn't seem to represent the current situation at the surface especially well: This is the sounding for here for example, where the current air temperature is -0.4C according to the nearest Traffic Scotland webcam. While I would expect that to rise as the cloud approaches it does make me a bit dubious of the current profile. Probably the best bet for nowcasting for myself is to check @Benvironment's measurements up the hill and if the wet bulb temperature is below freezing there it's probably going to stick as snow down here

Aye still quite marked disagreements further out - seems to be broadly a northerlyish cold trough setup until New Year with some colder uppers likely to come in just before Hogmanay and the chance of localised features cropping up but beyond that difficult to pin down - the high seems to move closer to us for a bit but whether it settles over Greenland or links up with the Russian high (or, presumably, some sort of milder option!) is really anyone's guess at this stage, or at least something that would require a bit of poring over the ensembles/teleconnections only for it to change again tomorrow!

The GFS shows how marginal it's likely to be tonight - with showers moving into Fife and the Lothians from 6-9pm it also shows this at the 925hPa level: Surface temperatures look fine enough away from the immediate coast but it's really a bit above the surface, where the flow is more of a direct easterly for a time, that the marginality sneaks in. The WRF soundings will probably be most useful in placing this in relation to the precipitation because the GFS grid is too coarse but difficult to call either way. Unfortunately my weather station picked a bad time for the battery to die so until the new ones arrive tomorrow I'm flying blind on conditions here (other than the weather station at the other end of the village on Wunderground).

I should also say, it's just nice to see proper wintry weather again even if there's not much snow - the Lomonds look stunning today with a pretty thick frost and yesterday evening's dusting intact: I can also report that Innsbruck is experiencing similar frustration at a near miss, with 40+cm in South Tirol and the Trentino but very little making it to the other side of the Alps aside from the pesky foehn winds and stupidly low humidity (10% on the nearby mountain!!).

Good to see some potential from the easterly (well more of a northeasterly) later on. There's a bit of a limit on how much we can really get from it as the upper cold pool, which is already quite shallow, will be eroded overnight, but still some decent potential this evening. That on the face of it would be fine enough for snow particularly if there's a bit of intensity to ward off what looks to be a very marginal layer around 925hPa. Later on in the evening we are likely to see the snow level move to higher altitudes as the surface cold erodes a bit, but this seems to coincide with the showers dying out anyway as the flow turns more northerly. At that stage I think it's unlikely to be a huge problem as clearing skies will bring surface temperatures back down too, so possible some areas can get up to an inch or so and retain cover through tomorrow as well.

As someone about 5 miles east of there that sounds about right But also that does seem a fair estimate, certainly it looks like it's not getting much to the east of that longitude to the north at the moment. If you squint, I guess you could argue it's maybe got a bit more of an eastward advance going than modelled but that's almost certainly wishful thinking.

Yeah it does look as though it's going to end up a bit too far west for the likes of us unfortunately, should turn to snow as the heavier stuff moves in particularly inland but certainly seems the less snowy scenario for most of Scotland. However I wouldn't get too disheartened - the cold is in place now, Edo rightly points out that the North Sea snow machine is still running (even if it is just warming up). The ECMWF is absolutely worth a look if you're a fan of snow near the east coast - some very tasty precipitation and cold enough uppers especially with a slack flow around New Year Forecasts | ECMWF WWW.ECMWF.INT ECMWF is the European Centre for Medium-Range Weather Forecasts. We are both a research institute and a 24/7 operational service, producing global...