Storm Forecast
Valid: Sun 03 Aug 2008 06:00 to Mon 04 Aug 2008 06:00 UTC
Issued: Sat 02 Aug 2008 23:01
Forecaster: SCHLENCZEK

SYNOPSIS

Zonal upper flow pattern has established in western and central Europe. An upper trough over northern UK will translate eastward during the period with a strong 50 m/s jet streak at its southern flank. Ahead of the cold front associated with this low pressure system, a strong southwesterly flow at lower and upper levels advects warm and moist air into France, Germany and the Benelux countries. Recent GFS runs indicate that a frontal wave will develop over the British Channel during the afternoon that will rapidly move eastward into northern France and Belgium where some instability should be present.

Some instability has formed over the central Mediterranean and parts of Italy in an environment with weak pressure gradients and weak vertical shear. Briefly organized storms are expected to develop over northern Italy during the day with isolated large hail / strong gusts not ruled out.

Further east, hot and moist air is present over southeastern Poland / southern Belarus and western Ukraine. The right entrance region of a 60 m/s jet streak will overspread the region where moderate instability will form during the day. Storms should evolve in the vicinity of the approaching cold front. Unsettled conditions are expected over southeastern Europe in the wake of a weak upper trough.

DISCUSSION

...Northern France, Belgium, extreme western Germany...

Advection of warm and moist air should lead to dewpoints around 15°C over France and Belgium during the afternoon and some hundred J/kg of CAPE will form. Although mid levels are quite warm, convective initiation should be warranted as a frontal wave will overspread the region in the evening. Deep layer shear in order of 30 m/s and strong low level winds with 20 m/s at 850hPa in the vicinity of the wave will aid storms to organize into a bowing line, capable of producing severe gusts. Storms at the leading edge of a linear system as well as discrete cells will probably become supercellular as 150 - 200 J/kg SRH3 should be available. Low LCL heights and strong LLS support an isolated tornado as well. Despite meager CAPE, a level-one threat is warranted.

...Western Ukraine, southwestern Belarus...

Ahead of the cold front, diurnal heating should create about 1 kJ/kg MUCAPE in an environment with 10 - 15 m/s deep layer shear. Although forcing is expected to be rather weak, some diurnally-driven multicells with an isolated large hail / severe gust threat will probably develop. Expected storm coverage should justify a low-end level-one threat.