LomondSnowstorm

I got caught in a deceptively heavy shower on my walk today near Auchinstarry Marina and a lighter one that was a bit sleety on the way back through Croy, don't think it quite made it through to Coatbridge properly but it's a decent sign for when it turns properly later on.

I do fear Freuchie is in line for a really good snowfall, the one time I'm away somewhere that's not stupidly snowy (wasn't quite so bad seeing pictures of a couple of inches last year when I was in the Alps or in 2016/17 when I was in Stockholm ).

That's understandable, I think sometimes on the forum we've developed terms that even meteorologists wouldn't really understand out of context Basically, you want the eastern edge of the low (black line) to be tilting from SE to NW so that the warmer air from the south is going directly north of the low, which will strengthen the block and mean the low won't be able to move northeast and bring the milder air in with it: You'd also want the low to then link up, in some way, with the low over Italy, somewhere around where the red line is. You'd also want to see less of the purples to the east of Greenland too to allow the high to build a bit further north. These things are driven by a number of other factors but for looking at how each model changes that's a decent way to see whether what's happening is better or worse. In terms of actual weather though that chart is quite a snowy one for eastern Scotland and that's probably the main thing for now

I've been holding off since it's pretty difficult to pin down details with convection, but I'll do a summary this evening. I think in short I'd say - don't get your hopes up too much the further west you are tomorrow, specifically, because it's possible we end up with this 'discontinuity' in the flow where it goes quite abruptly from easterly to southeasterly over the middle of Scotland: The UKV for what it's worth does continue to build showers through the day and eastern areas will certainly get frequent snow showers but further west it'll likely be a bit more sporadic because of this. However, by Monday morning this should be resolved, and we will get a bit more of a northerly tilt to the flow and a flow that goes from west to east, meaning more snow and streamer possibilities for central Scotland: Beyond that I'd say things are likely to continue similarly to Monday, some potential for streamers or slightly more organised stuff especially following the next low undercutting on Tuesday but I'd think somewhere like Cumbernauld should do reasonably well since the flow will be pretty close to a straight easterly, or a slightly northerly tilt, so any streamers that do form in the Forth are quite likely to be heading in your direction. It's worth noting though that the isobars are going to be more 'southerly' than the flow itself - you can see today for example that the showers are moving through pretty much perfectly east-west despite a southeasterly tilt to the isobars:

The Scandi high on Thursday/Friday seems to be increasing in strength run by run, the GFS 6Z makes one of the closer attempts I've seen at extending and, with a few tweaks, potentially even intensifying the easterly. This is what I'd say is the key point to keeping the easterly going: There's a lot going right here, with a negatively tilted trough and some signs that the monster low in the Atlantic is trying to disrupt, with that little secondary low forming to its east. With a slightly stronger Genoa low that sits in place a bit longer, and the disruption a little bit earlier/further southeast, we would see a linkup, with the secondary low sliding under, the main low staying out in the Atlantic (as seems likely now anyway) and this flow continuing (there's some pretty nice looking convective snow potential through Wednesday and into Thursday btw, just as good as anything early in the week): What we get following it isn't Atlantic driven by any means, and the cold air actually comes back in from the southeast after this too, but it really really wouldn't take much for it to turn into something epic:

I'd probably go with 200m for sea level too actually, I've seen 200-300m quoted too depending on the situation but it probably comes from living in places around 50-100m altitude where 300m is generally fine for snow.

It'll come in fairly gradually through the day but by 9pm tonight our marginality issues should be over: Rain should start turning to snow inland south of the Tay from about 3pm onwards too:

I'd think Perth's in a decent position for a covering of snow - it'll certainly be cold enough for a few days and unlike a lot of us who wait apprehensively for a bit of a northerly element to the flow to emerge Perth does pretty well from due east or ESErly winds.

The pub run gets even closer to extending the easterly, a bawhair away from it:

That's a slightly better looking hemispheric picture from the ECM long term too - if you can get a bit more undercutting and a little more clearance over the Arctic to allow a linkup again with the Scandi high we could be in for a reload by the end of next week...

ECM looks fine, again if I'm being picky could do with maybe slightly lower heights but you can't really sniff at charts like this 72 hours out: Or this 96 hours out:

I would say, regardless of the nuances of the flow for the rest of us, that you're very likely to see at least some snow from this setup - it would be really hard to imagine it missing you much as it normally tries

I'm assuming it's relating to their snowfall on Sunday - it does look like the northern/western extent of it has been scaled back a wee bit, which is indirectly a bit worse for us as far as the strength of the flow goes if the low just ends up being much further south, but if it's just that the band doesn't reach London but instead pivots a little further east it makes really no difference to us at all.

They have their moments but I have to say the UKV, which is a pretty incredible model as far as resolution goes, has had a couple of bad misses this winter. I think if there's broad alignment on the details then it's very good, but of course when you get such volatility with details resolution doesn't make up for having the low in completely the wrong place, for example.

