?Here is a summary of the NWP output for noon on Thursday.
Issued 0546, 12th December 2010

In summary, Thursday into Friday will see an active cold front sweeping
across the UK, bringing a period of rain which is quite likely to turn to
snow as the very cold air aloft digs in. This will herald at least a few
colder days, with northern and western areas at risk of some snowfall.
Further south and east heavy snow is less likely but snow is possible
anywhere in the UK. Into next week the ECM and its ensembles shows a steady
warming trend, while G(E)FS keeps things much colder. Interestingly the 18z
GEFS suite showed a similar setup to the 12z ECM, but it's reverted to a
colder outlook this morning. It remains to be seen whether the 0z ECM also
goes back to showing a colder output.

Analysis and evolution to T+120 via GFS
A high lies to the NW, leading to light to moderate northerlies. The high
moves over the UK tomorrow, with light winds for all. There's little change
on Tuesday, although some showers may affect SE England as a weak trough
moves in. On Wednesday the high recentres to the west, resulting in
northerlies for all.

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 remarkably large ridge over the North Atlantic,
extending from the Azores to beyond 80N west of Greenland. The jet roars
southwards just to the west of the UK, with the UK on the cold side of the
jet. At the 500hPa level there's a large upper high over Greenland with an
upper low near Norway. The upper flow is strong NNW'ly over the UK. MetO has
NW'lies aloft as an upper low heads towards the UK, while ECM has upper
westerlies with its upper low further north. Upper NW'lies cover the UK with
GEM, again with the low to the north. JMA shows a weaker upper low and
westerlies aloft.
At the surface, GFS brings low pressure to the NE and cold NW'lies over the
UK. There are cold NW'lies with MetO too, this time with an elongated low to
the east. ECM shows a trough moving southwards, with westerlies in advance
and NW'lies following behind. There are NW'lies for all with GEM as a trough
moves away to the south and JMA brings a trough over the UK with SW'lies in
advance and NW'lies following behind.

Evolution to T+168
ECM shows low pressure to the NW on day 6 with WSW'lies across the UK. The
winds back SW'ly for most on day 7, although northern Scotland lies under
SE'lies from a trough.
GFS has complex low pressure over the UK on day 6, leading to light winds.
On day 7 low pressure lies to the east and south, with a col and light winds
persisting for most.

Looking further afield
On day 8 ECM brings SW'lies for all with low pressure to the west. There are
further SW'lies for most on day 9 as the low fills to the west of Scotland.
On day 10 easterlies cover the UK ahead of a trough to the south.
Days 8 and 9 with GFS bring further light winds as the col persists. By day
10 southerlies pick up ahead of a low to the west.

Ensemble analysis
http://www.meteociel.com/cartes_obs/...&ville=Londres
The GEFS shows a marked change to cold conditions at the surface from Friday
onwards, with the cold likely to persist for a week.

ECM ensembles
http://www.meteogroup.co.uk/meteo/ep...tt6-london.gif
The ECM ensembles show much milder weather than GEFS, with temperatures only
a couple of degrees below average (and near average in the 10-15 day range).