Firstly, I'd like to have a look at what happened in the west of Scotland today. Basically, a level of upper air had a temperature above freezing , causing rain to fall despite the low ground temperatures [url="http://expert.weatheronline.co.uk/daten/proficharts/en/nae/2010/12/25/basis06/ukuk/t925/10122509_2506.gif"][color="#284b72"]http://expert.weathe...122509_2506.gif[/color][/url]
This was actually picked up on by the NAE at 12pm yesterday, so perhaps we should've been more on the ball with that one [url="http://expert.weatheronline.co.uk/daten/proficharts/en/nae/2010/12/24/basis12/ukuk/t925/10122506_2412.gif"][color="#284b72"]http://expert.weathe...122506_2412.gif[/color][/url] .
However, it was Christmas Eve, so I'll give myself a bit of slack there! However it would be careless if the risk of this occuring again was not examined.
Tomorrow has all the hallmarks of a complete mess forecast wise - models differing in their view of uppers and dewpoints at just the moment when things are most marginal.
The easiest question to answer is probably when - around 5pm for areas west of Glasgow, 7pm for any to get to Edinburgh and about 10pm by the time the heavier stuff makes it east according to the NMM and NAE
[url="http://expert.weatheronline.co.uk/daten/proficharts/en/nae/2010/12/24/basis12/ukuk/t925/10122506_2412.gif"][color="#284b72"]http://expert.weathe...122506_2412.gif[/color][/url]
The where is also explained - heaviest across western Scotland, including Galloway, and the central belt, with Grampian and eastern borders areas seeing very little up to Monday midday [url="http://expert.weatheronline.co.uk/daten/proficharts/en/nae/2010/12/25/basis12/ukuk/rsum/10122712_2512.gif"][color="#284b72"]http://expert.weathe...122712_2512.gif[/color][/url]
What will precipitation fall as is a toughy, so I'll attempt to firstly ascertain what COULD be falling out of the sky tomorrow. I think living things such as frogs and mice can be ruled out straight away, as can dust and sand.
Freezing rain? It must be said that there is indeed a risk of this in East Ayrshire, East Lothian, Kintyre, the Borders and the Northwest Highlands [url="http://expert.weatheronline.co.uk/daten/proficharts/en/nae/2010/12/25/basis12/ukuk/t925/10122621_2_2512.gif"][color="#284b72"]http://expert.weathe...2621_2_2512.gif[/color][/url] with little change in the affected areas by 9am on Monday morning [url="http://expert.weatheronline.co.uk/daten/proficharts/en/nae/2010/12/25/basis12/ukuk/t925/10122709_2512.gif"][color="#284b72"]http://expert.weathe...122709_2512.gif[/color][/url]
How about snow? Any chance of that? Well with the NMM (which I can't post on here) suggests lowland central areas will see almost exclusively rain with dewpoints about 1-2C, though inland and upland areas above about 200m should see snow, at least to start with. The NAE is different - snowfall is for a wider area from central Fife to Stirlingshire to Dunbartonshire (and obviously including M74 corridor and the Highlands) [url="http://expert.weatheronline.co.uk/daten/proficharts/en/nae/2010/12/25/basis12/ukuk/prty/10122700_2512.gif"][color="#284b72"]http://expert.weathe...122700_2512.gif[/color][/url] .
D.Ps look good for most of inland Scotland, though as I already mentioned some areas would see freezing rain rather than snow [url="http://expert.weatheronline.co.uk/daten/proficharts/en/nae/2010/12/25/basis12/ukuk/taup/10122700_2512.gif"][color="#284b72"]http://expert.weathe...122700_2512.gif[/color][/url]
Minimal change at the surface through Monday morning in terms of dewpoints according to the NAE [url="http://expert.weatheronline.co.uk/daten/proficharts/en/nae/2010/12/25/basis12/ukuk/taup/10122712_2512.gif"][color="#284b72"]http://expert.weathe...122712_2512.gif[/color][/url] so if you start with wintry precipitation it's likely that you'll stick with it and if you start with rain you're very unlikely to see it change back barring evaporative cooling with intense precipitation. As for accumulations, who knows really! With temperatures just above freezing, wet snow would still lie reasonably on top of old snow, but of course rainfall would compress snowfall into an icy mess. [url="http://expert.weatheronline.co.uk/daten/proficharts/en/nae/2010/12/25/basis12/ukuk/tmp2/10122700_2512.gif"][color="#284b72"]http://expert.weathe...122700_2512.gif[/color][/url]
I would favour Northern and Western Perthshire and places like Moffat and Lockerbie to see significant accumulations and coastal areas, both east and west of Scotland but especially west, to see a complete thaw. In between that is all to play for - some will come out with more snow on Monday morning than tonight, others will have much the same, many will have an icy pack and a few may see a total thaw, but, predictably, inland and upland is the place to be if you want to see snow from an atlantic front.
