Here is a summary of the NWP output for 12z on Tuesday.
Issued 0620z, 18th January 2007.

The models show surprising differences as early as T+120, all due to the
handling of the upper block near Iceland. If, as GFS shows, an Omega block
forms then the cold will be prolonged across England and Wales. If, on the
other hand, an Omega fails to materialise then milder air aloft should reach
all areas by the end of the working week. Regardless, Scotland and Northern
Ireland are likely to see milder conditions return earlier, with SE England
hanging onto the colder air for longer. It's still too far out to say with
any certainty which places will see precipitation, but the GFS has tinkered
with the idea of running some very cold (-40C at 500hPa) air ove eastern
parts of the UK, which would doubtless result in hefty localised snowfalls
were that to come off. As ever, more runs are needed.

Analysis and evolution to T+120 via GFS
http://85.214.49.20/pics/Rtavn001.png
http://85.214.49.20/pics/avnpanel1.gif
WSW'lies cover the UK, with complex low pressure over Scandinavia and a
large high over Iberia. The winds become SW'lies tomorrow as a trough
approaches from the west. By Sunday the Azores High ridges towards
Greenland, forcing the Icelandic low eastwards. A trough covers southern
Scotland and Northern Ireland, heralding much colder conditions. Ahead of
the trough winds are WSW'lies, with NW'lies behind. A secondary low fills
over the Celtic Sea on Monday, leading to light winds across 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://85.214.49.20/pics/Rngp1201.gif
The jetstream chart shows a near textbook "Omega" block to the NW of the UK,
with the British Isles under strong upper northerlies and NNE'lies. At the
500hPa level the Omega shape is also present, with the centre to the SW of
Iceland. MetO shows something that's close to but not quite an Omega, this
time with the centre to the SSW Of Iceland. ECM is again similar, again with
something approaching an Omega centred to the SSW of Iceland. JMA is similar
again, while NGP shows more of a trough from the east undercutting the
Icelandic block. Finally GEM shows a similar setup to GEM, with an upper
high to the west of the UK.
At the surface GFS brings NW'lies and WNW'lies over the UK with a high to
the WNW and lows to the NE and east. MetO brings NW'lies across the UK,
again with a high to the WNW. ECM has the high to the west instead, with a
ridge over the UK. Much of England and Wales sees northerlies or NNE'lies,
with NW'lies elsewhere. JMA shows NNW'lies and a high to the west, while NGP
brings strong NE'lies over much of the UK. The Canadian run also shows a
high to the west and this time it brings NNE'lies for most.

Evolution to T+168
ECM shows the upper high sinking SE'wards over Ireland, with mild air aloft
being dragged over the UK. On day 6 NNW'lies and NW'lies cover the UK,
followed by more of the same on day 7 with the high by now over Ireland.
GFS brings a renewed pulse of very cold upper air over eastern areas on day
6, as the high declines to the SW of Iceland and a low deepens and moves
swiftly SSE'wards over Germany. By day 7 the high moves rapidly SSE'wards to
the west of Ireland, leaving the UK under NNW'lies.

Looking further afield
Beyond T+168, ECM shows the Azores High influencing the UK. Day 8 sees
WNW'lies with the high over the Celtic Sea, followed by SW'lies as it moves
over the English Channel on day 9. By day 10 there's little change for
England and Wales, but strong WSW'lies pick up across Scotland and Northern
Ireland.
GFS shows the Omega block persisting on day 8, albeit centred west of the
UK. At the surface there's a large high over much of the UK, with some very
cold air trapped underneath in SE England. Scotland sees westerlies on the
periphery of the high. The high retrogresses on day 9 as lows undercut it
far to the south, resulting in NW'lies and NNW'lies for the UK. Day 10 sees
the high build to the west with NNW'lies for England and Wales and NW'lies
elsewhere.

Ensemble analysis
(http://expert.weatheronline.co.uk/da...ia/ensdia.html - last
night's 18z)
The ensembles still show a marked colder spell for next week, with the
majority of members bringing milder air across the UK by Friday. However, a
minority of runs (including today's operational GFS) keep things cold into
the following week.