Phil,

.... apologies for not coming back on your question sooner; after a
working life of shift-work, I look forward to shutting down the old PC
and drifting away with a good book ... I should have realised my post
would raise questions ;-)

Paul has done a fine job in my lamentable absence .. all I can add is to
say that we normally portray jet-streams (in text books) as nice simple
ribbons of high-velocity air, with well defined entrances (speeding-up)
and exits (slowing-down) of the air. On the flight briefing charts, they
look quite straightforward - long black lines with arrows and wind
feathers. In reality, the changes in velocity along a particular jet
length can be quite marked, and the *stronger* the jet, the more
'effect' will such variations have.

I'm not sure the purists will like the analogy I'm going to give but
here goes ...

You are on a 'bus in slow town traffic and you want to get off at the
next stop; you ring the 'stop' bell and begin walking down the aisle.
The bus driver has to change speed, direction for some reason (if only
to let you off), and you feel the change and compensate by steadying
yourself - for most people, this is easy to do and the 'forcing' on your
body is minimal.

Your on a high speed train and want to visit the buffet. It's on a
100+mph stretch of the East Coast mainline. The train is bowling along
quite happily and as you make your way along the gangway, no problems
with maintaining position. Then, the driver sees an unscheduled
'single-amber' (the next will be red); he/she has to take sharp action
to bring the train down to a reasonable speed; you might be on a curve;
you feel *this* one and perhaps lose the contents of the tray you were
carrying! The accelerations on your body are great and the 'forcing' is
significant.

The stronger the jet (and 180+ is strong), the greater will be the
effects for a *slight* change in strength - perhaps a localised zone of
stronger winds buried within the general jet structure, a jet-streak.

These changes will lead to changes in the dynamics of the column of the
atmosphere below: development occurs: the faster the jet, the greater
the change, the more vigorous the development. The more vigorous the
development, the greater the probability that stratospheric air, and
mid-tropospheric air with higher wind speeds will be squirted down to
the near-surface - a recipe for some of the strongest gusts. (I did an
empirical analysis of the relationship between jet strength and maximum
gust in any given event at Strike; for a 180 kn jet, this gives 90%
gusts to around 75kn, 190 kn jet to around 80kn; isolated stronger gusts
will occur.) Don't forget though, we are dealing with *forecast* jets;
the models are very good at these sorts of things, but errors can occur.

Then of course, there are the 'outside' factors: a short-wave upper
trough (we usually talk about these in mid-tropospheric terms, i.e.
700mbar upwards, but they often (though not always) have a surface
'reflection') will alter the environment significantly - not only
altering the jet-stream orientation/strength, but as Paul mentions, this
feature will have it's own dynamics to aid development.

Now, put yourself in the shoes of the senior staff at Exeter or Toulouse
or Dublin etc. You simply can't work these things out from first
principles; the models are the only 'tool' you have to work with. And of
course, there are many: The usual suspects will be UKMO, Arpege, GME on
this side of the pond (both 'global' and 'limited area' versions), and
the GFS and NGP from North America. All have strengths and weaknesses,
and all will have done well in past events, but in this one ...... there
comes the judgement. I have no doubt that the 'envelope' of
possibilities is quite large, hence the uncertainty. Other models will
also be viewed (e.g. JMA, GEM, but the ones mentioned will be key).

I've said it before, I wish someone would do a 'fly on the wall' tv
programme for one of these events, then people might see just what skill
and 'angst' is floating around the Operations Centre over the 4 or 5
days leading up to such occasions - and realise that even with all the
model help, it is NOT a case of fire up a workstation, run a couple of
loops, sip a cup of tea and issue the warning: the doubts and
uncertainties are many ... this particular event being one of only many
that the Chief and his team has to cope with in the course of a career.
And then then comes the night itself .....

Martin.