Here is a summary of the NWP output for 12z on Saturday.
Issued 0611z, 16th January 2007.

In summary...
The weekend will see winter finally show its hand across the UK. A front
will move in from the NW, heralding a blast of Arctic air which, in
association with a displaced Azores High, looks like lasting for much of the
following working week. With winds from the NW initially and a probable
swing around to the NNE or even NE during next week, along with 850s
hovering below -5C, there's the *potential* (note: *not* certainty) that
there'll be fairly widespread wintry precipitation. If there's no
substantial change in the models, it's worth keeping an eye on your local
BBC / MetO forecasts for the weekend and beyond.

Analysis and evolution to T+120 via GFS
http://85.214.49.20/pics/Rtavn001.png
http://85.214.49.20/pics/avnpanel1.gif
A mixture of WSW'lies and SW'lies covers the UK, with a weak ridge to the
west and a shallow trough over the UK. SW'lies and SSW'lies affect the UK
tomorrow as a front moves in from the west, followed by further SW'lies on
Thursday as a low approaches from the west. By Friday the low moves away to
the east, leaving the UK under westerlies and a weak ridge.

T+120 synopsis
http://85.214.49.20/pics/Rtavn12015.png /
http://85.214.49.20/pics/Rtavn1201.png
http://85.214.49.20/pics/Rjma1201.gif /
http://85.214.49.20/pics/Rukm1201.gif
http://85.214.49.20/pics/Recm1201.gif /
http://85.214.49.20/pics/Rngp1201.gif
The jetstream chart shows a strong jet across the eastern Atlantic, England,
Wales and much of western Europe. To the south of Greenland a marked ridge
is forming. At the 500hPa level there's a large upper low over Scandinavia,
with a ridge forming in the western Atlantic. MetO shows a similar pattern,
albeit with higher heights over Scandinavia, while ECM has less of a ridge
in the western Atlantic. NGP is very similar to ECM, while JMA has higher
heights over southern Egnland and not much of a ridge at all to the west.
GEM shows a weak ridge forming in the west Atlantic and a westerly gradient
aloft for the UK.
At the surface GFS brings strong to gale force WSW'lies over the UK, with a
secondary low to the NW of the UK. MetO is similar, albeit with the low to
the west of Scotland instead. Both models show the Azores High being pulled
NW'wards as warm air advection occurs to the west of Greenland. ECM has low
pressure to the NW and WSW'lies for all, again with a northwards ridging
Azores High. NGP is similar to ECM, as is JMA; both show a low to the NW and
WSW'lies for the UK. GEM brings westerlies for all with complex low pressure
to the north and WNW.

Evolution to T+168
ECM shows a dramatic height rise to the SW of Iceland as an upper ridge
builds strongly. For the UK, that means NW'lies and NNW'lies on day 6 as
pressure builds to the west. On day 7 the Azores High recentres itself to
the south of Iceland, feeding northerlies and NNE'lies over the UK.
GFS also shows a marked change, with a strong upper ridge forming to the
west. On day 6 NW'lies and WNW'lies cover the UK as the Azores High merges
with a Greenland High. On day 7 the high builds to the SW of Iceland and low
pressure forms over the North Sea. A mixture of northerlies and NNE'lies
covers the UK as a result.

Looking further afield
Beyond T+168, ECM shows the Azores High rotating on day 8, leading to a
ridge over Scotland and light winds there. Elsewhere winds are moderate
NE'lies, followed by NNW'lies for all on day 9 as the high declines slowly
to the west. Day 10 sees the high persisting to the west with NW'lies and
NNW'lies for the UK.
GFS shows the upper high slowly declining and heading SE'wards. At the
surface, day 8 sees high pressure SW of Iceland with a ridge NE'wards over
Scandinavia. NE'lies and NNE'lies cover the UK as a result. On day 9 a ridge
from the main high affects the UK, with light winds for all. In the light
northerly drift sub -10C 850 air reaches London and the eastern half of
England. The cold air lingers in the SE on day 10, as the ridge sinks
southwards. Further north much milder air returns from the north, having
circled the Azores High which still lies to the west.

Ensemble analysis
(http://expert.weatheronline.co.uk/da...ia/ensdia.html - last
night's 18z)
The ensembles still show a mild week for most, followed by a sudden abrupt
drop in 850s on Saturday. The ensemble mean is -5C or below for London from
Sunday until Thursday includive and is followed by a gradual rise back to
normal. It's notable that the cold spell has gone from 48 hours in the
ensembles, when it first appeared, to 5 days or so now.