Storm Forecast
Valid: Thu 09 Apr 2009 06:00 to Fri 10 Apr 2009 06:00 UTC
Issued: Wed 08 Apr 2009 19:58
Forecaster: GATZEN

SYNOPSIS

To the east of a subtropical high over the northern Atlantic, Europe is dominated by rather low geopotential. A new trough is forecast to spread southward over western Europe during the period, feeding the trough across the Mediterranean. Ahead of the new trough, a weak ridge is forecast to amplify across central Europe, and relatively warm and moist air mass spreads into western central Europe. To the north-east, a small short-wave trough affects the Baltic Sea region. Latest model output indicates quite weak deep layer wind shear over most of Europe during the period, limiting chances for organized severe convection.

DISCUSSION

East central Europe

A small upper trough moves across the southern Baltic Sea on Thursday. At its south-western flank, a 20 m/s mid-level jet streak is forecast to spread into northern Poland. At low levels, a cold front is expected from the Baltic States and central Poland to northern Austria at 12Z, and will propagate eastward during the day. East of this cold front, latest soundings indicate lapse rates around 7K/km in the lowest 3 km and a low-level mixing ratio around 6g/kg, leading to weak instability in response to diurnal heating. Given some moisture flux convergence ahead of the cold front, mostly uncapped instability around 500 J/kg is forecast to develop from the southern Baltic States to eastern Poland, and further south into northern Austria. Showers and thunderstorms will likely develop along the cold front.

Current thinking is that cells will have at least some potential to organize in the northern portions of the instability region given increasing mid-level winds ahead of the approaching trough. With SSE-erly surface winds and SW-erly winds above the boundary layer, around 10 m/s vertical wind shear is expected in the lowest 3 km, that will assist for more stable cold pools and more upright updrafts especially at the south-eastern flanks of the cold pools, where intense convection is not ruled out. Limiting factors are relatively weak forcing and winds as well as poor veering profiles. Storms are forecast to cluster more linear along the merging cold pools with some intense updrafts to the south-east later on. The strongest cells may produce isolated severe hail, but threat seems to be too low for a categorical risk level given weak instability and vertical wind shear. Severe wind gusts are also not ruled out completely, but the moisture profile and wind field seem to be not very supportive. Storms are forecast to move into the Baltic States and Belarus, where decreasing low-level moisture and inverted v-profiles may pose an increasing threat of strong wind gusts, while weakening convection will likely become outflow-dominant.

Further south, weak vertical wind shear and forcing are present in the range of the cold front. Along the main convergence, thunderstorms are forecast during the day that will likely be outflow-dominant and unorganized. Strong wind gusts are forecast as well a locally intense precipitation. Isolated severe hail is not ruled out with the initial convection, but indications are too weak for a level 1 threat at this time. After sunset, instability is expected to decrease rapidly and storms will likely die.