On 2013-10-24 12:25:06 +0000, yttiw said:

On 2013-10-24 12:05:49 +0000, Stephen Davenport said:

On Thursday, October 24, 2013 12:56:10 PM UTC+1, Togless wrote:



Interesting, thanks Stephen. Looks like I chose a good week for not
cycling to work. Could be a bit breezy.

===
To say the least. But equally it would not surprise me at all to see
the models back away from this either. It's the sort of development
that models often times do not handle well until 1-2 days before the
event -- understandable given that much hangs on how it will hook up
with the jet, and that at the moment it is only nascent.
GFS 06 has now made the low shallower and taken it farther south, more
along the Channel, so that would give a lot less wind. However, I have
less trust in the GFS 06 and I'm not going to jump onto individual
runs. EPS leads us to put about 45% risk of severe wind storm, so it's
a high threat. And even so there should still be periodic strong winds.

Stephen.

IMO it is not so much a problem of how the models handle a left exit
jet feature, but more how accurate they are in the timing and decay of
the large upper vortex currently to the west of Biscay, and its
associated surface. If they get that wrong then the potential/or not
for explosive deepening changes dramatically.

Now folk may have been thinking to themselves, what is this nutter
waffling about? Decay of large upper vortex to the West of Biscay? Is
he mental, when the developing low is coming screaming across the
Atlantic on a 100+ knot jetstream?

However, the turth appears to be that while computer models are very
good at forecasting explosive deepening of low pressure areas when the
conditions are right, and also (usually) very accurate at predicting
jet streams, what they seem not to be very good at, is predicting the
decay of large and slow moving upper lows with not a great deal of
circulation around them.

I do not know why this is, but I suspect it might be because of what I
like to call 'the strong dynamic forces' and 'the weak dynamic forces'
(mainly because it sounds similar to the strong and weak magnetic
forces in physics). In a rapid deepening low situation, the strong
dynamic forces so completely overwhelm the weak ones that slight
inaccuracies become irrelevant. However, with decaying upper vortices,
or for that matter, sharpening upper troughs leading to disruption, the
weak dynamic forces have much more influence over the scenario and it
is this which the computers find difficult; possibly because until
very recently the resolution of the models was too large to cope with
these more subtle and small scale effects.

What has this to do with the forecast for tonight and tomorrow? Well,
the upper vortex mentioned above was a little slower to decay than the
computer predicted, which meant that the whole development scenario
changes slightly in favour of a later deepening and the severe gale
area to be more centred over eastern England, than the Southwest.

But we shall see. After all, I am only a crazy man with mad theories.


A few days ago, one run of the GFS had an uncannily similar scenario to
the Oct 87 storm, with a low of 965mb just NE of the Wash late on
Monday. It appears that there are many attention seekers who will take
one run in isolation, and issue media warnings of armageddon in the
hope that once every decade they correctly forecast a catastrophe, and
can bask in their "glory" through many more years of failure.

As Spock would have said on Star Trek - " It's forecasting Jim, but not
as we know it ".