CONVECTIVE FORECAST
VALID 06Z WED 13/08 - 06Z THU 14/08 2003
ISSUED: 12/08 22:46Z
FORECASTER: HAVEN

General thunderstorms are forecast across SOUTH AND CENTRAL FRANCE...NORTHERN ALPS...S-GERMANY...CSECH REPUBLIC...ITALY...NRN IBERIAN PENINSULA...SWRN-SCANDINAVIA...UKRAINA

SYNOPSIS

Meandering jetstream between approx 55N and 60N with upper ridge over S-Sweden and trough over the British Isles. These features are both continuing eastward during the forecast period. Another upper trough over extreme southeast-Europe moves slowly to the Black Sea. North of the jetstream an upper low between Iceland and Scotland is moving towards the northern NorthSea, while another upper low east of the Ukraina is moving N-NE.
At lower levels...coldfront Wednesday at 06Z from E-Germany to cntrl-France moves east and is expected from Lithuania to N-Austria to SW-France around midnight. East of the coldfront a dry airmass is in place, near and behind the front pockets of moister air can be found.

DISCUSSION

...SOUTH AND CENTRAL FRANCE...NORTHERN ALPS...S-GERMANY...CSECH REPUBLIC...
Main problem will be the absence of sufficient deep moisture in the lowest layer of the atmosphere. This will very likely keep MLCAPE quite low, in the order of a few hundred J/kg. Nonetheless... forcing by the coldfront and orography will initiate a few thunderstorms, which will be isolated due to the rather dry thermodynamic profiles and near zero UVM.
Steep lapse-rates and the rather dry lowest 3 km of the atmosphere should support downdrafts, locally exeeding the severe treshold of 50 kts. With 0-6 km shear generally below 30 kts, dynamic pressure perturbation forces will not be sufficient for stormrotation. However... in the evening vertical windshear in the lowest 850 hPa as high as 30 kts over cntrl-Germany and the Csech Republic could be supportive for rotating storms if the low-level windfield is favorably modified by local orography. Large hail and damaging winds could occur with any supercell that forms. Tornado threat is quite low due to the expected high LCL. Because of the low expected coverage of the storms, a slight risk doesn’t seem necessary at the moment.

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