Storm Forecast
Valid: Thu 04 Sep 2008 06:00 to Fri 05 Sep 2008 06:00 UTC
Issued: Wed 03 Sep 2008 20:08
Forecaster: GATZEN

SYNOPSIS

Intense long-wave trough amplifies over western Europe. Resulting strong mid-level jet extends from eastern Spain to the Alps and central Europe and further to Baltic Sea and western Russia, and a frontal boundary is located at low levels. To the east, weak upper trough moves across western Black Sea.

DISCUSSION

Southern France, northern Italy

The frontal boundary is forecast to form another wave as QG forcing spreads eastward ahead of a short-wave trough that curves around the base of the long-wave trough. This is expected to result in strong moisture return from south-eastern France to north-western Italy. This will lead to unstable conditions especially over southern France, where cooler mid-levels are compared to Italy. QG forcing is forecast and convection will likely go on or re-develop along and south of the frontal boundary. Limiting factor may be cool low-level air mass near the frontal boundary. Convection that develops is forecast to organize into small bow echoes and mesocyclones given strong vertical wind shear and veering profiles (20 m/s DLS and 150 m²/s² 0-3 km SRH). Isolated large hail or severe wind gusts are not ruled out. As low-level vertical wind shear is also rather strong especially in the afternoon and evening hours when south-westerly low-level jet is forecast to increase ahead of the cold front, a few tornadoes are forecast. Convection is forecast to spread eastward into Alps and northern Italy during the day, where strong low- and deep layer vertical wind shear will also be present. While forcing will be lower, more isolated convective cells are forecast. These will be capable of producing large hail and tornadoes. As the upper trough approaches in the evening and night hours, thunderstorms will likely spread southward into central Italy, where threat of isolated tornadoes and large hail is forecast to continue through the night.

South-eastern Spain

Both GFS and ECMWF indicate that some convection will form in the range of the trough axis that moves eastward across eastern Spain. Weak instability will likely be present over the land during the day given moist low-level air mass originating from the Mediterranean. Limiting factor is quite warm mid-level air mass that will likely create a strong capping inversion over the sea. Current thinking is that some thunderstorms will develop around noon over the mountains that will move north-eastwards. Strong vertical wind shear will be present and large hail and isolated tornadoes are not ruled out. Convection will likely weaken over the sea there low-level forcing is forecast to be too weak.

Alps to Slovakia

Ahead of the new vort-maximum, unstable air mass is forecast to advect into the Alpine region. Although QG forcing will be rather weak during the day, a few convective cells may develop. Strong vertical wind shear will be favourable for bow echoes and mesocyclones capable of producing large hail and severe wind gusts. Thunderstorms will likely move into Czech Republic and Slovakia during the period, where severe threat will likely continue.

Parts of Poland, Belarus, southern Baltic States, western Russia

Along the cold front, frontal waves are forecast to travel north-eastwards. Some instability may develop, but low-level will be rather dry over most places. Best potential seems to exist in the northern area, where mid-level air mass is coldest. Strong vertical wind shear will be present over all of the region, and severe storms may develop, capable of producing severe wind gusts and a few tornadoes.