LomondSnowstorm

Excellent work as ever Ed, as in depth and detailed as we've come to expect from a Netweather LRF, which is certainly no mean feat given Stewart's record and reputation on here. Certainly this seems like a particularly difficult winter to forecast in the sense that, while there are, on the face of it, very strong signals pointing towards a positive Arctic Oscillation and nothing particularly strong pointing towards an SSW, almost all of the composite charts I've seen (including mine) are very much favourable to low heights over Europe, certainly mid to late winter, along with positive height anomalies to our north. Anyway, all the best and I too am rooting for option 2 for February, not least so we can add a severely cold February to complete the set of recent extreme cold months (December 2010, January 2010 and March 2013)

Yeah, although it was only the early part of the month that was warm (which is pretty exceptional because there's usually limited variation in stratospheric temperature from late spring through to autumn). After that the strat cooled pretty rapidly without any actual SSWs or CWs and it was that which eventually led to the reformation of the vortex once the massive initial tropospheric forcings ran out. The Berlin site has the longest dataset but because it's (I believe) only for the North Pole it's not 100% accurate. I think there might be a more comprehensive chart somewhere, certainly I think GP posted a strat chart from 62/63 last winter at some point that might be worth digging out, but given the synoptics I'd be amazed if the strat wasn't warmer than average and if the strat vortex wasn't in a mess at multiple points that winter. 78/79 had a very cold strat early doors before a CW brought it up to around average but if you look at the blocking pattern (the same goes for December '81) it was more a case of a southerly tracking jet stream with a deep mean trough over Europe, although the blocking pattern was favourable in that the bulk of it was centred over Greenland. The positive for us is that we do appear to be in a phase, as we were in the late 70s and early 80s, where the jet is prone to being further south than normal and that gives us a big advantage regardless of the strat conditions. Certainly what we've seen so far is pretty positive - Scotland managing to stay very much on the cold side in spite of the blocking setup and the strat being far less than optimal, which is a decent omen for later on in the winter. I personally think it's got something to do with the SSTs, which as even the MO pointed out hint at a negative NAO (Chiono's forecast also goes for a negative NAO, as does mine) in spite of other signals pointing towards a positive Arctic Oscillation. Anyway, I'm slightly annoyed that our nice exclusive Arctic NWerly might turn out to be a more general Britain-wide northerly outbreak due to increased amplification on the western flank and a shunt eastwards of the pattern: Still, *if* it verified like this it would be a very impressive one at that: Certainly some very cold air bottled up in the Arctic at the moment and all the models are at least toying with the idea of throwing it in our direction, even if it is only a brief blast. -12C uppers from any direction would give good snowfall potential with major instability, although last week the whole thing was shortened somewhat by the premature eastwards movement of the high along with a dreaded warm sector.

It is indeed, associated with a negative NAO.As for the Omega block, I'd say our one wouldn't exactly be a classical omega block (yet) although with such massive amplification upstream I wouldn't be all that surprised if we did end up with the core of the blocking transfering eastwards to Greenland (at least temporarily). Either way, that anomaly chart is an absolutely textbook example of cold zonality - core of the cold over the US, big Aleutian ridge, ridging from Iberia up to Newfoundland and low heights towards us. I reckon the margin of error is probably a bit better than with typical cold spells too, because ultimately these are hemispherically large scale features rather than just the odd shortwave or a transient icelandic block as we saw last year. The cold is sourced directly from the PV too, so that also gives us more room for manoeuvre, and any energy which does disrupt south of the trough would only act to dig the cold further south. My one concern is that heights build over central Europe, but with troughing there presently and with the dominant feature likely to be the Aleutian ridge the vortex getting squeezed to our side by D10 looks almost inevitable. The 2m temperature anomaly drops below 5C on the 4th December for Edinburgh and doesn't get back above that for the remaining 8 days:

12Z GFS throws down the gauntlet in FI with an absolute stoater of a run: Culminating in a proper vortex split and screaming Arctic northerly: One of the more extreme options on the table but certainly not beyond the realms of possibility and it would deliver a hell of a lot of snow,with sub -5C uppers pretty much continously from +192 onwards.

Having re-read it I think it's more that we can't keep going on with a setup where the Pacific/Aleutian Ridge coexists with a positive Arctic Oscillation/strong vortex and that if we don't see the vortex split properly the Ridge will eventually get bulldozed, replaced with a trough and then it's goodbye to cold until later on in the winter when the +QBO wanes a bit. I don't entirely buy that analysis but it is a possibility. It ties in quite well with that anomaly chart I posted showing that basically we need the ridge on the other side of the pole to hold for as long as possible to give us a shot at cool/cold zonality. If it does eventually wane by, say, mid December it wouldn't be a great omen for cold for much of the rest of the month (although we could still just about squeeze out a UK high and some surface cold if we were lucky) but on the other hand it would tie in pretty well with my LRF

They're all pretty respectable for the end of November, with the mean sitting around -3/-4C for most of FI. Bear in mind also that -5C uppers in a northwesterly flow with low heights is a lot more useful than -5C in an easterly flow, so in spite of minima probably being significantly higher than we've seen in the HP of the last few days the risk of snowfall from these kinds of setups is quite good. The +192 mean highlights the cool/cold zonality potential, with the Atlantic and Pacific ridges 'squeezing' the vortex towards Scandinavia with a NW/SE tilted jet: The ECM ens is more muted and more HP dominated but there are similarities by +240: I've often wondered if there was some kind of pattern to cold zonality outbreaks because, in terms of longer term forecasting, predicting that we'll have a strong vortex is a lot easier than working out whether it will be favourable for cold outbreaks for Scotland, in particularly because most of these outbreaks are either transient or non-existent south of the Lake District, and are often associated with above average temperatures for most of western Europe and the mid latitudes. The composite map below shows most of the major cold zonality events of the last 40 years, with the Highland Blizzard of January 1978, most of the main snowfalls of January 1984, the Braer storm of January '93, the Christmas Day northwesterly of 2004 and early December 2011: Pretty much as I thought - the Aleutian Ridge (a likely major feature for this winter) is key to 'squeezing' the vortex over to our side, with Atlantic ridging also key to directing the cold air on the right trajectory. Anyway, that's essentially what we're looking for in the next week or two in the hunt for snow and cold. It's not 'pretty' in the sense that it involves a +NAO and a neutral/positive AO but actually for us it's very effective, and with the added bonus that you don't have the constant ramp/TOORP cycles on the MT because it's no use to anyone 'darn sarf'.

This is decent, gives the temperature anomalies, wave activity and 'z' values (basically the heights i.e. high z values mean blocking, low means troughing essentially) right up through the atmosphere all the way back to 1979: http://www.cpc.ncep.noaa.gov/products/stratosphere/strat-trop/ This is the 30hpa temperature graph for 65-90 degrees north, showing the warm strat early November 2010: You can get the rest of these from this link here: http://www.cpc.ncep.noaa.gov/products/stratosphere/temperature/ RE last winter, the blocking did occur hemispherically on an impressive scale, but lowland Scotland was pretty unlucky locally for most of January/February, and then March was the coldest month of the lot!

It's a fair point - we do need to look more holistically at the entire system and how it interacts rather than just looking at a relatively small part of it. When we talk about the stratosphere being important what we mean is more how it interacts with the troposphere and the rest of the global circulation system. And even then trying to work out how it affects our little bit of land is even more of a crapshoot and depends on a plethora of contradictory factors, all of which are linked to each other one way or another. Which is part of the reason we built computer models to try and work out the details and stop all our brains from exploding

It's all broadly interlinked, is the short answer - in 2010 the very cold start was largely a product of the troposphere I believe, although helped by the fact that the strat was anomalously warm in early November, which affected a split in the vortex before it could properly set up. It's relatively rare but even this year we've seen some early 'wobbles' with the vortex from that pumped up Aleutian ridge but the cold strat in October into early November seems (so far) to have made the difference in stopping the real blocking from materialising so far. I reckon some kind of coolish zonality is the solution guided by the mid Atlantic ridge the models will eventually latch onto, with occasional greater amplification of the ridge from (as you rightly see) a promising upstream pattern will bring the odd 'proper' northerly but the problem we have is that the vortex is becoming more and more established as it should do at this time of year so I'd say our best chance of real cold is still later on in the winter when full blown strat warmings are both more common and necessary to break up the established vortex. After that it's all about the positioning of any blocking, and with the current ML blocking tending to favour the mid Atlantic there are worse places to be starting from than this (provided this holds through December, which I broadly reckon it will).

It's pretty bad but in fairness the fact that it was December 2010 and we'd had a full month of almost nonstop snowmaggedon probably took the edge off a bit. Chuck that same story into a 'near-miss' winter and you've got the tragic tale of how even Christmas failed to deliver the only gift worth having - the gift of snow.

By the way, here's a handy way to tell if it's going to be freezing rain based on the infamous Christmas 2010 'why the swearing is not big or clever (touché netweather, touché) isn't it white' episode where Catch woke up on Christmas morn, eagerly ran down to the laptop to check the radar, saw a band moving in, saw the temperature at -4C, jumped around excitedly running in and out of the house trying to catch a better look at the lamppost for the snow only to find rain falling, at which point he ran back inside, had an extended TOORP on the thread and then threw the Christmas turkey in the bin in protest at the unfavourable 'lower uppers' (or at least that's how it happened in my head). It's for Albermarle which is, stupidly, the closest mainland station to Scotland where soundings are done (I'm lobbying for a weather balloon in every settlement populated by a member of the kilted thread post-indy) but it's similar for the one Northern Irish one, if not more emphatic, and marks the point when we started using 950hpas as a definite no-go for snowfall: The moral of the story is 1) don't ever bank on a technical white Christmas and 2) you can have a million and one factors in favour of snowfall but if just one layer of the atmosphere sneaks above 0C then you might as well have booked a month long cruise of the Med for all the snow you're going to see. I'll do a sleet example another day (probably that gut-wrenching 2nd/3rd February 2009 event or the many horrible near misses of January 2009) but for now I've got a night of cricket watching before I head off to Glesga the morra for some serious student politics (meet for 30-60 minutes then hit the pub, basically).

Cheers, almost exactly (maybe even the same) chart as we got in our meteorology lectures for the differences in precipitation type. I've certainly been enjoying the settled spell but unfortunately it looks like our luck may run out soon when the high sinks a bit too far south, allowing mild mushy drizzly nonsense over the top: It is a, dare I say it, a pattern worthy of the era of modernity, but that's not to say that amplification can't occur - early December 1990, part of a composite winter on my forecast and more recently picked up on by SK for its shorter term analogies, is an interesting example of this. 1st December 1990 - not much doing: 8th December 1990 - fairly notable snowfall: Both the ECM (posted earlier on) and the GFS (in this case the pub run) do try and bring in a toppler, although not quite as impressive a toppler as the chart above in many respects, but that high is going to be a rather fickle friend over the next fortnight at least, provided it stays around our latitude and doesn't drift too far east: Signs of a strat warming beginning in Russia towards the start of next month too, which could make thing interesting combined with the limpet Pacific/Aleutian high. As others have pointed out, without these we'd be locked into a classic 'zonal' pattern but with the ridge causing temporary disruptions to the tropospheric vortex and also aiding wavebreaking into the strat (perhaps eventually building up into a more substantial warming) we still have hope for setups which, if they land in the right place for us, could be very interesting for snow and cold here. The anomaly to the 20th was -1.1(below 1st-20th average) and with the last three days coming in well below average that will have been pushed down to maybe more like 1.3/1.5C below average. However, with the incursion of a Tropical maritime airmass early next week we're going to have to hope for a toppler to come good before the month's end to keep the anomaly in the significantly below average category.

I think low ground snowfall is unlikely between 9pm and 6am, almost anywhere to be honest, but before then we might have a bit of luck with the leading edge which is already edging its way into the far west, and certainly the hills will get a decent dumping off this. By 6pm, the 950s look fine for most still, although very much borderline the further west you go: The warm sector won't be particularly potent at the upper air level either, so if this were a December 2009 scenario where the cold was so deeply entrenched even southwesterlies were sent homewards tae think again we'd have a very good shot of snowfall: However, we don't have that deep cold pool and with the strong winds the sub 0 air will get mixed out fairly rapidly, with nowhere below 2C at the 950hpa level by midnight: However, by morning things get a bit more interesting in that dewpoints are forecast to fall below 0C again with the cold front starting to undercut: The slight complication sadly is that the 950s haven't quite budged below 0C widely: which means sleet or freezing rain below 200m, with probably just enough elevation for snow above that. Anyway, currently DPs are excellent for snow across most of mainland Scotland, with only the far north and northwest coasts really affected by DPs above 0C: http://www.scottishweather.net/ so we may still be able to salvage a good hour or two of low ground snow before the freezing levels rise overnight and we're left to drool over Northern Strath's updates about blizzards and the like.

I've been trying to find the netweather groups bit but with no success so far. I'm a SACRA member, it basically means you're obliged to post something like this: every once in a while. It was better back in the day when the MT was a lighthearted place filled with rampers with low expectations instead of now when every second run is a TOORP on there. Mind you, maybe I'm being a bit nostalgic in my old age Remember the great Ian Brown/everyone else stand off in the run up to December 2009? Utterly brilliant showdowns, the drama does keep you entertained right enough, same with the SMvIB (and actually GP+Chiono v SM+IB) stuff last winter. Anyway, good to see that some people saw their first snow of the season but a bit annoyed that the NMM lead us up the garden path like that. I'll need to make a note of that for future reference - 'likes to tease east coasters during northwesterlies with fantasy snow.'

Hmm, well this is interesting - NMM high resolution brings a feature across the central belt between 9pm and midnight, which is hardly surprising given the current level of convection building in from the west: A good night for lamping I'd say...

In terms of snowfall January 1984 was pretty exceptional by all accounts, although synoptically it was a completely different kettle of fish to the majority of 'classic' winters: A very positive NAO with an uber-strong vortex centred over Greenland. Normally this would mean straight zonality but with Atlantic heights negatively tilted and a Russian High in place the jet was deflected south of the UK on a NW-SE trajectory. This is part of the reason I don't jump to the conclusion that zonality=mild because, while that's almost always true for southern England (barring reverse zonality) it becomes an increasingly weak link the further north you go. For northern Scotland the real key is having a northerly sourced airmass, be it Maritime, proper Arctic or (somewhat less so) continental, but even here Arctic northwesterlies can be pretty effective. Similar synoptics brought us the January 1993 Braer Storm and associated snowfall and a large number of December cold spells, most recently December 2011 which was pretty snowy for western Scotland.

Yeah, possible, provided the cold air undercuts quickly enough. Behind the front we're looking at a pretty complex setup with a shower northerly combined with essentially at Atlantic occlusion moving slowly southeastwards, so all in all it's a very messy setup:

The good thing though is that with the low held further east the colder air comes back at us more quickly. The worst solution would be a middle ground where the low is big enough and east enough to give an all rain event tomorrow night but not far enough east to allow the cold uppers to get back in.

Having a read of the strat thread it does appear that my 'strong vortex for December' forecast is going to be tested to the absolute limit with wave 2 activity still showing up strongly on the ECM this morning, encouraging a split of the stratospheric vortex. Whether it can succeed in producing an SSW(that'd probably be early December at the very earliest I'd imagine, if not mid-late December if the first lot of wave breaking doesn't do it) is interesting, but either way there's still likely to be a fair amount of disruption to the shape and eventually the strength of the upper vortex in the next few weeks which could have a major effect at the surface in time. Meanwhile the ECM op has come out with its second consecutive run showing a Greenland high and -5C uppers are currently making their way towards Na h-Eileanan Iar - an unexpectedly dramatic start to the 13/14 netweather season.

