LomondSnowstorm

First signs of the strat warming having a major tropospheric impact today? GFS 0Z gives proper blocking setup, albeit without deep cold uppers, in FI. Model consensus is trending colder with cooler uppers from the southeast, at the moment a no man's land for us between deep cold easterlies and an east/west battleground but if the heights continue to edge west on the modelling so too will the deep cold.

It's good synoptically but the lack of a proper cold pool to the east is slightly perverse given the time of year: Still, better than nothing, and the longer that Scandi High hangs around the better chance we have of a proper SSW by the end of the month and a shot at something genuinely impressive for cold.

I hope not either, although it would be typical if it was the mild bit that verified but not the cold Still got around a fortnight to get my 'beast from the east' started and with the strat warmings and lowering of zonal winds I'm not all that downbeat - reckon my temperature forecasts are once again going to be quite a bit too low though.

I think the delayed onset of the front is why the risk of snowfall has increased a bit - previously it looked like reaching us by 3pm but now that's extended to 9pm: 'Lower' uppers just about holding due to a slightly better tilted trough, with Glasgow westwards in particular having a shot of some early snowfall: before turning more to rain later on: The really intriguing bit is that uppers above 0C are overhead right now, draw up from the south, but slightly colder uppers will push in from the west with the front: By midnight this layer is fine, but lower down the less cold air has mixed in: Slightly unusual to see one layer warming while the other cools with frontal systems but this is an unusual one - we're currently in the 'warm sector' (believe it or not) although the warm front was incredibly weak and hasn't really managed to mix down to lower levels. When the cold front pushes in behind this evening it will cause the milder uppers to mix downwards while simultaneously bringing in a colder airmass From a geek point of view it will be fascinating to see how this unfolds - cold be a freezing rain-sleet-snow-rain event for some. edit: forgot to put in the actual fax chart:

950s flirting with the idea of battleground snowfall tomorrow afternoon, probably a combination of the time of day and above freezing 850s will do for us: Temperatures forecast to rise above 2C just before the precipitation hits which won't help us: The uppers are pretty dire for it which means cold or even freezing rain where DPs remain below 0C around the central belt: Models still flirting with the undercut but not quite delivering. You can see the slight improvements from yesterday at very close range between the 6Z today and yesterday's 12Z GFS: Higher heights ridging northwards and a more negatively tilted low gives us a better shot of getting the cold in. This is what we were looking at for that timeframe 3 days ago: The trough properly disrupting southeastwards with much better ridging north. We're not going to get back to that sadly, but we can get a bit closer to it and maybe force the second trough to go under properly if the first one can just get enough low heights southeastwards into Europe. The longwave pattern is basically fine, we just need the smaller scale features to work out to advect some of that cold air westwards.

Really? Last time I checked the bit you quoted was one of two possible outcomes for February, a month which isn't even within the range of the GFS yet. We may not end up with an easterly but we do have blocking to the northeast building for the first time this winter and with further wave attacks on the vortex and we could easily end up with an SSW, or at least some decent vortex disruption, by the end of the month, all of which could lead to a colder than average February but also potentially to the second, less cold, scenario outlined. Calling that potential from 2 months out is pretty good going, and I don't see why the work of people like Chiono should be dismissed so easily, especially when we're not even at the halfway point of winter yet. Most forecasts (including this one) basically went for a winter of two halfs and most got the first half about right - let's see what happens in the next 7 weeks before we start throwing toys out of the pram.

Let's put all our money on that one then - 1000% better than anything else on offer tonight

Slack cold air in situ over the British Isles at +96 on the ECM, cold enough for snow for northern and western Britain and Northern Ireland though most likely dry and cold here with no wind. Actually looks slightly better in some ways than yesterday by Day 5 with good amplification into Greenland and the low starting to fill a bit more quickly. Need to see a bit more 'energy' going under the block to the east to pull the cold air through but certainly not the worst chart I've seen, even if we do end up getting unlucky:

While it's no beast from the east, the ECM solution for Tuesday is very cold and although mostly dry there could still be the potential for snow from features developing in the still cold air: Uppers not that relevant because there's no wind but you'd be looking for -2C or lower I guess to be sure of snow: Thicknesses around 520dm so very cold, still air. Other models have it a bit less slack and potentially more sleety/rainy but the above wouldn't be too bad.

Yeah, overnight could be a bit interesting: Temperatures look fine away from the far west, potential for a dusting if you do catch a shower: Slight issue with the 950s being a bit high, but these appear to push down as PPN approaches between 3 and 6am: A wee bit of convective potential but you only need a bit to sustain the convection coming in from the Atlantic: Probably not much more than a cm or two even for higher ground (maybe 2-3 mm precipitation at best for the southern band of showers just north of Glasgow, more like 4-5mm precipitation maximum for the NW Highlands) but it's been an entire month since we had any lying snow here so even a little hint of it on the cars would be welcome.

The pub run has finally heard my plea for sanity and goes with the undercut. Pretty far from perfect but something to work with going into tomorrow:

Five clusters and some fairly notable differences within each cluster tells the story here - many options still on the table, a good number of them still cold and snowy, moreso the further north and east you go but certainly not exclusively. All to play for tomorrow.

