Storm Forecast
Valid: Thu 15 Jul 2010 06:00 to Fri 16 Jul 2010 06:00 UTC
Issued: Wed 14 Jul 2010 23:05
Forecaster: GATZEN

A level 1 was issued for south-western Scnadinavia mainly for tornadoes and severe wind gusts.

A level 1 was issued for the northern Allpine region, Czech Republic, and western Poland mainly for severe hail.

SYNOPSIS

At the periphery of a trough centred over the British Isles a strong south-westerly jet affects western and northern Europe. A negatively tilted short-wave trough moves northward into Scandinavia during the period while another short-wave trough crosses the British Isles. At lower levels, waves form along the frontal boundary over north-western Europe. To the south-east of this front, a warm and unstable air mass is present.

DISCUSSION

Southern Scandinavia

Ahead of the approaching short-wave trough, an easterly flow will advect some low-level moisture from the Baltic Sea region into southern Sweden and Norway. This together with some diurnal heating may help to destabilize the air mass in the range of the occluded frontal system of the frontal wave that will be present over southern Scandinavia in the morning hours. Strong forcing in the range of the negatively tilted trough axis as well as upslope flow will lead to strong forcing.

Along the front, strong vertical wind shear will be present that will cause favourably veering profiles ahead of the front, overlapping with expected instability, and a few bowing lines and mesocyclones are not ruled out along the front. In the wake of the occlusion, some additional cells may develop given the forcing of the short-wave trough. With very strong winds at the 700 hPa level, severe wind gusts may occur locally. Isolated tornadoes are also forecast as the low-level vertical wind shear is rather strong. Storms will likely decay rapidly after sunset.

Alpine region to western Poland

While the low-level cold front has moved away from the trough, it slows down over the Alpine region and western Poland. Model output suggests some frontal waves moving northward. Given low-level convergence along the front, storms will likely go on during the day. Latest model output suggests that rather strong vertical wind shear will overlap with the instability in the range of the front as least over the Alps. As a consequence, storms that develop during the day may become organized with the main threat being large hail.

The Chanel region and southern British Isles

Another short-wave trough yields a frontal wave across the Atlantic that travels into the British Isles on Thursday. Expect neutral lapse rates and rich low-level moisture in the warm air advection regime, some CAPE might build in response to diurnal heating. Latest model output does not indicate instability due to the warm air aloft and rather weak pre-frontal low-level moisture. However, at least a weak chance is present when the moisture increases from the west.

Along the occlusion of the wave that moves into the British Isles in the evening hours, a few embedded storms could develop. Given very strong low-level vertical wind shear in excess of 20 m/s in the lowest kilometre, tornadoes are not ruled out completely, and even strong tornadoes may be possible. Given the lack of instability predicted by the models, no level is given at this time.

Baltic States to Bulgaria

In the range of the eastern high pressure system, a broad region with moderate instability and some low-level convergence has developed. Expect widespread thunderstorms including a few severe pulse storms capable of producing large hail and excessive rain.