By Wednesday at 12pm the models agree that high pressure will have seen off the Atlantic threat [url="http://www.netweather.tv/gfsimages2/gfs.20101225/12/96/h500slp.png"][color="#284b72"]http://www.netweathe.../96/h500slp.png[/color][/url] [url="http://www.netweather.tv/ecmimages2/20101225/12/ecm500.096.png"][color="#284b72"]http://www.netweathe.../ecm500.096.png[/color][/url] [url="http://www.wetterzentrale.de/pics/Rukm961.gif"][color="#284b72"]http://www.wetterzen...ics/Rukm961.gif[/color][/url]
Yaaaay! In practice, of course, that doesn't mean we're back to cold and crisp weather. Maxima on Wednesday are hardly much to speak of [url="http://www.netweather.tv/gfsimages2/gfs.20101225/12/96/ukmaxtemp.png"][color="#284b72"]http://www.netweathe...6/ukmaxtemp.png[/color][/url]
but with mild uppers and a southerly flow we're likely to fall short of an air frost in most places [url="http://www.netweather.tv/gfsimages2/gfs.20101225/12/114/ukmintemp.png"][color="#284b72"]http://www.netweathe...4/ukmintemp.png[/color][/url]
However, it does look a 'cleaner' high than yesterday when a dirty raw southeasterly was being shown. The GFS suggests the flow will be slack by Hogmanay, allowing for a more widespread frost [url="http://www.netweather.tv/gfsimages2/gfs.20101225/12/162/ukmintemp.png"][color="#284b72"]http://www.netweathe...2/ukmintemp.png[/color][/url] though a rather unfortunate and purposeless easterly flow would, in this instance prevent eastern Scotland seeing a frost. [url="http://www.netweather.tv/gfsimages2/gfs.20101225/12/162/h850t850eu.png"][color="#284b72"]http://www.netweathe.../h850t850eu.png[/color][/url]
However, let's not bother with details as the other outcomes on offer for New Year's Day range from a cold easterly to the beginnings of a northerly to high pressure sitting just south of the UK.
The UKMO is promising, giving a colder easterly to southern England and would almost certainly allow for cold air to move right across the UK as the high retrogresses to Greenland [url="http://www.wetterzentrale.de/pics/Rukm1441.gif"][color="#284b72"]http://www.wetterzen...cs/Rukm1441.gif[/color][/url]
Note that an easterly from the initial low is unlikely to deliver this far north and that getting the high to move northwest is key to getting severe cold back.
The GFS ultimately fails on this front and allows the PV to reform to the north, postponing hopes of a cold spell for a fortnight at least [url="http://www.netweather.tv/gfsimages2/gfs.20101225/12/240/h500slp.png"][color="#284b72"]http://www.netweathe...240/h500slp.png[/color][/url]
The ECM is nice, making more of troughing over Scandinavia to allow the high to drift slightly northwest of the UK, and we'd probably be close enough to the centre of it to see a cold and frosty New Year as opposed to the milder, cloudier scenario we ended up with earlier in the month. On the face of it the ECM appears to be headed for a dud, the high drifting south [url="http://www.netweather.tv/ecmimages2/20101225/12/ecm500.192.png"][color="#284b72"]http://www.netweathe.../ecm500.192.png[/color][/url]
However, the troughing to the east and heights building to the north give a clue of the next step in the evolution - [url="http://www.netweather.tv/ecmimages2/20101225/12/ecmt850.240.png"][color="#284b72"]http://www.netweathe...ecmt850.240.png[/color][/url] [url="http://www.netweather.tv/ecmimages2/20101225/12/ecm500.240.png"][color="#284b72"]http://www.netweathe.../ecm500.240.png[/color][/url]
Stunning easterly with a Greenie High.
So, to sum up, anticyclonic weather will kick in midweek onwards, with frost returning to most by the weekend and the possibility of something more significant than mere surface cold into the first week of January.