Here is a summary of the NWP output for noon on Wednesday.
Issued 0514, 21/01/12

The end of the working week is likely to see unsettled and mild weather give
way to cooler conditions as a trough moves eastwards. By the weekend high
pressure is likely to build from the south, bringing settled (and chilly)
conditions to the south, with less settled weather further north. Thereafter
the models diverge, with ECM showing unsettled weather for all while GFS
maintains high pressure over or near the UK. The ensemble guidance is
unequivocably cold, with the majority of members of both GEFS and ECM
showing a cold (but not necessarily snowy!) spell on the way.

Analysis and evolution to T+120 via GFS
The UK lies under westerlies from a deep low to the north. Tomorrow there
are further westerlies, as is the case on Monday. Tuesday sees WSW'lies as a
weak ridge moves eastwards.

T+120 synopsis
http://www.wetterzentrale.de/wz/pics/Rtavn12014.png
http://www.wetterzentrale.de/wz/pics/Rtavn1201.png
http://www.wetterzentrale.de/wz/pics/Rukm1201.gif
http://www.wetterzentrale.de/wz/pics/Recm1201.gif
http://www.wetterzentrale.de/wz/pics/Rjma1201.gif
The jetstream chart shows a strong southerly flow over the UK with a trough
to the west. At the 500hPa level there are SW'lies, also with a trough to
the west. MetO is similar, while ECM has higher heights along with its
SW'lies. GEM and JMA also show upper SW'lies.
At the surface GFS brings a trough over Ireland with SSW'lies for the UK.
MetO has SW'lies, while ECM shows SSW'lies. GEM and JMA have SW'lies and
SSW'lies respectively, with all the models showing a similar pattern.

Evolution to day 7
ECM brings southerlies and SSW'lies on day 6 with a trough over Scotland. On
day 7 there are SW'lies for Scotland and Northern Ireland with light winds
elsewhere.
A trough moves eastwards on day 6 with ECM, leading to westerlies. The
Azores High ridges NE'wards on day 7, bringing light winds to England and
Wales. Elsewhere winds are SSW'lies.

Looking further afield
WSW'lies affect the UK on day 8 with ECM, as high pressure builds to the
south. On day 9 there are southerlies, followed by a trough on day 10. Ahead
of the trough there are strong southerlies, with WSW'lies further west.
GFS shows SSW'lies for most on day 8 with a ridge over the SE. The ridge
builds and moves NW'wards on day 9, covering all areas except NW Scotland
(which lies under SW'lies). On day 10 the ridge moves northwards, allowing
ENE'lies to affect the south of the UK.

Ensemble analysis
http://www.meteociel.com/cartes_obs/...&ville=Londres
The GFS shows a mild interlude on the 25th and 26th, followed by the same
two clusters as recently - either zonality or much colder weather, with the
colder weather option hosting the majority of members (in a 66/34 ratio).

ECM ensembles
http://www.meteogroup.co.uk/meteo/ep...tt6-london.gif
The ECM ensembles also show two clusters and again the colder cluster has
the most members: an 80/20 split in favour of cold from the 27th. The
operational was in the milder cluster on days 8, 9 and 10.