There's a couple of sticking points to this developing, the low over Svalbard (which only really appears at D4 or so so could be overcome I'd imagine) and the height rise over Iberia, but certainly wouldn't take much to trend in that direction. I remember at the beginning of this spell we saw the heights over Svalbard slowly clear run by run to allow the block to develop. The GFS also comes pretty close to keeping the easterly flow going actually:

GFS a bit better than yesterday's 12Z with lower heights and a stronger flow: UKMO still a bit 'limp' looking, for the reasons outlined previously I think (not enough on that low in the Channel on Sunday): Quite interested in how it tried to carve out more of a Scandi high though, that might be something that could develop and create a bit of a longer lasting easterly: Doesn't quite manage it on this go but you can see how, with a bit of a westward shift, we could end up with a longer easterly:

It is only yellow for now but a pretty good yellow warning all things considered, extending across the central belt through to Wednesday lunchtime and '2-5cm daily accumulations widely, up to 10-15cm' in some areas.

I *think* the key to this is how far north the Channel/Pas de Calais low gets. The runs that have it a bit further north have a bit more of a northerly tilt to the flow and slightly more pepped up precipitation e.g. the ECM: the UKMO doesn't do it so much, so the flow is a little to the south of east generally: The GFS is better on this as well, and the 6Z places this low a bit further north than yesterday's 6Z too: And the ICON has started to develop it a bit more in recent runs as well: Basically there's good reason to believe the UKMO/UKV will be towards the lower end of what you can expect snow wise especially south of the Forth even if you assume it isn't underestimating convective precipitation. Just for illustrative purposes, here's how the ICON 6Z ends up (this is just accumulations since 6Z so we don't end up with inflated totals because of what the models think has fallen already, and there's almost nothing at low levels until late Saturday so it isn't distorted by the current frontal 'no):

Saturday night should be a decent palate cleanser: Although, I can't promise it'll be entirely dry:

Fair point - it gets close but it doesn't quite reach the Black Isle until early Tuesday:

The ECMWF even delivers the rarest of things - easterly snow for both Ayrshire and the Black Isle, in the one timeframe!: It does try to break it down but even at this range struggles to erode the cold pool and instead delivers a pretty nice frontal snowfall (for those of us who aren't permanently scarred by this week): Southerly and fairly slack winds with embedded cold does normally give you a better chance but that is really just for fun at this stage and to illustrate how difficult it could be to dislodge the cold once it's established.

ECMWF looks pretty good at least up to Wednesday (probably a bit far out then to be too worried about the exact placement of that low, but it does seem that that's where the next potential breakdown or at least easing of the flow comes in), this for Tuesday to Wednesday: I'm having trouble with my usual ECMWF precip data but the site itself shows at least some precipitation right down the east coast from Sunday morning right through, and on most of them it gets at least some way inland: On a global model that's really as much as you can ask for - it's not going to really show much beyond that well but convective snow developing and moving inland somewhat for 3 days straight is a good sign certainly.

Starting to see signs of snow pushing further west on the ICON through Monday: We don't have the ICON archives but I did find this from the 2018 thread for the first Tuesday of the easterly (27th February), which had considerably less day to day uncertainty in the build up: The reality was the initial front sweeping in from the east (10am to midday): It wasn't actually terrible in terms of capturing what happened but certainly worth bearing in mind that features can, and will, crop up at short notice.

Some impressive stuff from the 12Zs again. UKMO: Not much point posting the uppers since uniformly -10 to -12C throughout, with the -12C coming closest around +120 (I'd think Tuesday on that would be the snowiest day for most of us with streamers developing through the central belt but Monday also looks pretty decent for that too). GFS: Certainly can't complain but if I did have a criticism it's that it could do with bringing the next low, the one to the southwest at +120, a but further north and east, if we could get that we'd have a bit more of a northerly tilt and probably would sustain the thing longer too as well as increasing our snow chances. The GEM: This is more what I'm meaning, and the +12C line gets to us by +120 too, again I'd think pretty snowy. All in all a good set of runs so far, and better than yesterday's 12Zs certainly.

Difficult to say really - 2018 was more of a 'pure' easterly than November 2010, similar to this one it started more southeasterly: The upper air temperatures for that one though were on another planet from really anything we'd seen since January 1987, just a crazy depth of cold: 2010 didn't have anything like the same depth of cold as far as upper air temperatures go, but was more directly Arctic sourced than 2018, and had a slightly more northerly tilt generally to the flow: I kind of thing this one is somewhat in its own category - the blocking isn't really anything like as strong (albeit probably more extensive over the Arctic), the uppers are sort of in between the two in terms of depth but maybe closer to 2010 and it's being sustained more by a weak and very southerly tracking jet rather than just a big yellow Greenland high: Certainly on the east coast I don't remember either being especially dry, hugely snowy but if anything a little disappointing because of how the breakdowns went without a nice anticyclonic period to finish. 2009/10 was more of a drawn out period with a few big snowfalls but also mostly sunny crisp days to enjoy the snow. Not that I was around at the time but this setup looks far more like something out of the late 70s, like this for example: