LomondSnowstorm

There's been so much binning of runs this evening that I doubt there's much left to look at

Cheers, but it wasn't actually that, it was a site which (at least claims to) have one of the newer MO high-res model outputs, either UKV or MOGREPS. I'm not sure how genuine it was/is but it would be good to find it again - here it is: http://www.weatherweb.net/wxwebchartsukmoukpcp.php It's in the form of a slideshow although it could just be directly lifted off the MO's website. Did you see Ian Fergusson's comments earlier RE how the BBC/MO website forecasts are generated compared to the tv forecasts? Intriguing. I always knew there was an incongruence there but I didn't realise it was because the websites just get 'fed' the raw data with no chief forecaster modification. Worth remembering that when people see arbitrary icon forecasts which don't match up with your expectations

In terms of details I'd put FI closer to +72 - after that this shortwave and associated warm sector are very tough to pin down so operational forecasting on Tuesday night into Wednesday is a bit of a nightmare. After that we have slightly more clarity of the larger scale picture with heights generally to our north/northeast and some variant of an easterly flow ranging from the potentially snowy to the dull and drizzly. After that, it becomes a bit tougher to call, but most likely cold and dry via high pressure situated close to Scotland.

UKMO comes out with an utter peach of a run: At +120 we're looking at uppers still at around -8C so the risk of snowfall, plus the shortwave at circa +84 (Tuesday night) is significantly smaller, further south and better positioned which would significantly reduce the warm sector in extent and would give an increased risk of snowfall. Very much looking forward to the fax charts tonight and I'll take a closer look at the NMM 6 (the very high res NMM) this evening once Monday afternoon comes into play.

I'll do an update post the 12Zs but we have a rather interesting situation on Tuesday night into Wednesday with the Atlantic trying to push in against our newly established cold pool. What this will bring and whether cold uppers will return behind it is very much up in the air at the moment so I'm hoping we get some clarity but depending on the timing that's where the major snow risk for most central areas will come into play. However, as Lorenzo pointed out we do have some troughs in the flow too which could also deliver, particularly early on Tuesday as the cold air really digs in. One of the most complex northerly 'toppler' setups I've ever seen and somewhat reminiscent of January last year when the cold couldn't decide whether it was coming or going. At the surface of course it looks like remaining cold for a long time to come but as ever snowfall is a different kettle of fish, so as ever it's going to come down to nowcasting. A few hints on what to remember: NMM is decent with precipitation but tends to overestimate coastal modification like its parent model the GFS NAE is sketchier all round (does anyone have the link to that site which shows what appears to be the UKV model?) but useful for things like the 950hpas yr.no is a class act and shows the ECM in high resolution, but wundermap also shows something which appears to be a higher resolution version of the ECM This site gives you the global NMM which is, I believe, a less UK specialised version of Netweather's NMM, although the two often have quite different outputs http://www.meteocenter.eu/WRF-forecast/NW/North-West-Europe.html

My take on the MT mood after a retrospective reading of this evening's capers: 'Aaargh, crazy horrible GFS, big run needed from the ECM' 'Hey, SM just posted to say it'd all be fine in the end so I suddenly feel reassured even though we're looking at exactly the same charts as before.' 'Boo, Ian Brown keeps talking every run down and cold baiting through inflammatory use of the word sinker.' 'Yay, ECM isn't so bad in the short term, it's all gonna be fine' 'Boo, not quite getting there, starting to panic now, but SM's still pretty happy so all's well' 'Ooh, looks like a reload coming from the north, big SM/IB standoff, let's back SM unless we're feeling like being contrarian and then we'll cheer on IB' 'Aaargh, indecisive, ends up not that great but with just enough hope to hold out for something colder, standoff continues' 'Ok, so we're finally seeing some sense, the ensembles are still basically alright and the other "adults" are back to reassure us. Bottom line is we're going to have to accept short term downgrades in the south for long term gain' 'Time for a nice stress-free pub run to see how our new hypothesis turns out: AAAAAHHHH SNOWMAGGEDDON WINTERWONDERLAND NOVEMBER2010 1947 1947 1947 1947 1947' *gives up reading the model thread*

Although the longer term outlook has shifted somewhat (and rapidly) towards a mid lat high over a not all that brilliant easterly (which is causing the mother of all mid November TOORPs) the outlook for us has barely changed (for the moment)- still cold, still cold enough for snow and still with decent convection likely on Tuesday/Wednesday. The specifics are a bit up in the air after Tuesday and the overall pattern has been flattened preposterously over the last 24-36 hours but we're hopefully at the timeframe where the northerly at least will remain, certainly not the first time though that we've seen bizarre looking features (in this instance what looks like a late season tropical storm which has a pathological hatred of mid Atlantic heights) spoil some decent synoptics, if that is what happens. Honestly, I'm not *that* fussed, not least because the upgrades were so stunning and so surprising, but we're all sensible enough to take anything that comes our way this early as a nice wee bonus. We'll probably get some falling and maybe even lying snow for lowland Scotland, and hopefully a good top up for the ski resorts, and then have at least a few days of dry settled conditions at the very worst before we see which way the heights go.

Very encouraging signs indeed, I did notice that, in spite of the model variability in the medium term, the ECM ensembles this morning remain as promising as they were last night, if not moreso. The common theme from pretty much all the output is low heights remaining over Europe, blocking emerging to our north and a vortex weakening or splitting with most of the energy heading towards Russia. And that's after a not unsubstantial northerly next week which could yet deliver a decent fall for many of us - not bad for mid November!

Not that much to report in the way of short term changes this morning, GFS wants to hold its shortwave further north thereby giving us a shot at the prize of proper frontal snow (as much as 4-6 inches possible inland) but these things usually flip around and even if we don't get this we're compensated with lower uppers and convective stuff instead. Longer term the UKMO has decided to come onboard for the northeasterly while the GFS is a bit less keen and the ECM goes a bit mental as it tries to sink the high then decides it wants another go at a northerly after all. ECM ensembles look almost identical to 12 hours ago though, so I think ultimately we're not in any better or worse position than yesterday, but it's going to be a few days before Thursday onwards gets properly resolved. Fortunately, we don't even need to worry about that yet because we've got a good enough cold spell lined up next week as it is, and that's mostly what we should focus on until the longer term becomes clearer.

I felt I had to do the honour since Mondy/BFT isn't around at the moment Kinks in the flow are pretty good (unless you're called Stewart Stevenson sadly), troughs can be decent as well depending on the topography, I might have to dig out some fax charts from the northerlies than went right in past years to say whether it was the cold front or just troughs embedded in the flow. Basically, convection is enhanced because SSTs are 4-5C greater than they are later in the winter. It's not specific to northerlies and in fact it's why the November 2010 easterly was so exceptionally brilliant (and why the Great Lakes get most of their snowfall in November/December before the lakes freeze up) but it does help to get it over the Grampians.

I'll dig it out tomorrow after the 12Zs if I've got time but the NMM-18 (basically high res GFS which goes out to 168 hours on NW-extra) has been signalling that some of those troughs may actually still have a bit of life in them when they reach the central belt and actually have some temporary accumulations for central and eastern parts, particularly Wednesday into Thursday and on this run, Monday into Tuesday. Now, many of us have bad experiences with northerlies in terms of them being dry but more often than not they come good for central areas this early in the season and I'm hoping this one will prove no different. If past years are anything to go by I'd expect a good number of of us to end up with a covering at some point in the next week and almost all of us to see some falling snow. I'm particularly interested to see how northerlies pan out here in Edinburgh with the Forth 'resurrection' effect coming into play with troughs that have died out going over the Grampians, since we had almost no northerlies last year. Currently it's a balmy 10-12C off the coast of Scotland so we're going to be looking at a 20C temperature gradient from surface to 850hpa level and a 50C gradient to 500hpa. Now given that the skew-t for Lerwick itself was so promising with air temperatures at 5C imagine what 12C is going to look like - some very hefty showers and even thunderstorms are going to crop up, so I'd expect some really spectacular troughs heading down the spine of the country north to south. Actually, even better, 500hpa temperatures are progged even lower than I thought, below the magic -40C threshold, which means that we *may* even be looking at territory favourable to Polar Lows: These only occur in straight northerly flows but that's exactly what the GFS has for us on Tuesday evening: Too early to say, but keep your eyes on the satellite charts from Monday night onwards...

It was a superb spell, utterly top notch, thought I'd have to wait years to see snow as deep as it was in 2009/10 (which was just under a foot I believe) but less that a year later we had way more than that (I think my depths were conservative because snow kept falling off the measuring table!). Day after day it pilled in off the North Sea, with the temperature nudging slightly downwards right through to the Wednesday when it sat pretty much dead on 0C the whole day, and the first time 'snow week' off from school in memory. Just an amazing time, everyone should get a week like that at least once in their life, and because of January 2010 (a rather different but similarly excellent affair) I basically had two snow weeks in the space of 11 months.

Good to see some familiar names returning to the forum again, hopefully even more will be back along with some newcomers as the northerly gets into 'telly forecast' range. If you're a newcomer or just a more occasional poster from years gone by who's lurking then come and say hi and join in with the discussion - we're friendly folk, honest!

That's not a bad ensemble mean to be looking at in 5 days time: Quite a spread of solutions thereafter which is reflected in quite a 'fuzzy' mean but the key is that the Italian trough remains which gives us a shot a reload further down the line: The greatest spread over higher latitudes from southeast Greenland round to Novaya Zemlya is also a promising sign:

It's all a bonus anyway I suppose at this time of year, just nice to see some proper blocking on the charts this early in the season. Possible that the models are overestimating the heights over Russia too as they did in January/February 2012, which ironically would be a big boost whereas then it was a major hinderance!

You're right BUS, that was basically what I was warning about earlier - amazing synoptics for January aren't any use unless there's a cold pool to tap into. However, it's still worth keeping the heights around because sooner or later they're bound to try and set up for another go at the retrogression (although the GFS is inching ever closer to achieving that in one go, which would be nice). Otherwise, it's just a case of whether it's crisp and cold with a very limited chance of any snowfall or an easterly which squeezes every last drop of our cold pool until there's nothing left and have a slight chance of getting something epic if it holds. Either is excellent in terms of the longer term synoptics but the latter would feel like such a waste, to a much greater extent than the very cold and quite snowy but 'so gutted it wasn't 6 weeks earlier or it would've been worse than '47, '63 and December 2010 combined' spell this March. How it could redeem itself is by doing this: Which would make the week of dank drizzly 'North Sea' airmass worthwhile, but at the moment it looks like some kind of holding pattern will be required if we are to get retrogression to Greenland. It's worth remembering that we had a similar build up to November 2010, with a not all that cold easterly to start with before the cavalry showed up from the Siberian Arctic. This chart, for example, is pretty similar to much of what the ECM has churned out in its later stages: This was made irrelevant by the vortex dropping into Russia and two days after the verification date of that chart we were staring down the barrel of an exceptional cold spell: Anyway, there's little point worrying about FI for the moment when we're looking at a good 4 day cold spell at the very least Monday-Thursday next week.

850s up to(but not above) 0C are technically fine for snowfall under certain circumstances, mostly if the cold is deeply embedded at the surface and is cooling the lower layers of the atmosphere up to around 900hpa. 950hpas upwards MUST be sub 0C for snow to fall, which is why that's the parameter I tend to use, unless pressure is extremely low i.e. the Braer storm where 950s were probably around 1-2C but 950hpa was pretty much ground level. 528 dam thickness is probably a better indicator than -5C uppers, 1290 on the 850-1000 thicknesses is even better (anything below that and snowfall is almost guaranteed) and dewpoint at or below 0C/Wet bulb temperature below about 1C are also fine indicators but none are entirely fullproof so it's just a case of checking all the variables and seeing if they all tick the box. And then we have to hope there's some precipitation around. I'll run through a quick example for Lerwick on Monday midday based on the 12Z GFS: Uppers -6C *check*: Thickness 522 dam *check*: DP sub 0C *check* 850-1000 around 1295 *passable*: Now we check the skew-t for any nuances between 850hpa and surface level that might lead to a problem for snowfall, including it being too dry: Confirms DP is sub 0C (actually -1.9C), Temperature is a bit high *but* 0C line is about 950hpa so it should be ok, along with the fact that the cloud level is very low (maybe 900hpa, check out John Holmes' guide on the learning area for how to read skew-ts) and the CAPE is extremely promising (cloud development above 500hpa) so showers would be very, very likely to form, with a high chance of troughs embedded in the flow. Given this tendency for heavy showers air temperatures would be pushed down readily during showers since the relatively high air temperature is a function of ground heating by the sun so we'd expect pretty much all precipitation there to fall as snow, although probably not lie given the high initial air temperature. So, in summary, it depends

We're really firming up on the northerly now, GFS 12Z has sub -5C from Monday midday: to the end of high res: A whole week of snow potential, with winds varying from WNW through to NE. And from this position, every chance of a reload when the next chunk of PV decides to drop down to our east: (although with that slightly pessimistic caveat I posted earlier about chilly damp easterlies if/when the first upper cold pool dries up until the real cold arrives) I haven't seen charts this promising in November since 2010, although with the glutony of synoptics riches we've been treated to just in the last 12 months you'd be forgiven for thinking these are pretty 'standard' winter charts - they aren't. We may be on the cusp of yet another major cold spell, in spite of a cold stratospheric vortex and in spite of most of the signals going into this month indicating exactly the opposite for the first half of winter. The MJO is likely to help us out, with most forecasts putting it back to a low amplitude phase 8/phase1: (cheers for these ScotlandWx/Lorenzo!) Anyway, we'll have to wait and see how the UKMO and ECM shape up, but I haven't seen anything genuinely mild in the longer term in the last week and I suspect that's how it'll stay. It's coming folks. LS

In fairness, I think most of my generation at least know who Debbie Harry is, if not quite the significance she had on guys in the 70s. Anyway, slight issue (and one we really didn't have last March) is the lack of deep cold uppers to tap into. The northerly itself looks like bringing circa -8 uppers to our shores but after that any of the 'cobra run' setups that I've seen have involved a flow from the continent which is barely any colder than Scotland. Thus, the ECM setup of a decent northerly which is followed by a cold high sitting over us is for me the best option in terms of overall weather enjoyment and actually also in terms of chances of decent cold. 4C with endless Stratocumulus isn't my idea of fun, so a bitter but dry holding pattern while Eurasia gradually cools is for me a better bet than something synoptically impressive but actually pretty tame (unless we can get proper troughing into Scandinavia). Low heights over Europe are the key - if we keep those it really doesn't matter how close the high gets to sinking because it won't actually sink.

Jeezo, that reminds me of December 2010, the most stonking GH perhaps in the history of the archives: It was such a dominant high that the warm front that preceeded the trough brought the warmest temperatures of the whole month, melting most of the snow from the earlier epic cold spell (the dizzying heights of 8C-10C for most) before the cold front piled on in behind.

Classic ECM, superb synoptics but slightly watered down uppers. Still, what does it matter with temperatures like these on the ground: And the first signs of some North Sea convection popping up too in the northeasterly flow: The angle of attack will shift around a bit, but the main thing is that the potential is there and we have cross model agreement for our first proper Arctic incursion of 2013/14

The 12Z reminds me a little of this (only a little mind): But of course that was only the beginning, and just five days later: And the rest is history. Now I'm emphatically NOT saying we're going to see anything like the depth of cold we saw then (certainly it would be foolish at this stage to make any firm prounouncements beyond +144), but my point is, if we do end up with sustained blocking in the right place, a chunk of the vortex is liable to drop into mid-latitudes and then we're in business.

Excellent from the ECM, it usually has more moderate uppers than the GFS so we shouldn't be too surprised but the euros are looking excellent tonight:

Interesting methodology Phil, I effectively did the opposite with mine, including years which weren't *that* close a match and then weighting the anomalies to those which were closer, but with similar results actually, at least for the second half of winter.Best of luck and I hope your December forecast is closer to the mark than mine!

UKMO at +120 - circa -5C uppers (528dam thickness) moving across Scotland by Monday midday, with troughing embedded in the flow bringing a mixture of rain, sleet and snow. Temperatures around 6-7C but dropping rapidly by late afternoon and that's when the fun begins: +144 - uppers sub -8C across Scotland (sub 520dam thicknesses), showers probably would be temporarily easing as the wind shifts round to a northerly but maxima suppressed to 1-2C at most with certainly a covering of snow for the vast majority north of Inverness/above 400m west of the Cairngorms but also with the possibility of snow cover further south, particularly in the west. Everything that falls on Tuesday is likely to be snow, although with the caveat that moderation of uppers could still lead to a more watered down affair closer to the time. Tuesday night would give another potential shot of snowfall with similarities to this: The GFS doesn't quite achieve the same degree of ridging so isn't quite as impressive but on the ground there's not that much difference, with the main one being that the flow switches to a straight northerly much more rapidly. The cold plunge is very much still evident: After this point we have divergence (or possibly not since we don't see the UKMO post +144) with options varying from a rebuilding of the vortex over Greenland leading to a return of 'zonality' (not that this necessarily means mild of course) to the much vaunted retrogression to Greenland High nirvana and a series of COBRA runs forcing the little dancing man on Frosty's posts on the MT to have a heart attack.