Storm Forecast
Valid: Mon 20 Aug 2007 06:00 to Tue 21 Aug 2007 06:00 UTC
Issued: Sun 19 Aug 2007 21:57
Forecaster: GROENEMEIJER

SYNOPSIS

Monday at 06 UTC... the major feature on the forecast maps is a trough that has its axis over the British Isles and western France.
This trough is filled with relatively unstable air and should move eastward very slowly during
the next days while evolving into a slack cut-off low over southeastern France on Monday and Tuesday. At the surface, a low-pressure system should develop in the Gulf of Genua on Monday and later the low should "jump over" the Alps on Tuesday when cyclogenesis is expected over southwestern Germany.
An intense jet streak over Ireland at 06 UTC is expected to round the base of the trough on Monday evening. Downstream, to the east of the trough, the temperature and height gradients and hence the flow is not very strong but nevertheless advecting moist, warm air northward.

DISCUSSION

Western and southern France...

In the left exit region of the intense jet forecast to cross the region during the evening, strong vertical motion is expected. This should lead to the formation of a stratiform cloud shield with unstable air in its wake in which a number of convective showers are expected. Given the very strong wind shear, convection will likely be organized into a few lines. The passage of these lines may be accompaied by gusts around the severe limit of 25 m/s. Additionally, one or two tornadoes may develop with the first convective lines that may profit from sufficiently helical inflow to support storm rotation.

Italy, Austria, north-western Balkans, Hungary, Czech and Slovak Republics, Poland, extreme eastern Germany...

Within the large warm-air advection regime over the level 1 area, moderate amounts of weakly-capped CAPE around 1000 - 1500 J/kg should be able to form in many places.
A weak shortwave trough that is initally located over north-eastern Italy and moves north-north-eastward reaching northern Poland on Tuesday morning.
Rather widespread storms should form ahead of the trough and somewhat lower coverage is expected elsewhere. The kinematic environment looks moderately favourale for severe storm development with deep-layer wind shear of about 10-15 m/s. A number of strong multicells should be possible locally causing a few large hail and strong/severe wind events. Storms should diminish after sunset expect for northern Poland where the trough will likely keep storms continuing overnight.