Re: Update on cold situation 29/12/08

On Dec 29, 5:15 pm, "Will Hand" <w...@xxxxxxxxxxxxxxxxxxxx> wrote:
Well as predicted the cold weather has duly arrived now despite 850hPa
temperatures and 1000-500 thickness being well up (as explained earlier).
And it is going to stay cold and probably get colder as deep cold air starts
to advect SSW'wards later this week.

We now have a distorted omega block with the jet splitting. One portion
heads north from Hudson Bay rounding Greenland maintaining the
Greenland-Iceland block, the other part heads east along roughly 50N. Both
these jets will have a part to play in the outlook.

The northern portion will help maintain the block by bringing lower PV air
northwards but then as it comes south again to east of Iceland this will
spin up depressions due to high PV coming south and the latitude effect.
The southern portion will trigger depressions in the normal way which should
mostly dive SE'wards into Biscay/Spain. However, ...

Next weekend into Monday the northern part has every chance of developing a
North Sea low veering the wind and advecting sub 516DAM air into UK bringing
widespread heavy snow into eastern and northern areas and later the SE. I
rate this as a 50% chance. The southern jet will always offer the
possibility of milder Atlantic air pushing into SW England as a low runs
along the Channel followed by another surge of very cold air from the north.
This would give rain in the SW with heavy snow above 250m asl but heavy snow
over Wales and Midlands and later SE England. I rate this as a 30% chance..

So up to the weekend mainly dry and cold with occasional flurries of light
sleet or snow and a lot of cloud with a few breaks at times, but then by
next Monday ...

50% chance heavy snow coming down from northeast
30% chance rain turning to snow coming up from SW
20% staying mainly dry everywhere.

So there you have my theme, staying cold with an increasing risk of heavy
snow from the weekend onwards.
Northern Ireland, NW England possibly seeing least. Low lying parts of SW
England likely to have rain or sleet rather than snow unless the system from
the northeast develops more.

I shall not update on every toss and turn of the GFS or other models it is
understanding the broad picture that is the key and that is what I and Joe
Bastardi always aim to do.

Ciao,

Will
--

Thanks for the update Will, much appreciated. I must admitted I still
associate a ‘true’ cold spell as meaning deep cold. The deep cold
gives the instability for snow showers along the east coast etc and
maintains a lower max in the later winter, etc.

One question I would like to ask is how thickness levels affects the
chances of dry snow at sea level locations in a maritime climate such
as ours? I remember the times living in East Kent the only time we got
dry snow was when thickness levels were low, e.g. 1st Jan 1979.

.

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