Storm Forecast
Valid: Thu 26 Jul 2007 06:00 to Fri 27 Jul 2007 06:00 UTC
Issued: Thu 26 Jul 2007 16:01
Forecaster: GATZEN

SYNOPSIS

Intense trough has spread across east Europe, and hot subtropical air mass has been replaced by rather warm and dry low-level air. Weak upper ridge follows over central Europe, while another trough migrates over British Isles into North Sea region. The Mediterranean will be affected by a weak westerly flow at the northern edge of subtropical air over Africa. Mediterranean air mass is stable due to rather cold water surface temperature and poor low-level moisture.

DISCUSSION

British Isle

Upper trough axis is expected to move across the British Isles during the period. Very strong south-westerly upper jet is located at the southern edge of this trough, and associated strong DCVA is forecast to spread from Ireland westward. Latest model guidance shows that instability will develop underneath the trough axis. Limiting factors will be relatively weak low-level moisture. But given rather cool 850 hPa temperature and some surface heating, low-level buoyancy may be relatively good. Showers/thunderstorms that form underneath the southern part of the trough will likely be affected by strong deep layer vertical wind shear. Given steep low-level lapse rates and locally high low-level buoyancy, as well as strong synoptic wind field especially at the 850 hPa level, a few tornadoes are not ruled out. Chance seems to be too low for a risk level, though. Further southeast, strong deep layer vertical wind shear and QG forcing is forecast ahead of the trough axis and along the cold front. Latest model guidance shows that instability will be quite low in the warm air mass. Current thinking is that instability may realize due to surface heating, and a strongly-forced convective line may develop along the cold front. With strong vertical wind shear, a few tornadoes are not ruled out along the leading gust front. Chance seems to be weak, though, given rather poor low-level moisture and uncertainty of instability.