LomondSnowstorm

Yeah it hasn't been as heavy as the radar shows here either. I think the wind is the main culprit - aside from blowing some of the snow away the effect of it has been to dry the lower layers of the atmosphere, so as it falls some of the snow is evaporating. Additionally, the melting effect isn't having quite the same impact on cooling because we don't have near saturated conditions as was shown on that NMM sounding from yesterday. If the intensity keeps up then we can start to see bigger flakes and more reaching the ground/settling but it is an extra barrier that isn't necessarily well represented by models like the ECM when projecting snow totals.

Does look like some very heavy stuff moving northwards at the moment particularly south of Glasgow, you'd imagine that would do something to lower the temperature enough for proper snow to 100m or so but could do with a break in the wind ideally since that's keeping temperatures up a bit even as far west as here. That should come tomorrow, but for now we'll just have to deal with it.

I'm tracking that at the moment, looks like the heaviest pulse is about to move over you and, hopefully, keep going northwards a bit... UKV for a couple of hours time: Does suggest we will see some heavier precipitation (my guess is snow) in central areas this afternoon before it transfers north. Does look like a lot of snow potentially for Stirling northwards and maybe higher parts of Fife as that's where the intensity is likely to be strongest. Watch out for a second pulse moving through this evening too: Marginal in places but the freezing level still looks fine enough certainly at around 300m:

I'd think initially yeah, the occlusion sinking south as the cold air undercuts behind it is what they're suggesting, something like this: You'd then see the showers coming in behind, but also fair to say the intensity of precipitation on the front would be significantly enhanced by North Sea convection too.

I'm not sure I'd have any finger nails left if that narrow path was what we were waiting on The 6Z yesterday was not dissimilar to that though so I suppose it's not too bad, we do still seem to have a little wiggle room as far as paths to a good easterly go but we just don't want that low on Thursday getting any further north or west or there might be problems (god knows we've seen too many of these vanish). The ECM ensembles continue to look very very solid: I do think the bump Thursday/Friday in the operational may be more likely than not given how many of the higher resolution models are going for it at the moment but notable that the majority of runs don't go for it and instead put us straight into the freezer on Friday.

Of course, I was more meaning, uhm, some of us who for some reason happen to be temporarily displaced in the west and thought they were suddenly immune to that sort of thing It is snowing here but pretty light and not settling yet, the radar is starting to pep up but I do think as Joe says we could end up with a bit of a precipitation shadow in central areas. In Innsbruck the equivalent setup would be called a 'Südstau', where the main precipitation is stuck at the ridge to the south and struggles to get over, but you'd think it could manage to get over the Pentlands a bit better than the 2000+m peaks there

I think we all forgot how much easterlies can modify temperatures, and that really looks like what's happened overnight - further west it's not too far off what you're looking for (most DPs are around or below 0C but with temperatures maybe 2C going by wundermaps) but without strong precipitation and with cloud cover it's not really ideal conditions for most of us. However, with the heavy stuff starting to move north I do think things will improve - look at how temperatures on the Solway Firth have dipped since the front arrived there: So for now anyway it's the modification, which can be overcome with heavy precip for most of us, rather than any frontal change that's causing the issues. Quite how far north the occlusion gets I'm not sure, the UKV doesn't really have much heavy stuff for central areas until afternoon: But the latest ICON does have some moderate stuff arriving by lunchtime which looks more in line with the radar: It'll be pretty patchy and altitude/intensity dependent until tomorrow night, but there's a window there where the colder uppers come back in and we're likely to see more widespread settling snow for all of us for a time: We'll see how the low develops on Thursday but it may turn a bit dicey for a day or two until the easterly proper can arrive. Anyway the longer term is still basically fine, the GFS threw a bit of a wobbly but doesn't seem to have been followed by the others and up to Sunday night it's a pretty decent run in any case. I think I'd give it a day or so, until the secondary low is properly dealt with, before having too much certainty in the easterly. I would say that things being a tiny bit further north than yesterday, as they are on the UKMO could work out even better for us as far as increasing snow totals go:

Approximately 40m difference in altitude (I guess more/less depending on where exactly they pick), pretty big difference in outcomes. It's not necessarily unreasonable but I don't think it'll be as clear cut as 3 days of snow there, 3 days of sleet here for example. I suppose in 2012/13 we did have a couple of events like that where really marginal altitude changes had massive differences in outcomes, with a strong easterly wind too, but I do think the situation is a bit more dynamic and things like precipitation intensity will make a bigger difference.

