Storm Forecast
Valid: Thu 16 Aug 2007 06:00 to Fri 17 Aug 2007 06:00 UTC
Issued: Wed 15 Aug 2007 17:48
Forecaster: GATZEN

SYNOPSIS

Amplified upper long-wave trough slowly propagates eastward. The trough axis is orientated nearly north-south and is expected to reach from northern North Sea to western France on Thursday. At the eastern flank of the trough, a very strong jet streak from Scandinavia to eastern France will continue to move northward. East of this jet, a sharp ridge is expected, while low geopotential will remain further east over east Europe. At low levels, a frontal boundary stretches from eastern Spain to the Alps, western Poland, and eastern Scandinavia.

DISCUSSION

Southern Alps, eastern Austria, Czech Rep. and Slovenia, Poland, Baltic States, Finland

Ahead of the frontal boundary, a narrow tongue of warm air is advected northward. This air mass is characterized by rich low-level moisture as indicated by latest observations, while mid-level lapse rates are rather low. This will likely result in weak instability as indicated by latest GFS model run, but locally moderate CAPE is not ruled out especially in the range of the Alps. During the day, the frontal boundary propagates eastward over Baltic Sea region, Poland, and Czech Republic. To the south, the frontal boundary remains along the Alpine region. Showers and thunderstorms seem to be quite likely in the range of the frontal boundary due to low-level convergence and upper divergent flow in the range of the anticyclonic entry of strong upper jet streak. Thunderstorms that form may organize given strong deep layer vertical wind shear. Locally large hail and severe wind gusts are forecast with the stronger storms. Further north, more favorable hodographs are expected over Poland, and Baltic States/Finland in the range of a strong low-level jet ahead of the cold front, and chance for mesocyclones is higher, posing an increasing threat of tornadoes. Expected weak low-level buoyancy is a limiting factor, and severe events will likely be isolated. Thunderstorms may merge into clusters over Baltic States and Finland in the evening hours along the frontal boundary, while convective activity is expected to weaken further south as instability decreases.

Sweden, Denmark

At the cyclonic flank of the jet streak, several vort-maxima will likely lead to strong DCVA. Affected air mass is expected to be slightly unstable due to rather steep mid-level lapse rates and some low-level moisture in the maritime air mass. Showers and thunderstorms that form will likely organize given strong vertical wind shear, and small bows and multicells are forecast. Isolated severe wind gusts and probably a tornado are not ruled out, but threat seems to be quite low given weak instability and low-level forcing.