Re: Update on cold situation 29/12/08


<alanmgay@xxxxxxxxx> wrote in message
news:310839fb-c5f8-46aa-ba39-f04154335c9c@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
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.

It is not the thickness which relates to a mean temperature in a deep layer
but the wet-bulb temperature vertical profile which is important. For powder
snow the wet-bulb has to be at least below freezing all the way up through
the atmosphere. The colder the better. I guess with very low thickness
levels this would occur as the sea would not have chance to modify and warm
the lower layers to give positive wet-bulbs. Powder snow is more common at
altitude in the UK.

HTH

Will
--


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