Here is a summary of the NWP output for noon on Thursday.
Issued 0500, 20/11/11

A change is on the way. Although there will still be mild SSW'lies over the
UK on Thursday, a trough will move eastwards and this will lead to more
zonal conditions with frontal systems moving across the UK in the following
days. Temperatures will fall to nearer normal values in the south and in the
north it will be cold enough for snow over the higher ground.

Analysis and evolution to T+120 via GFS
Southerlies cover the UK as the result of a weak ridge. There are further
southerlies tomorrow, followed by more of the same for many on Tuesday.
Wales and SW England lie under a shallow low, however. On Wednesday a col
cover England and Wales, while Scotland and Northern Ireland lie under
SW'lies.

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 trough to the west and a strong jet surging
northwards over the UK. At the 500hPa level there are SSW'lies with a zonal
flow upstream. The other runs all show SW'lies instead aloft.
At the surface GFS has a trough moving eastwards. Ahead of the trough there
are southerlies, with westerlies following behind. There are SSW'lies on the
other runs with a secondary low far to the SW.

Evolution to day 7
ECM brings a trough across the UK on day 6, followed by colder WSW'lies.
There are further WSW'lies on day 7.
GFS shows a weak ridge over the UK on day 6, with a small low to the west.
The low deepens as it moves eastwards on day 7, leading to moderate to
strong westerlies for the UK.

Looking further afield
On days 8 to 10 with ECM there are SW'lies for all as low pressure moves
slowly eastwards to the north.
GFS brings SW'lies on day 8, followed by WNW'lies for most on day 9 as low
pressure deepens over the North Sea. On day 10 there's a col over southern
Scotland, Northern Ireland and northern England. To the north there are
westerlies and to the south, SW'lies.

Ensemble analysis
http://www.meteociel.com/cartes_obs/...&ville=Londres
GEFS shows a mild outlook to the 27th, followed by a somewhat colder spell
of weather.

ECM ensembles
http://www.meteogroup.co.uk/meteo/ep...tt6-london.gif
ECM shows a slight fall in temperature after the 25th.