Storm Forecast
Valid: Wed 20 Aug 2008 06:00 to Thu 21 Aug 2008 06:00 UTC
Issued: Tue 19 Aug 2008 19:26
Forecaster: GATZEN

SYNOPSIS

Long-wave trough remains over north-western Europe, yielding a westerly mid-level jet over western Europe and a south-westerly jet from central Mediterranean to Baltic States. The latter also extends into the lower troposphere over the Baltic States, where winds will likely reach more than 20 m/s at the 700 hPa level. Most of Europe is dominated by a cool or relatively dry boundary-layer air mass.

DISCUSSION

Baltic region

Along a cold front that moves eastward at the eastern flank of the upper trough, a frontal wave is forecast to develop over Poland in the range of a strong jet streak and associated DCVA. Warm air mass will spread southward in the range of this wave. Latest soundings indicate that lapse rates and boundary-layer moisture are quite poor over eastern Germany and Poland, but mid-level height falls in the range of approaching short-wave trough may lead to weak instability east of the frontal boundary.

At the surface, a weak low is expected to develop, and low-level convergence will be quite weak. However, forcing along the frontal boundary will likely result in initiation. Showers and thunderstorms will rapidly move north-eastwards given strong south-westerly winds, while low-level winds will be weak from the south or south-east. This will result in moderate low-level storm-relative helicity and updrafts may rotate. Isolated supercells will pose a threat of some large hail as well as severe wind gusts. A tornado is not ruled out especially hear the frontal wave where low-level buoyancy and vertical wind shear will be most favourable. Storms will move quickly to northern Baltic States and will weaken during the night hours.

Southern British Isles

At the flank of the long-wave trough, a weak frontal boundary moves eastwards over southern British Isles. To the south of this frontal boundary, southerly winds are forecast that will advect moist low-level air mass into Ireland. Given low-level convergence along the boundary and rather high low-level buoyancy due to increasing moisture underneath the cyclonic flank of the upper jet, thunderstorms are forecast to develop during the day. Weak vertical wind shear is expected and weak tornadoes are forecast. Later in the period, low-level convergence spreads eastward, but low-level moisture seems to be significantly lower. However, showers and thunderstorms are not ruled out, and an isolated weak tornado is not ruled out. Convection will weaken in the evening/night hours.

Northern Adriatic

Moderate vertical win shear and some instability due to moist low-level air mass will likely assist for some thunderstorms during the afternoon and evening, capable of producing isolated large hail.