After a week where temperatures of -2C have been regarded as a slight warm up for most of us and where snow cover still lies thick on the trees days after its initial fall, the old adage of all good things must come to an end enters the fray. It should have been plainly evident to anyone who has lived in this country for more than two years that spells like this one are not only rare but exceptional, and even the classic winters did not manage to sustain cold like that for over a fortnight. The breakdown of this particular bout of cold has been on the cards for over a week now, initially progged as a Christmas Eve breakdown but eventually changing to a battleground scenario between embedded surface cold, aided by high pressure towards Scandinavia, and the first Atlantic low to get as far northeast as Iceland in a month. The general set up by midday on Sunday seems almost certain to happen http://www.netweather.tv/gfsimages2/gfs.20101223/18/66/h500slp.png http://www.meteociel.fr/ukmo/run/UW72-21.GIF?23-18 http://www.meteociel.fr/modeles/ecmwf/run/ECM1-72.GIF?23-0
The trough to our west is the obvious feature, introducing less cold air from the Azores (initially) and creating a frontal boundary between itself and the cold air to the east. As has been said on the model thread many times, the tilt of the trough is key - southeast-northwest would be ideal as it would allow us to tap into colder air from a frozen continent. A flow from the south would allow lower dewpoints to move northwards, keeping snow loss at a minimum and any precipitation as snow. The worst case scenario, and one which it would appear we may be heading towards, is a southwest to northeast tilt, dragging unmodified Tropical Maritime air up from the Atlantic.
http://www.netweather.tv/gfsimages2/gfs.20101223/18/66/h850t850eu.png
The latest GFS gives us a SSW flow, blowing away the cold air rather quickly http://www.netweather.tv/gfsimages2/gfs.20101223/18/90/h500slp.png
The cold air over the continent is trying hard to get into the UK, but merely clips the tip of Kent http://www.netweather.tv/gfsimages2/gfs.20101223/18/90/ukpaneltemp.png
Needless to say that almost all of the precipitation on low ground from this point onwards would fall as rain http://www.netweather.tv/gfsimages2/gfs.20101223/18/99/ukprec.png
Superficially, this chart looks very similar to the 18Z, but the differences on the ground are stark http://www.netweather.tv/gfsimages2/gfs.20101223/12/96/h500slp.png
The difference is a few degrees in the angle, but it's enough to get that continental air in http://www.netweather.tv/gfsimages2/gfs.20101223/12/102/ukpaneltemp.png
so that when heavier precipitation moves in to all areas it has a fighting chance of being snow http://www.netweather.tv/gfsimages2/gfs.20101223/12/111/ukprec.png
The UKMO is somewhere between the two, probably allowing enough of a continental element to bring snowfall to inland areas as the current BBC forecast suggests http://www.meteociel.fr/ukmo/run/UW96-21.GIF?23-18
The latest Fax chart will be of much interest. As we approach NAE and NMM timeframe I'll update further, but as I've noted slight changes mean huge differences in the outcome.
A quick Christmas update: http://expert.weatheronline.co.uk/daten/proficharts/en/nae/2010/12/23/basis18/ukuk/prty/10122512_2318.gif
Looks like central Scotland could see a technical as well as semantically white Christmas.