The most recent ICON is quite satisfying, if maybe not as snowy for high ground from the frontal stuff - the low splits and the westernmost one just sort of decays: and then the floodgates open: The GFS is very consistent again in bringing the beast in without too much damage: The milder air has a couple of shots at us but always seems to stall just before it can break through into the central belt properly: To be fair to all areas something like the GFS is probably best because the northeast takes a pounding on Thursday from the low, and we can live with a bit of potential slush before the easterly comes in further south.

Cheers! Makes it easier that you've usually posted all the interesting UKV charts anyway and I can just riff on that It's genuinely astounding to me that they used one outlier operational run to base an entirely weekly forecast on. At least when the Met Office based forecasts did that nonsense with UKMO runs that were a bit out there the warnings would at least match up - utterly bizarre to present that along with a warning for snow.

I did wonder if anyone had checked the lamp post yet - it's such a messy hazy picture, with showers coming in from the east and this band from the west, and with the radar not really doing a great job consistently picking up on it, that it's hard to say what's actually happening. The showers look real but the band is very much a movable feast. Going to be a confusing couple of days with so much going on and so little certainty on where things *should* be.

There really is - I think something I've failed to mention so far is that we do appear to have some convective stuff (albeit relatively organised looking) ahead of the front for Fife and Tayside tomorrow, in the morning at least, and I think this partly explains the 'gap' between the precipitation there and the main front. On the UKV the front seems to largely stall to the south until Wednesday morning when it comes north then westwards: Marginal stuff, probably snow inland but a bit difficult to tell due to the time of day: On the other hand you have the ICON with some heavy snow tomorrow evening for central areas: Compared to I think the difference may actually be explained by the angle of attack and by topography - the UKV seems to have the front 'jump' over the central belt to a large extent whereas the ICON has it converging there. I'm not too sure which is more likely, certainly both are possible, but it certainly makes a difference as to whether you'd be looking at an amber warning for tomorrow evening and where you'd place that.

ECMWF Forecasts WWW.ECMWF.INT It's quite impressive how much they've made freely available now, you can also get Extreme Forecast Index plots, their weekly 30 day forecasts etc. as well!

My hunch is we're looking at lower totals than modelled for the 'first half' of the potential snowmaggedon, and higher totals, maybe much higher, once the easterly proper sets in, and I think that's true for everywhere east of Glasgow. In part because, if we have -10C uppers or lower, you're going to see snow accumulate much more quickly - while I think we'll see some totals to relatively low levels tomorrow it'll still be a bit of a slog with temperature above freezing, whereas from the weekend onwards: Quite spectacular cold forecast here with a number of ice days.

I was just waiting for you to respond to that

The 'big 3' at +144: It's odd to say there's this much consistency at +144 when there's really not at +72, and that still bothers me, but if it holds tomorrow (and the shorter term clears up) I think we can start to get pretty excited...

Absolutely, and the closest thing you'll get to a modelled Forth-Clyde streamer on a global model:

The ECM doesn't like to make things simple sadly: If we're comparing like for like then the low is significantly further south than yesterday's offering, and arguably is trending towards the GFS (certainly it's a hair away from UKMO, but it does at least delay the arrival of the -8C uppers by a day or so, at least: On the other hand, I mean, this isn't bad for 48 hours work: After this it's moreso high ground accumulation up to Saturday evening and slight lower ground melt: But in the end it does work out fine: We're really nitpicking here but it's still pretty damn good

