Storm Forecast
Valid: Thu 30 Jul 2009 06:00 to Fri 31 Jul 2009 06:00 UTC
Issued: Wed 29 Jul 2009 21:11
Forecaster: GATZEN

A level 1 was issued for extremely northern Germany, Denmark, southern Norway, and southern Sweden mainly for severe wind gusts.

A level 1 was issued for southern Switzerland and north-eastern Italy mainly for large hail.

SYNOPSIS

Low geopotential over the northern Atlantic continues and associated strong south-westerly jet still affects western Europe. A negatively-tilted and quite intense short-wave trough is forecast to move north-eastwards across the North Sea on Thursday, while a subtropical high over Morocco ridges into the central Mediterranean, the Baltic States, and further into Finland. Low geopotential is present further east including cut-off low over the northern Black Sea. At lower levels, cool air masses spread into the North Sea region, France, Germany, and most of Scandinavia behind a cold front, while relatively warm air masses are present over the Mediterranean and eastern Europe.

DISCUSSION

Southern Scandinavia region

In the range of the intense short-wave trough, models agree about a dry intrusion moving across the North Sea into southern Scandinavia in the morning hours. Strong QG forcing is expected, and a strong ageostrophic circulation does likely lead to warm air advection. The cold front is forecast to cross northern Germany and Denmark in the morning hours and will spread across southern Scandinavia at noon.

Models indicate that strong QG forcing will likely overspread the cold front leading to weak instability in the range of the cold front. A narrow convective line is forecast to spread across northern Germany and Denmark into southern Norway and Sweden and will reach southern central Sweden in the evening. Given rather cold equilibrium temperatures at the cyclonic flank of the upper-level jet streak, thunderstorms are forecast in the north-western portions of this convective line.

Given strong vertical wind shear along the convective line, severe wind gusts are forecast, especially in the range of bowing lines. Tornadoes may develop as rather large horizontal cyclonic vorticity may develop in the range of mesoscale wave along the convective line. The main limiting factor is the rather cool boundary-layer and uncertainties about the development of low-level moisture.

In the wake of the cold front, models agree about some instability at the cyclonic flank of the mid-level jet streak over the southern North Sea, and showers and thunderstorms will likely develop, capable of producing small hail and stormy gusts mostly below severe limits in the range of rather strong vertical wind shear.

Southern Switzerland and north-eastern Italy

Models agree that an upper-level jet streak will spread across southern France in the range of the delta regime of the main westerly jet. This will likely be associated with some QG forcing especially over north-eastern Italy, where weak temperature advection is forecast.

Given rather moist low-level air mass and quite steep mid-level lapse rates, instability will likely develop in response of diurnal heating. QG forcing and some upslope flow will likely enable deep moist convection. Stronger cells will be capable of producing large hail especially in the western portions of the Alps, where vertical wind shear will be strongest. Further east, convection will likely be weaker over most of the Alps, but locally hail and extensive rain is not completely ruled out.