Skywarn Austria Forum

A level 1 was issued for northern Spain, southern France, Switerland, Austria, northern Italy, Slowenia, northern Croatia, Hungary mainly for large hail and excessive precipitation and to a lesser extent severe wind gusts.

DISCUSSION
Pyrenees, southern France, Alps, northern Balkans

Mid-level ridge will cover this area and large-scale lift will be weak. However, rich low-level moisture is present that is overlapping with steep lapse rates especially in the western portions, but diurnal heating is also expected to cause instability in the east. CAPE will range around 1500 J/kg in the afternoon. Initiation is forecast over the mountains in the noon and afternoon. Pulse storms and multicells are forecast to produce large hail and excessive precipitation. Local severe wind gusts are not ruled out. Later in the day, clusters of storms may also spread into the lowlands, capable of producing large hail and severe wind gusts and also excessive precipitation. Storm activity will be rather isolated and widespread severe weather is not forecast.

This Mesoscale Discussion is issued to highlight latest developments of a severe thunderstorm outbreak in parts of central Europe.

... E Germany ...

On Thursday morning, an intense elevated MCS has developed across the North Sea that currently enters Germany. Latest radar imagery indicates a rear inflow jet and leading bow echo. Wind gusts reached 34 m/s at Helgoland island and even a tornado was observed there despite a very cool boundary layer (2m temperature of only 13°C). It is expected that this storm system will become long-lived and will even intensify as it encounters increasingly warm and moist air and roots down to the surface. In addition, new storms have already formed to its southwest that will merge with the system later on.
The MCS rides a warm front extending from Hamburg to Berlin and further southeast. To the northeast, a dry and cool boundary layer will be present, whereas from the west, humid and unstable air masses are advected as indicated by latest Bergen and Meiningen soundings. Latest observations (09z) indicate 2m temperatures from 22 to 29°C and 2m dewpoints from 8 to 17°C, respectively, from NE to SW across the highlighted area. Additionally, with south-easterly surface flow, strong vertical wind shear and favorably veering profiles are present in the range of the frontal boundary.

We expect that the storm system will propagate even stronger to the right, where a lot of CAPE (possibly up to 2000 J/kg) is present. There is a high risk of widespread severe to extreme gusts in its path, probably maximized in the provinces of Sachsen-Anhalt and Sachsen. An embedded supercell at the SW-ern end poses an additional risk of large hail and a tornado which might even become long-lived or strong!
It is well possible that the storm system will still survive and move into the Czech Republic after 12z.

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... S Austria ...

The 12z Vienna sounding shows an overlap of robust CAPE (1500 J/kg) and enhanced 0-3 km shear (15 m/s) under the fringe of a NW-erly mid-level jet downstream of the subtropic ridge. Mid-level flow is weaker on the southern side of the Alps, but southerly upvalley and upslope winds stil maintain vertical wind shear of the same magnitude. CAPE is regionally even higher, as evaporation after two days of intense afternoon storms have pushed 2m dewpoints up to 20°C.
Due to a capping inversion and a lack of synoptic-scale lift, only isolated storms are expected in the next few hours, but they turn multi- and supercellular with a primary risk of large hail.

Conditions are similarly good in N Italy but the stronger cap is expected to suppress storm formation.

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