Storm Forecast
Valid: Thu 18 Aug 2016 06:00 to Fri 19 Aug 2016 06:00 UTC
Issued: Thu 18 Aug 2016 00:15
Forecaster: GATZEN

A level 1 was issued for eastern France, southern Germany, Switzerland, north-western Itlay mainly for large hail and severe wind gusts as well as excessive rain.

A level 1 was issued for northern France into Belgium mainly for excessive rain.

A level 1 was issued for central and southern Austria, northern and central Italy, and western Balkans mainly for large hail and excessive rain.

A level 1 was issued for Bulgaria and central Ukraine mainly for excessive rain and to a lesser extend large hail.

A level 1 was issued for western Russia mainly for tornadoes, large hail and excessive rain.

SYNOPSIS

High-over-low blocking is present across northern Europe, with an extensive polar trough centred over the southern Baltic Sea. Its mid-level circulation affects almost complete Europe except for the western parts influenced by the flow ahead of an Atlantic trough. The trough is filled with polar air masses, but at the south-eastern periphery of the trough, low-level have warmed especially over the Alpine region and from the southern Balkans to south-western Russia. This is associated with steep mid-level lapse rates. Overlap of these with rich low-level moisture extending along the frontal boundary results in quite substantial instability at places. Thunderstorms have formed on Wednesday and are also expected during the forecast period.

Main convective activity is expected where short-wave troughs move along the edge of the main circulation, affecting eastern France and later on the Alpine region as well as Italy and the Balkans, and also the Ukraine. Thunderstorms will be concentrated along frontal boundaries and where topography assists in storm initiation.

DISCUSSION

Eastern France into Benelux, western and southern Germany, western and central Alps

Storms are expected from France into the Alpine region, where low-level warm air advection ahead of an occlusion coexists with a passing short-wave trough. Vertical wind shear increases in the afternoon due to strengthening westerly winds, and storm organization becomes better, with multicells expected. Supercells are also not ruled out from eastern France into southern Germany, where favourable veering profiles evolve in the evening hours. These could produce large hail and severe wind gusts. Further north, weaker flow close to the short-wave trough centre and associated slow moving storms can produce locally excessive rain.

Italy to Balkans

A quite pronounced short-wave trough amplifies over Italy. Thunderstorms are most likely over the Italian Apennines but may also spread into the low lands, especially in the northern regions where the capping inversion is weaker. Later in the day, the trough will support intensification of storms over the Balkan mountains as well. Most storms will be relatively weakly organized and partly elevated given limited vertical wind shear as well as inversions below the mid-level steep lapse rates. Stronger storms may produce large hail and excessive rain as weak flow will lead to slow-moving storms.

Bulgaria to western Russia

Storms that form along the frontal boundary from Bulgaria to western Russia can move parallel to it allowing for stronger and long-living updrafts. Some large hail is expected and especially excessive rain. Main threat is forecast over the Bulgarian mountains and the central Ukraine. Over western Russia, strong southerly flow and veering low-level profiles can support mesocyclones capable of producing tornadoes and large hail. Excessive rain can be expected when larger clusters of storms manage to form. Main limiting factor is rather weak low-level buoyancy to the west of the main warm front.