DT12Z 17th runs are very consistent. The low running towards Greenland now
looks like passing to the east of it ensuring high development over the UK
and indeed with centres just to the east. Cold northerly plunge taking place
still into eastern Europe. Exact location of high cell on Christmas Day
still uncertain but now much more likely to be east of UK than to west, it
is now a matter of whether it subsequently pushes north or not.
Interestingly after Christmas both EC and GFS hint at retrogression and the
possibility of a very cold east or northeasterly with snow showers in
eastern areas. So here are my revised probabilities for Christmas Day:

Anticyclonic east or southeasterly, becoming colder during the day, but dry
with some sunshine - 60%
Anticyclone over right UK with fog and frost - 30%
Anticyclonic easterly with stronger winds in south with wintry showers into
SE - 10%

The really interesting stuff will be on Boxing Day and beyond, more on that
as it becomes clearer.
The DT12Z GFS is a cold outlier but now supported by other ensemble members
and the ensemble mean temperature trend is firmly downwards. Also supported
by ECMWF. But let's not get carried away :-)

Cheers,

Will
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"Will Hand" wrote in message
...
Trends are becoming clearer and more consistent today, at least up to day
before Christmas Eve.
During the weekend a deepening low moving northwards from Newfoundland
towards Greenland will finally distort the upper flow and amplify the
developing high just to the west of the meridian over the UK. The
behaviour of this low will be key for this amplification. Meanwhile
another depression scoots across the north Atlantic rounding the UK ridge
and dives into Scandinavia on Monday plunging very deep cold air
southwards into eastern Europe and Russia. By Tuesday we should have an
anticyclone somewhere over UK, but where will the centre be? The low which
ran up to Greenland will continue to be key to this and how it interacts
with the Greenland plateau, models never handle these things well. If it
tends to the west of Greenland then we will have retrogression during
Christmas week, ending up with a bitter northerly. If it tends to east
then high will stay over UK and possibly sink a little southeastwards -
more normal. GFS on one run took lows right across Greenland which is
dynamically unrealistic, so I do not trust GFS as much in this sequence.
So there you have it, high pressure now looking very likely but where will
the centre of gravity be? This will be key to the weather and
temperatures. Any sort of easterly after the deep cold plunge on Monday
will be very cold indeed, south or southeasterlies will be milder of
course and a high right over us will give us problems with fog. So here
are my probabilities in which I still have low confidence in for Christmas
Day. They will change as the situation becomes clearer, ECMWF, for
example, is trending colder with each run with 528DAM now over Denmark in
a north-easterly DT12Z today (16th).

Bitterly cold northerly with snow showers in eastern areas otherwise
sunny - 5%
Mild southwesterlies, cloudy and dry, with drizzle in Scotland - 10%
Anticyclonic with persistent fog and frost - 40%
Anticyclonic east or southeasterly, chilly with sunshine and early frost -
40%
Bitterly cold or east north-easterlies with snow flurries in southeast -
5%

Ciao, :-)

Will
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