Storm Forecast
Valid: Thu 05 Dec 2013 06:00 to Fri 06 Dec 2013 06:00 UTC
Issued: Wed 04 Dec 2013 22:49
Forecaster: GATZEN

A level 2 was issued for Scotland, the central North Sea, Denmark, southern Sweden, eastern Germany, and north-western Poland mainly for severe convective wind gusts.

A level 1 was issued for southern Poland, the Czech Republic, and surroundings mainly for severe convective wind gusts.

SYNOPSIS

Continuing height falls across central and eastern Europe on the one hand and increasing geopotential across the northern Atlantic result in a west-north-westerly flow over most of Europe on Thursday morning. A frontal wave that was located to the south of Iceland on Wednesday 18Z will move south-east and is expected over southern Norway on Thursday, 12Z. Located in a favourably coupled jet streak position, rapid intensification is forecast by latest models and the centre pressure will drop by 40 hPa within 24 hours. A strong winter storm is therefore expected to affect an area from the northern British Isles to the southern Baltic Sea region. Intense cold air advection will be dominant in the wake of the storm. Across southern Europe, subsidence is expected through-out the forecast period.

DISCUSSION

British Isles, North Sea, southern Scandinavia, Germany, Poland, and surroundings

In the morning hours, the North Sea low will move into southern Norway. Its cold front will cross most of the British Isles and the North Sea. Ahead of the cold front, a narrow tongue of boundary-layer moisture with 0-1 km mixing ratio around 5 g/kg will spread from the British Isles to southern Scandinavia. Given moist-adiabatic lapse rates and rather warm sea surface temperatures, weak instability is forecast ahead of the cold front, allowing for a narrow cold frontal rain band with embedded shallow convection. The contribution of deep moist convection to the wind gust occurrence is expected to be weak to the south of the jet core.

From the jet maximum northward, strong DCVA will lead to QG lift and the low-level saturated air mass will become deeper. Thunderstorms are forecast to develop along the cold front from Scotland to the central North Sea until noon. Given very strong vertical wind shear, convection will organize and shallow multicells with linear segments are expected. Especially in the northern portions, rapid movement of these linear segments will result in an increasing surface wind gust threat that warrants a level 2.

In the afternoon, most prominent convective development is expected along the cold front. Especially from Denmark to the southern Baltic Sea, thunderstorms are forecast given the strong DCVA at the cyclonically sheared flank of the mid-level 50 m/s jet streak. A quasi-linear mesoscale convective system capable of producing high or extremely high winds is not ruled out if this scenario will become true.

Together with the strongest DCVA, the convective activity will spread south-east in the evening, where the main limiting factor is the dry boundary layer. The mostly occluded warm sector will only allow for a mixing ratio up to 4 g/kg. So it is questionable if cold pools will develop due to the lack of intense convective precipitation. A maintaining quasi-linear convective system becomes unlikely for this reason.

Current thinking is that a broken line of storms is the most likely scenario over eastern Germany and Poland. Local downdrafts will increase the wind gust threat especially in association with persistent multicells that may evolve small but rapidly moving cold pools. Wind gusts in excess of 33 m/s are forecast with this convection along the cold front, and a level 2 is issued. The threat will spread across eastern Germany into Poland during the evening hours. Although tornadoes are not ruled out, the weak low-level buoyancy is expected to limit the potential strongly.

Together with the strongest DCVA, the cold frontal convection is forecast to spread into the western Ukraine at the end of the period. Although moisture will further decrease, showers capable of producing severe wind gusts are still not ruled out.

In the wake of the cold front, strong cold air advection is expected. The warm sea surface of the North Sea will allow for weak CAPE in the cold air mass. Especially in the range of short-wave troughs travelling south-east, showers and thunderstorms are possible, but will likely be isolated due to the unfavourable QG situation. The convective downdrafts are not forecast to enhance the severe wind gust strength significantly given strong turbulent mixing in the lower levels even without convection.