A small look at the soundings for just before the main front arrives, based on the NMM. This is for Coatbridge at 6am: the precip map: Going through some of the key variables: Freezing level - check (under 350m) Wet bulb freezing level - also check (less than 300m from the surface) Surface temperature - fine enough (around 1-1.5C or so, bearing in mind the surface has a lower pressure than 1000hPa) Dew point - meh (0.5C or so) Twet - not bad (1C or so) Uppers - around -6C at this point so fine Overall you'd probably expect it to be snow, but the reason I'd maybe be a little more confident comes from this paper (it's a bit lengthy but a good read, maybe for a time when there's less nowcasting to do!) https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&ved=2ahUKEwjS3Z_OosnuAhXtVRUIHXBnA_QQFjAAegQIBRAC&url=http%3A%2F%2Fwww.diva-portal.se%2Fsmash%2Fget%2Fdiva2%3A968860%2FFULLTEXT01.pdf&usg=AOvVaw1b5qEOXtTdFZZOiLiyY4UL Basically,the conditions are perfect for the 'melting effect' on latent heat (I think I've described it as evaporative cooling in the past but that's slightly different) whereby the snow melting at higher levels uses up heat energy and causes the temperature to drop to near freezing over a layer of maybe a few hundred metres. I'm sure we've seen this a couple of times or more this winter so far, and it's particularly pronounced with persistent, moderate to heavy precipitation as we're likely to see early tomorrow, and also with a near saturated surface layer as we're likely to have (near 100% humidity). The one caveat to this is primarily areas of low temperature advection, and this is where it might not be enough closer to the coast (I'll post a sounding for Fife and maybe the Lothians if it's different after I've had my tea) or if the easterlies are very strong, and of course if we get the full warm sector at any point it won't be sustainable. However, I just wanted to give a bit of theoretical basis to what we're looking for tomorrow.

It's quite amazing isn't it - the showers from the east are actually bumping into the band from the west over the Campsies at the moment!

The 12Z GFS looks markedly better than the 6Z for Thursday's warm sector drama: It's a very good run: The 12Z UKV is significantly better up to Wednesday night again, with the -8C isotherm as far south as Fife by the end of it: and snow showers starting to come in: The UKMO I'm not quite so sure about though - we don't get that secondary low splitting and heading eastwards and as a result we end up with the slightly flabbier low a bit further north: and as a result we end up with a fair chunk of us in a bit of a warm sector until it clears east: Still you can't be too unhappy with a run that ends with this:

It does look like that outlook is based on the 00Z UKMO/03Z UKV, as you'd expect from the Met Office I guess, but if the UKV 'upgrade' in the earlier part of the run leads to a 12Z UKMO more in line with the earlier GFS or ECM then I'd imagine they'll extend the warning to cover the vast majority of Scotland (or I guess maybe wait until tomorrow to do so to avoid having to make too many changes to a warning that's a few days out still).

It's really quite something to watch on the radar - the first band from the west moving eastwards as some still fairly decent convection off the North Sea moves west! I'm quite heartened by the amount of showers we've seen coming in off the North Sea well past when it was forecast to largely die out - bodes well for the weekend...

On my phone so can't post charts as easily but the 9Z UKV is certainly an upgrade on earlier runs, with the easterly coming in earlier and the front not getting as far north in the initial push. As a result the wet bulb freezing level is closer to 300m or lower for most and you would be talking about a more clear cut situation with proper snow north of D+G and the Borders and away from the coast. Not quite so clear on the implications of what follows it but certainly puts us in a better position with cold air flooding back in by Wednesday afternoon.

It's a mix I think - some of it is convective: But with that low cycling nearby (which I think the GFS is a bit on its own with that far on but who knows where we'll be by then) there are some fronts in the vicinity, I think an initial cold front as the lower uppers move in and then the wrap around occlusion moving back north around the low. I'd think the 'big' models like the GFS and ECM aren't really going to pick up on the convective stuff very well in general though, and some of the runs where the low clear are probably a bit drier than they'd be in reality because they won't pick up on features like streamers (and equally they might overestimate frontal precipitation too). What I would say is there's very high confidence that we're going to see sub -8C uppers with an easterly type flow at some stage from Thursday to the middle of next week: The details are difficult to iron out but most of the time you're going to get pretty widespread low level snowfall for most of Scotland with that setup.