Here is a summary of the NWP output for noon on Wednesday.
Issued 0528, 29th Dec 2007

Britain will be on a knife-edge as the battle rages next week. There will be
a large high to the NE and Atlantic lows will approach the high and (most
probably) there'll be trough disruption too. The million dollar question is
whether the cold to the east will make it to the UK before the Atlantic
systems push it back east. At the moment the "big three" (ECM, MetO and GFS)
all show snow at some stage for parts of the UK on the 3rd, although with
MetO the bulk of the cold stays firmly to the east. If it's not quite cold
enough to snow it's likely there'll be plenty of rain around, with low
pressure not far from the SW. It'll be worth paying attention to media
forecasts and/or the charts online over the coming hours and days, as this
has the potential to be very messy in places.

Analysis and evolution to T+120 via GFS
http://85.214.49.20/pics/Rtavn001.png
Low pressure lies to the NW with SW'lies and WSW'lies across the UK.
Tomorrow the low moves away to the NE, with a trough over the North Sea.
NW'lies and WNW'lies cover the UK as a ridge moves eastwards. On Monday the
ridge covers the North Sea, with southerlies and SSW'lies for the UK as a
result. Southerlies persist on New Year's Day as pressure builds over
northern Scandinavia and low pressure fills to the west of the UK.

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://www.meteociel.fr/modeles/nogapse_cartes.php
The jetstream chart shows a strong jet over the North Atlantic and a
diffulent block to the west of the UK. At the 500hPa level an upper low
covers the North Sea and is moving westwards towards the UK. MetO has a weak
ridge aloft over the UK, while ECM shows an upper low heading westwards from
the Netherlands. In all three cases, there's a large upper high over
northern Scandinavia. GEM shows an upper ridge from the UK to Scandinavia
with upper SSW'lies. NGP has an upper high over NW Russia and a trough
extending across the UK from the west.
At the surface GFS brings a massive high over NW Russia, which ridges
westwards to the north of the UK and southwards to Greece. Lows lie to the
WSW of the UK and over Biscay, with strong to gale force easterlies and
widespread snow across the UK. MetO shows a low to the west and SE'lies and
ESE'lies over the UK. ECM has easterlies and SE'lies with low pressure near
Biscay and a ridge from the Russian high to the NE. GEM shows SSE'lies with
a trough to the west, while NGP has a trough over the Irish Sea. Ahead of
the trough winds are SSE'lies, with westerlies following behind.

Evolution to T+168
ECM shows the upper low moving westwards over the North Sea on day 6,
bringing easterlies and NE'lies across the UK, along with plenty of snow. On
day 7 the low moves northwards as a SW'ly sets in across the UK.
GFS brings NE'ly gales over the UK on day 6 as low pressure fills over
Biscay. Temperatures are borderline for snow with the raw output showing a
slushy mixture of heavy rain and heavy snow over the UK. By day 7 the winds
become NNE'lies as low pressure fills over France.

Looking further afield
Days 8 to 10 with ECM see low pressure anchored to the NW with strong
WSW'lies across the UK.
A weak low covers the North Sea with GFS on day 8, with light westerlies and
WNW'lies for the UK. On day 9 high pressure builds over Ireland, with
NW'lies for the UK. Day 10 sees the high move eastwards, allowing NW'lies
to affect eastern England and SSE'lies to cover Northern Ireland and western
Scotland.

Ensemble analysis
(http://91.121.94.83/modeles/gens/gra...ille=Lond res)
The ensembles show a reasonable chance of colder air aloft reaching the UK
around the 3rd, but the operational was one of the colder runs.
Significantly, both the operational and control show a relatively prolonged
cold spell for London, while the majority of other ensemble members show
more of a cold snap (if that).