Here is a summary of the NWP output for noon on Monday.
Issued 0523, 17th December 2009.

The new week will start with low pressure close to or over the UK. With cold
air entrained, there's a risk of significant snow as milder air tries to
push slowly northwards. It may well not make it to Scotland for another
week, but further south rain looks likely by Tuesday for southern England
and by Wednesday for northern England. However, there's a very fine
balancing line and it would be no surprise to see the first milder push (on
Tuesday) shunted away to the east instead. The European models seem to be
more progressive with getting rid of the cold air, while the GFS keeps it
around for longer.

Analysis and evolution to T+120 via GFS
NE'lies cover the UK, with a slack low over the North Sea. The low deepens
over northern France tomorrow, bringing NE'lies across most areas. Saturday
sees lighter NNW'lies as the low moves eastwards and fills, followed by
stronger NW'lies on Sunday as a new low moves swiftly southwards over the
North Sea.

T+120 synopsis
http://91.121.93.17/pics/Rtavn12015.png /
http://91.121.93.17/pics/Rtavn1201.png
http://91.121.93.17/pics/Rukm1201.gif
http://91.121.93.17/pics/Recm1201.gif /
http://www.meteociel.fr/modeles/nogapse_cartes.php
The jetstream chart shows a strong southerly zonal jet over the Atlantic,
with amplification over Europe and a SW'ly flow over the UK. The 500hPa
level sees a deep upper low over Scotland and NOrthern Ireland, with SW'lies
aloft elsewhere. ECM also shows an upper low over Scotland and Northern
Ireland, with SW'lies aloft. MetO has an upper low to the west instead, with
SSW'lies aloft. GEM has an upper low over Scotland and NI as per ECM and
GFS, while JMA has an upper low to the NE instead and westerlies aloft.
At the surface, GFS brings a cold, slack low over the UK. Winds are light
for most, with the strongest being very cold NE'lies over northern Scotland.
ECM has a low over Northern Ireland, resulting in easterlies for northern
Scotland and SW'lies elsewhere. MetO shows milder SE'lies for all, with a
low to the SW, while GEM has a mixture of southerlies and SE'lies with a low
west of Scotland. JMA has SW'lies and WSW'lies due to a low over southern
Norway.

Evolution to T+168
ECM shows southerlies on day 6 with a low west of Scotland. On day 7 the low
fills, with SE'lies and easterlies spreading across the UK.
A small, deep, cold low lies to the north of Scotland on day 6 with GFS,
bringing blizzards to the far north. Elsewhere winds are cold and light
westerlies. On day 7 the low moves northwards, with southerlies and SW'lies
for the UK.

Looking further afield
ECM shows easterlies for most on Christmas Eve, with a low over Wales.
Southerlies affect the UK on Christmas Day, followed by SW'lies for Boxing
Day.
GFS brings a ridge from the east over the UK on Christmas Day, with ENE'lies
for England and Wales and SE'lies elsewhere. The air aloft is mild, however,
so it wouldn't be snowy for most. On Boxing Day strong SSE'lies affect the
UK, followed by strong to gale force southerlies on day 10.

Ensemble analysis
(http://91.121.94.83/modeles/gens/gra...ille=Lond res)
After the weekend, the ensembles show a gradual warming over Christmas and
beyond.