Mesoscale Discussion
Valid: Wed 03 Jan 2018 09:00 to Wed 03 Jan 2018 12:00 UTC
Issued: Wed 03 Jan 2018 08:56
Forecaster: PISTOTNIK

This mesoscale discussion is issued to summarize and interpret latest observational data with respect to an ongoing risk of severe to extreme convective wind gusts over parts of central Europe.

... NE Switzerland, S Germany ...

A pronounced convective line with intermittent lightning activity and bowing segments has formed along the cold front. It is placed along the NW-ern rim of the highlighted area at 09 UTC, is quickly moving SE-ward and will cross S Germany and NE Switzerland in the next two hours. Widespread wind gusts in the 25 to 32 m/s range have been measured so far, locally even considerably more (e.g., 38 m/s at Nancy, E France, and 35 m/s at Waibstadt, SW Germany).

Diurnal heating and prefrontal WSW-erly downslope winds will create an increasingly deep, drier boundary layer as the cold front approaches the Alps. The risk of extreme wind gusts will then increase even more, as downbursts due to evaporational cooling add to the downward mixing of momentum. The strong flow in the warm sector (30-35 m/s at 850 hPa) is already mixing down to the surface in many parts of Switzerland, where 2m temperatures up to 18°C (!) were recorded at 08 UTC. It is unclear how far eastward this deep mixing will manage to proceed in the given short time frame, but similar Foehn winds could develop very quickly along the German Alpine rim. If this verifies, it is not ruled out that maximum wind gusts at the convective line will locally approach or exceed 40 m/s in a corridor from Zurich across Lake Constance and Munich to Passau. The critical time frame is between 09 and 11 UTC, when also latest high-resolution model output agrees on the most extreme gusts. The risk is highest near the dryline (i.e., the northern rim of the warm and dry Foehn air), but each bowing segment that forms anywhere along the convective line should be closely monitored.

In addition to the main risk of severe to extreme straight-line winds, a tornado is not ruled out in case a more discrete updraft crystallizes somewhere within the line. Any tornado that forms in this environment of >15 m/s 0-1 km shear could become strong.

The convective line should start to weaken after 11 UTC when it impacts the Alps, but scattered lightning activity and severe wind gusts will continue some distance into Austria.

... W and central Germany ...

Postfrontal convection, including two strong line segments, have also formed in W Germany and are quickly moving E-ward. Despite the slightly weaker wind field, gusts can still locally exceed 25 m/s with these storms.