Ok, a quick(ish) summary: Monday 12pm - cold front will have moved through, leaving in its wake some scattered showers, uppers just starting to get good enough for snow with the dividing line roughly around the Highland/lowland boundary fault: Coastal areas northwest of the line still might struggle in lighter precipitation but generally speaking 950s on the way down by this point: Decent shower activity starting to build, with the skew-t for Lochinver in the NW highlands signalling cloud bases under 900hpa and up above 500hpa: By 6pm the cold air will be starting to build in across central areas, with 950s just on the cusp of being good for low ground snowfall and probably enough within the heavier showers: Even more impressive convection for Lochinver, not quite thundersnow but certainly enough for convection to develop widely across the northwest Highlands: A similar story for the far northeast corner, where northwesterly winds will blow in showers off the Moray Firth: To a lesser extent the same will be true further south, with showers managing to get into Galloway,Strathclyde and Argyll with ease up to about Harthill, but with not that much chance of showers making it further east (outwith organised troughing). Accumulations are likely above 200m but may reach low ground further south, particularly between 5 and 10pm around the Firth of Clyde, but are likely to be slight i.e. no more than 1-2cm for low ground: As the night progresses, the 'organic' convection will largely die back to the coasts, but a trough will move southeastwards across northern areas overnight, bringing an increased risk of more significant accumulations for the likes of Hairy Celt, Northern Rab and Northern Lights: Uppers look good, particularly with the -10Cs moving in behind: and 950hpas shouldn't cause much bother: However, there is a risk at the immediate coast that the high SSTs combined with an onshore wind will push DPs above 0C (dreaded coastal modification), which is an issue moreso for Moray and the Aberdeenshire coast but also potentially in the Black Isle: However, no such problems further south, other than perhaps air temperatures being above 0C when the trough is likely to hit: The more concerning prospect is just that the trough moves too far east as the mid Atlantic high nudges in from the west, or that the Grampians 'eat' all the snow. A combination of these factors mean that the best chance of seeing wintriness south of the Angus hills is Edinburgh southeastwards, where the trough could still have enough power left to give the first snowfall and potentially the first lying snow of the season: However, details on this are likely to change through the day tomorrow when I'll hopefully be putting out a wee paint map about it all. I'm aware we still haven't got through Tuesday, when everything falls as snow until evening, but I've got to get to KB for 9am tomorrow to hand in my Physics hand-in so I'd probably be best calling it a night there. In summary - snowfall from lunchtime onwards northwest of the Boundary Fault, accumulations likely during the evening when temperatures drop to near 0C for many parts west of Glasgow and exposed to a northwesterly wind, nothing *that* serious until the trough comes into play in the early hours of Tuesday and gives the potential for a few inches for inland northern parts including Inversnecky and NL's home patch, less marked southwest of the Cairngorms with the best chance of 'lowland' snowfall being north of the Tay or south of the Forth, moreso the further east you are, Paint to come tomorrow.

This is certainly a good profile to be going on with on Tuesday: Any chart that shows precipitation through the day Tuesday as anything other than snow is nonsense IMO - whatever you see on the precipitation charts is going to fall as snow. It might not lie, but if you haven't seen snow so far this winter your best chance of seeing it so far is either Monday night or Tuesday.

It is a tough one to call certainly, and it's changed a lot due to that damned overblown shortwave and associated warm sector (which sadly looks to be progged further east, although a lot can still change in the next 60 hours before it hits). Anyway, I'll wait for the 12Zs to roll out before I give my assessments, certainly some potential based on the fax charts with troughs in the flow (although as always these too vary a lot in their placement): by Monday 3pm the coldest uppers are starting to push in, although 950s remain a touch on the high side (although close enough to 0C for heavier precipitation to fall as snow): by Monday night we should be out of the marginal zone and all precipitation is falling as snow: Some showers making it into central areas too by evening: A note about skew-ts - given that convection is going to develop out in the Atlantic, in the Minch, Moray Firth etc. it may be more worthwhile looking at skew-ts for the coastal NW highlands and the Moray coast to see if we can get some decent convection built up which can then sustain moving southwards. The skew-ts for lowland parts will be important too but if troughs move over between three hour skew-t charts then it might not be flagged up so well. Anyway, we'll see how that looks once the 12Zs are all out.

Anyway, the new +96 fax is a beauty (as expected given the forewarning by Ian): With the 528dam line straddling the centre of the country and cold air coming back at us from the north behind the shortwave this creates the risk of perhaps disruptive (if transient) snowfall for inland eastern areas and higher ground in particular. A forecaster's nightmare if ever there was one!

Now this is where it gets interesting - Ian F posted on the MT that the UKMO are going with a largely unmodified UKMO for what is now +84 (ie the +96 Fax chart). If the Chief forecaster is correct then things are going to get pretty interesting for most of us Tuesday night into Wednesday and onwards...

Last night being a prime example when the senior forecaster went against the GM's take on proceedings at +96 instead going with a heavily modified solution (I realise that's the wrong chart now because the faxes have updated but take a look anyway, the long awaited new +96): The reason they mostly look like the GM is, I'd imagine, because model agreement is usually pretty reasonable up to +120 at least to the point where modifications aren't that noticeable. A few times during similar 'high Shannon entropy' events last winter they did modify their faxes pretty heavily from the GM.