This development is going to be very difficult to get right looking at
the broad-scale pattern. With the models predicting jet-strength of at
least 180 kn, the potential for gusts 80 to 90 kn are quite credible.
The difficulty is judging the phasing of the low-level baroclinic
disturbance (given the multiple possibilities offered by the models)
with the upper forcing (due to the jet structure - a good way south
atm), and I suspect that even 18 hr or less ahead, things will still be
uncertain. The best anyone can do is to prepare, and check closely with
short-range forecasts as we get nearer - but expect marked fluctuations
in advice as new data comes to hand. No doubt December 1999 will be
uppermost in the minds of MeteoFrance! (And remember, we [UK] only just
missed out on those events.)

Martin.

"Martin Rowley" wrote in message
...
This development is going to be very difficult to get right looking at
the broad-scale pattern. With the models predicting jet-strength of at
least 180 kn, the potential for gusts 80 to 90 kn are quite credible.
The difficulty is judging the phasing of the low-level baroclinic
disturbance (given the multiple possibilities offered by the models)
with the upper forcing (due to the jet structure - a good way south
atm), and I suspect that even 18 hr or less ahead, things will still be
uncertain. The best anyone can do is to prepare, and check closely with
short-range forecasts as we get nearer - but expect marked fluctuations
in advice as new data comes to hand. No doubt December 1999 will be
uppermost in the minds of MeteoFrance! (And remember, we [UK] only just
missed out on those events.)

Martin.

Spot on, as ever, Martin. The model variability is causing quite a few
headaches
but the potential is there as reflected in the 12Z GM and many of the
ensembles.
On this occasion the low on the finalised T+72 was modified and taken
slightly further
south.

Before all that the NW looks like it will take a bit of battering tomorrow
night
http://imkpc3.physik.uni-karlsruhe.de/pics/brack0a.gif
with forecast gradient winds are in excess of 60KT.

A busy day what with all that and BBC SW/David Braine in doing a bit for the
Spotlight programme
which went out earlier. I believe they may show some of it on News24 and
possibly some other footage
on Sunday they recorded for Countryfile.

Cheers,
Jon.

But remember. If anybody rings you. there won't be a hurricane! ;-)

Victor

"Martin Rowley" wrote in message
...
Snip

No doubt December 1999 will be
uppermost in the minds of MeteoFrance! (And remember, we [UK] only just
missed out on those events.)


.... it had me thinking of December 1999 too. Jan 25th 1990 also got a
mention during the course of the day.

Jon.

"Martin Rowley" wrote in message
...
The difficulty is judging the phasing of the low-level baroclinic
disturbance (given the multiple possibilities offered by the models)
with the upper forcing (due to the jet structure - a good way south
atm),

Martin,

Time to show my ignorance of the science!

What exactly do you mean by the above. I understand the jetstream is moving
the whole thing at 180kts. Is the phasing the 'where and by what amount' the
low will deepen at the surface ? and please clarify 'upper forcing'

Thanks

Phil

"Phil Layton" wrote in message
...

"Martin Rowley" wrote in message
...
The difficulty is judging the phasing of the low-level baroclinic
disturbance (given the multiple possibilities offered by the models)
with the upper forcing (due to the jet structure - a good way south
atm),

Martin,

Time to show my ignorance of the science!

What exactly do you mean by the above. I understand the jetstream is
moving
the whole thing at 180kts. Is the phasing the 'where and by what amount'
the
low will deepen at the surface ? and please clarify 'upper forcing'

Thanks

Phil

Not that i wish to answer for Martin, but here is what i think he is
referring to :-

The low is currently moving off the eastern seaboard of the USA, it is a
realtively shallow affair, to the NE of this off the NE coast of the USA is
an increasingly powerful Jet forecast by sunday to exceed 180 Knts,
underneath this jet lies a tight low level barcloninic zone (this is a tight
divide between the cold showery polar air over Labrador and the warmer
tropical air of the atlantic. As the low moves NE it will encounter this
jet, it is the interaction of this jet and a shortwave upper trough moving
out of canada that will determine how this feature develops. If the Surface
Low, Jet and upper trough happen to engage at the right time (typically if
the surface low moves into the left exit on the cold side of the the jet)
the low could deepen very quiclkly.

The Forcing is provided on the forward side of the shortwave, and also by
the powerful upper air dynamics.

I hope that is of some help

Paul

"PJB" wrote in message
...
As the low moves NE it will encounter this
jet, it is the interaction of this jet and a shortwave upper trough moving
out of canada that will determine how this feature develops. If the
Surface
Low, Jet and upper trough happen to engage at the right time (typically if
the surface low moves into the left exit on the cold side of the the jet)
the low could deepen very quiclkly.

The Forcing is provided on the forward side of the shortwave, and also by
the powerful upper air dynamics.

Thanks Paul, I'm OK on the low level baroclinic zone. Could you just amplify
the last sentence. What is a 'shortwave Upper Trough' is this just a
'trough' of low pressure, but at altitude and not necessarily at the
surface?

Phil

"Martin Rowley" wrote in message
...
This development is going to be very difficult to get right looking at
the broad-scale pattern. With the models predicting jet-strength of at
least 180 kn, the potential for gusts 80 to 90 kn are quite credible.
The difficulty is judging the phasing of the low-level baroclinic
disturbance (given the multiple possibilities offered by the models)
with the upper forcing (due to the jet structure - a good way south
atm), and I suspect that even 18 hr or less ahead, things will still be
uncertain. The best anyone can do is to prepare, and check closely with
short-range forecasts as we get nearer - but expect marked fluctuations
in advice as new data comes to hand. No doubt December 1999 will be
uppermost in the minds of MeteoFrance! (And remember, we [UK] only just
missed out on those events.)

Yes, I already e-mailed an "alert" to relatives and friends in our part
of France (Limousin), only to find that they've been alerted already.

Philip Eden

"Phil Layton" wrote in message
...

"PJB" wrote in message
...
As the low moves NE it will encounter this
jet, it is the interaction of this jet and a shortwave upper trough
moving
out of canada that will determine how this feature develops. If the
Surface
Low, Jet and upper trough happen to engage at the right time (typically
if
the surface low moves into the left exit on the cold side of the the
jet)
the low could deepen very quiclkly.

The Forcing is provided on the forward side of the shortwave, and also
by
the powerful upper air dynamics.

Thanks Paul, I'm OK on the low level baroclinic zone. Could you just
amplify
the last sentence. What is a 'shortwave Upper Trough' is this just a
'trough' of low pressure, but at altitude and not necessarily at the
surface?

Phil

Yes Phil a shortwave is a trough of low pressure at a certain height (say
300mb) it is called a shortwave becuase it is much smaller than a typical
"long wave" upper trough which stretch for thousands of miles (such as when
we are in an omega block - that is the longwave pattern) a shortwave is a
much smaller feature, but it can provide the required forcing to deepen a
low very quickly.

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.