19/12/08

Still plenty of consistency.
Good signal for high centred in North Sea on Christmas day with a noticeable
east wind in the south.
Thereafter a retreat into Scandinavia is signalled with the upper pattern
very conducive to blocking with a low becoming established over the Azores.
ECMWF is still my model of choice as it has been pretty consistent using the
time-lagged ensemble technique, thanks to Jon's website.

As usual here are my revised probabilities for Christmas Day:

Anticyclonic east or southeasterly, becoming colder during the day, but dry
with some sunshine, although a fresh and raw wind in southern areas.
Most risk of fog would be over Scotland - 80%

Anticyclone centred right over UK with fog and frost - 10%

Strong cold easterly in south with drizzle, snow grains or light snow,
otherwise dry and cold - 10%

Outlook is for it turning very cold with outbreaks of light snow in the
south and some eastern areas in an easterly airstream.

Will
--

"Will Hand" wrote in message news:...
No real surprises today.
Still anticyclonic and turning colder.
Ensembles reflect variability nicely.
Here are my revised probabilities for Christmas Day from subtle trends:

Anticyclonic east or southeasterly, becoming colder during the day, but
dry
with some sunshine, although a raw wind along south coast - 70%
Anticyclone centred right over UK with fog and frost - 20%
Strong cold easterly in south with drizzle, snow grains or light snow,
otherwise dry and cold - 10%

Will
--


"Will Hand" wrote in message
...
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
--

"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
--