In fairness, they were all wrong the day before by a long way as well before flipping yesterday morning and then flipping again today, it's just flipping mental There are at least 10 very different solutions within the 20 run suite by +144, including westerlies and one where the PV drops into Scandi with a GH giving us a massive northeasterly, so safe to say it isn't having the best of days

I'd avoid the rollercoaster for a while to be honest - models as ever are doing their best to turn a standard undercut into another crazy ride. The latest ECM contrives to keep the Scandi high, keep low heights to our south yet somehow end up with one of the warmest easterlies in history: It all comes from the models' collective inability to deal with this massive round low in the Atlantic, which it decides should just drift slowly southwards, weakening all the time until there's nothing left of it: That low has ONE job - disrupt southeastwards. The only thing it has to do to usher in an easterly is go under the block, following the low that went before it and following the global NH west-east flow of air, and somehow the GFS and ECM decide that this time, it's going to sit there, glaikit, not even bothering to move. I don't buy it frankly - either the high sinks or the low disrupts eastwards, and I think the latter is a lot more probable all things considered. So, in summary, get back to your hot dogs because there's no point looking too hard at something which isn't likely to be resolved for at least another 24 hours.

Bit of a straw clutch here but it's certainly true that the models are struggling with how the lows out west interact with the block, and I wonder if we've been here before with the previous low - on Tuesday the ECM 12Z was showing this for Tuesday 14th: Taking 48+ hours to disrupt the trough southeastwards: But by tonight's run we see a cleaner undercut on the first low, albeit with the second one then proving more problematic: By the time it disrupts it basically stalls, failing to get any further east: I do wonder if, with another 24 hours of modelling, we'll see the models getting a better grip on the second low and maybe even getting a decent undercut. The other thing of course is that the Scandi high is modelled a bit further east on this run, but with better trough disruption under the block you'd expect the heights to back a bit further west anyway.

A few degrees more negatively tilted troughing on the ECM compared to both the UKMO and the GFS, slightly better WAA as well (although not as good as yesterday's to this point). Should have more of a chance at the undercut although it depends on how rounded or otherwise the low is modelled: (not jumping to early conclusions, just pointing out slight but potentially significant differences early on).

Good for you, all I meant was that a good number of people in the Northwest of England, Southwest Scotland and Wales saw their biggest snowfall of 2012/13 (for some their biggest in decades) in mid March, so, you know, plenty of time left for snow still

EURO4 hinting at some Saturday snow: 12Zs will be interesting - more promising thicknesses than on most of the near misses we've seen this winter plus it's coming from the west.

I haven't really seen much evidence of a sustained eastward shift of the block to be honest, in fact the low getting held out to the west is perhaps a bigger problem for us than the block being too far east (at least at the moment).We need to remember where we were two days ago in the modelling and that we have made some massive strides forwards - comparing Monday night from the ECM 0Z two days ago compared to now shows a massive shift westwards with much more jet energy going under the block: With the blocking in place to the northeast and at least some jet energy getting under the block (helped by an emerging Euro trough) most of the differences are based around how far south and west the pattern can get rather than if there'll be an easterly flow at all for our longitude. Living further northeast than most members on here I'm not as despondent as some on here by the northwards shift of the pattern but equally we do want to see a shift towards more trough disruption to allow the cold pool to back westwards. These are fine margins, and as long as we have the basic 'trough to the south, high to the northeast' then the boundary between snow and rain will be fluid even into the mesoscale model range. Currently northeast Scotland away from the immediate coast is the place to be but at this range the Margin of error is big enough that I wouldn't rule out snow for any part of the UK within the next 8 days. I'm also not all that concerned with the idea of a 'return to zonality' post Day 10 - it's worth noting but at that range a lot can, and very often does, change at that kind of range. We still have over half of winter left, with the second half always progged as being of more interest for HLB than the first, so even if this one doesn't work out there's still plenty of time left to get your share of snow. Last year it took until March, two winters ago it took us in Eastern Scotland until early April, so it's far too early to get hung up on the first real chance of an easterly this winter so far. The only thing I will add that's a bit concerning is the potential for further heavy rain in the south - that's not what anyone needs, so for that reason I hope we can shift the troughing further south.

Good call, although maybe wait a while before you go back on again or things could get pretty messy...

They all basically support the undercut as far as I can make out - just a few favour the slower, potentially more rewarding but also more high risk option. As I said, at some point the pattern will get shunted southwards, usually at pretty short range, so having the coldest air initially just slightly further north works for me. UKMO brings in a battleground intially with -4C uppers and southerly winds which could signal decent frontal snow (could be some of that Saturday as well and also Monday but this needs to be determined closer to the time). Nothing's really changed and the ECM Ens have upgraded, presumably because some of the rush which went very wrong very early have left the scene as that option became near impossible, so almost all are at least toying with a proper undercut. Let's see where we are by the 12Zs

I'm pretty pleased with it all really, strange just how far northwest the pattern has drifted which is counterintuitive since it usually seems to go south with easterlies and east with blocking setups. Hopefully it either holds or makes slight southward adjustments (or at least just have some sensibly modelled trough disruption which the GFS seems incapable of) between now and verification. PS: if you look on the MT you'll see Ian F and Snowballz effectively hinting that there's an SSW in the offing from their GloSEA5 - caps off about as good a day's model watching as you will ever get. Now all we need is it to verify!

I'm certainly not too 'perturbed' by the Ensemble mean: All academic for now though - tomorrow's runs will be fascinating once again...

We now end up with an attempt at a Greenie high and Scandi trough after all that: Still, as I said earlier today with the 6Z the slower evolution suits me fine given a model propensity to sink the pattern southwards once we hit mesoscale range. Actually, any of the operational runs look fine.

It's an utterly blockbuster run, with a killer Arctic shortwave, a stronger low than on any previous op. with less trough disruption early on and the weakened heights settling in over Greenland. However, we still have a negatively tilted trough, a wedge of HP and, eventually, the undercut: The shortwave may end up bringing in a northerly but that's such a convoluted evolution it would almost certainly give Nick S a heart attack if we had to follow it all the way through