Storm Forecast
Valid: Wed 03 Sep 2008 06:00 to Thu 04 Sep 2008 06:00 UTC
Issued: Tue 02 Sep 2008 20:07
Forecaster: GATZEN

SYNOPSIS

An intense long-wave trough is situated over the North Sea and British Isles. A strong mid-level jet extends across France, Benelux, Germany, and Baltic Sea. An embedded short-wave trough and associated jet streak will travel from northern France to north-eastern Germany reaching southern Finland on THU morning, where winds will reach 70 m/s at the 500 hPa-level. To the south-east, broad high geopotential is divided into two upper highs by a weak mid-level trough west of the Black Sea and southern Balkans. Warm air has spread across southern, central, and eastern Europe that will destabilize over the Mediterranean, in the range of the Balkan trough, and near the frontal boundary from southern France to eastern Germany and Baltic States. Unstable maritime air mass is located near the cyclonic flank of the jet stream from British Isles to the North Sea and southern and central Scandinavia.

DISCUSSION

Southern France, northern Italy, Alpine region, and Germany

Rich boundary-layer moisture has developed over north-east Mediterranean as indicated by latest Barcelona sounding and surface observations over southern France. Although wind will be from the west in the morning hours, strong moisture return is expected on WED in the range of a developing southerly low-level jet ahead of extending short-wave trough over Spain and approaching cold front over central/western France. In the range of the frontal boundary and over northern Italy, rather cool mid-level air mass will likely allow for instability. To the south, warm air mass above the boundary-layer will be a limiting factor for sufficient instability. Current thinking is that convection will develop in the morning hours in the range of the first DCVA maximum spreading from southern France to northern Italy. Later in the period, instability is expected to increase south of the frontal boundary, and given strong quasi-geostrophic forcing ahead of another approaching jet streak over Spain, convection will also initiate from central France to western Alps, spreading eastwards during the evening hours. With strong vertical wind shear due to southerly low-level winds and strong mid-level jet (up to 25 m/s 0-6 km and 15 m/s 0-3 km vertical wind shear) multicell clusters and embedded mesocyclones are forecast. Large hail may occur initially with more isolated mesocyclones, while severe wind gusts are forecast with bowing segments in the range of multicell clusters. Given strong low-level vertical wind shear in the range of the low-level jet, tornadoes are also forecast near the frontal boundary over France and western Alps. Although low-level vertical wind shear decreases to the east isolated tornadoes are not ruled out over Germany along the frontal boundary. Convection will likely merge into a more elevated MCS over Germany given strong QG forcing. Potential for severe storms is greatest over France, where an upgrade to level 2 may be warranted when low-level instability will be strong, indicated that strong tornadoes may become more likely.

Northern Poland and Baltic States

Ahead of the first vort-maximum, convection will likely develop along the frontal boundary from northern Poland to southern Baltic States in the late morning hours, when sufficient instability will develop. Widespread fog and therefore rich low-level moisture is present, and lapse rates will likely improve in the range of the approaching short-wave trough. Given strong vertical wind shear around 15 m/s 0-3 km and 10 m/s 0-1 km vertical wind shear, severe thunderstorms capable of producing tornadoes and isolated severe wind gusts are not ruled out. Limiting factor is rather cool boundary-layer and associated weak low-level buoyancy, and later observations have to confirm this scenario with isolated tornado threat.

Denmark, Sweden

To the north of the upper jet, maritime air mass is expected to destabilize due to rich low-level moisture and steep low- to mid-level lapse rates. Strong QG forcing is forecast and showers and thunderstorms will likely develop. Given strong low-level winds and associated low-level vertical wind shear, a few tornadoes are forecast during the day. Severe wind gusts are not expected to be likely, though.