Storm Forecast
Valid: Wed 09 Jul 2008 06:00 to Thu 10 Jul 2008 06:00 UTC
Issued: Tue 08 Jul 2008 20:52
Forecaster: GATZEN, TUSCHY

SYNOPSIS

Large-scale flow over Europe is dominated by a broad long-wave trough over the north-eastern Atlantic and a subtropical high over northern Africa. To the north, strong westerly jet-streams affect the Mediterranean and central Europe during the period. A frontal zone divides hot desert air mass over northern Africa and maritime air mass in the range of two trough centres west of the British Isles and from Scandinavia to Germany. Ahead of the western trough, a surface low moves into the British Isles. Its warm front will enter central Europe and moist maritime air mass will advect north-eastward. Wavy cold front is expected to extend from England to the Bretagne at the end of the forecast period.


... Parts of United Kingdom ...

After an intense warm air advection stratiform rain event over most parts of southern UK during the morning and midday hours, the attention then turns to the cold front, which enters SW - UK during the evening hours ( about 17 - 19 Z ). Warm sector downstream of this front has BL dewpoints around 15°C and overall weak lapse rates at all levels. However, forecast soundings from SW and S - UK reveal an atmosphere, which does not need that much modification for deep convection which is reflected in some low - end to locally modest instability release by latest GFS outputs. For now we think that best chances for organized thunderstorms will be west of a line London - Liverpool as 25m/s speed max at 700 hPa approaches from the SW and a compact vort max crosses the area from the WSW. In addition, the surface front has a near normal component to the background flow with a rapid NE - ward advance while a slower eastward shift over S / SE - UK can be noticed where front becomes more or less parallel aligned to the background flow. GFS also has persistent signals of a well defined intrusion of stratospheric air just behind the cold front and exact timing and placement of this intrusion in regard to the surface front could also be crucial for thunderstorm initiation and degree of coverage.

The warm sector is strongly sheared with veering wind field, very low LCLs and SRH-1 values well above 200 m^2/s^2 , increasing from central UK to the south. Any storm, which manages to evolve along the east / northeastward propagating cold front will have the chance to produce a tornado and strong to locally severe wind gusts. Confidence in this scenario is not yet that high to go with a level area but we have to monitor this areas during the day for updating and / or upgrading. The risk of isolated thunderstorms will rapidly spread east / northeastwards, resulting in such a broad thunderstorm area.

... E - Romania, Moldova and S - Ukraine ...

Remnants of a decaying EML and a pool of very moist BL ( also reflected in latest synop reports with dewpoints at or above 18°C ) result in a concentrated area of modest to high instability release with MLCAPE values around 1500 J/kg. Both DLS between 10 and 15 m/s and veering with height are not strong, but steep lapse rates and that much instability should be adequate for numerous large hail reports. The tornado risk is not excessively high, but rapidly decreasing LCL values along the coast and offshore in combination with some LL shear and locally strong LL CAPE release could result in an isolated tornado report. A cluster of storms should evolve betimes, moving slowly NE - ward with a flash flood risk also for S - Ukraine.

... Most parts of E - Europe ...

An extensive area with modest instability release and weak shear will be found over E - Europe. An upper trough crosses the area from the NW during the forecast period and scattered to widespread thunderstorms will evolve in a weakly capped environment with a marginal hail and gusty wind risk. There will be a corridor where interface of better shear / some instability could result in a couple of organized storms with an isolated large hail and tornado risk mainly over S - Poland, E - Slovakia and W - Romania, but coverage of storms should be quite low. Thunderstorms will go on during the night under the favorable thermodynamic environment of the eastward shifting upper trough although the risk ceases from west to east.

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