On Dec 11, 8:31*pm, "Eskimo Will" wrote:
"Neil" wrote in message

b.com...









On 11/12/2011 18:06, Eskimo Will wrote:

"fred" wrote in message
....
how accurate (in %age terms) is the GFS at 120h ?

Never mind the GFS detail Fred, look at the synoptics. A circa 200 knot
has been consistently forecast to slam into England and Wales on Friday.
Associated with this will be a deepening depression of the order 940 hPa
with a wind field wider than last Thursday's storm due to a different
deepening mechanism (Norwegian left exit and high specific humidity
entrained as opposed to Shapiro Keyser - confluent trough/right entrance
runner). This means that the damaging storm force winds will be spread
over a wider area. Precise track depends critically on when the
deepening phase will begin. High risk area has to be northern England to
central Scotland for the most damaging winds, but the MetO will refine
that in due course. If I was you Fred I'd prepare for a storm of greater
ferocity, and longer lasting than last week and hope for the best. My
personal feeling is that northern England should be very worried too and
all parts of the UK will at least have gales, except perhaps the far
north on the northern side of low, but even here the northerly could be
fierce. This is a serious low system and should not be under-estimated..

http://www.lyneside.demon.co.uk/Hayt...antage_Pro.htm
Will Hand (Haytor, Devon, 1017 feet asl)
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With regards to the track of this system, I've seen elsewhere comments
such as as 'as the pressure drops, the low will veer North' or something
like that at least!

My question is why? Why will a deepening low pressure system move North
rather than South?

Occasionally they do move south. What happens is that the low distorts the
thickness field through differential advection of warm and cold air.
Depressions are steered along the thickness (mean temperature) gradient.
Sometimes if the warm advection is stronger this distorts the thickness
pattern in such a way as to veer the thickness gradient and the low moves
SE, but mostly yes they do turn left. there are other dynamics going on such
as conservation of vorticity etc, but I'll leave that.

HTH

Will
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Taking it a bit further.
As air moves north the earth's vorticity(spin) increases and for the
air to conserve its total vorticity(spin) it must decrease its
relative vorticity.
The latter being related to its curvature. Eventually acquiring
negative relative vorticity which gives anticyclonic curvature and the
turning to the south we are talking about.

So as Will explained, its the thermal wind field combined with this
effect.

Nothing is simple. Friction plays its part as well as the air slows
and encounters land masses. This at least straightens the trough slope
in the vertical which happens anyway as the air in the depression
mixes.

The difficulty in predicting comes from any late tightening of the
thermal field from entrainment which will affect the track.
This is the problem with predicting the track of the intense
depression of Friday.

Can be sub grid scale, but also poorly understood and parametrerised
feedbacks in the models

Len Wood
Wembury
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