Here is a summary of the NWP output for noon on Saturday.
Issued 0528, 29th December 2009.

By the end of the weekend high pressure will be influencing the UK, trapping
the cold surface air and leading to widespread hard frosts. Into next week,
as usual the models are split. GFS goes for a snow-to-rain event across much
of England as a low pushes warmer air northwards, while ECM shows a front
moving SE'wards with snow likely in the north.

Analysis and evolution to T+120 via GFS
Easteries cover much of the UK, due to a low to the WSW and a trough to the
south. The trough moves over southern England tomorrow, bringing ENE'lies
elsewhere. On Thursday colder air moves south, allowing NE'lies to affect
the UK. By Friday northerlies and NNE'lies affect the UK, with low pressure
to the south.

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 and zonal southerly branch of the jet,
with a more northerly zonal branch over the UK. an easterly jet streak lies
to the NW. The 500hPa chart shows a large, stable cut-off high over
Greenland, elongated on a west-east axis. A weak low lies to the east of the
UK, with an upper col for the UK. ECM has a weak upper trough over the UK,
from the east, with again a large high over Greenland. MetO has an upper col
over the UK, while GEM brings a weak trough from the east. JMA shows a weak
trough too.
At the surface, GFS has a SE'ly or ESE'ly flow for many with a weak high
over East Anglia. ECM has easterlies for Scotland and a col with light winds
elsewhere. MetO brings light ESE'lies with a trough to the SW, whilst GEM
hgas a low over western Scotland and SW'lies for most. JMA also has a low
over western Scotland, again with SW'lies for most.

Evolution to T+168
ECM shows a ridge over the UK on day 6 with light northerlies. Day 7 sees
the ridge move southwards, with light winds for all areas except Scotland -
here winds are westerlies.
GFS brings easterlies on day 6 with a deep low to the SW. The low moves
eastwards on day 7 with strong ENE'lies for all.

Looking further afield
ECM shows a trough crossing the UK on day 8, bringing SW'lies in advance.
The winds become westerlies on day 9 as the trough moves eastwards. By day
10 a weak ridge builds with light winds for most.
GFS has NNE'lies on day 8 as low pressure moves over northern France. A
northerly flow covers England and Wales on day 9 as a ridge moves over
Ireland, with WSW'lies for Scotland and Northern Ireland. Day 10 sees
pressure build over all areas except Scotland, which lies under westerlies.

Ensemble analysis
(http://91.121.94.83/modeles/gens/gra...ille=Lond res)
66% of the ensembles show a prolonged cold spell, the others (including the
operational) show a cold snap instead.