Storm Forecast
Valid: Wed 22 Jun 2011 06:00 to Thu 23 Jun 2011 06:00 UTC
Issued: Tue 21 Jun 2011 20:32
Forecaster: GROENEMEIJER

A level 2 was issued for SW, central, and east-central Germany mainly for severe winds, and secondarily for tornadoes, large hail, and heavy rain.

A level 1 was issued for parts adjacent to the level 2 area, in Germany and NW Poland primarily for severe wind gusts and rain, in other areas of the level 1 primarily for large hail.

SYNOPSIS

Within a strong WSWly flow over the western part of the continent, a mid/upper-level shortwave trough is expected to translate northeastward rapidly during the forecast period. This system is preceded by a well-developed cold front, along which a baroclinic wave will develop.

DISCUSSION

... much of Germany ...

At 06 UTC, the cold front is expected near a line from the eastern Benelux towards far southwestern France. The air-mass ahead of the front is fairly moist with dew point temperatures of 16-18 C expected to develop during the day. Although the lapse rates are not particularly steep, this should suffice to produce on the order 500-1000 J/kg of MLCAPE per ECMWF and GFS as temperatures rise to 25-28 C during the day.

A strong 40 m/s jet streak is expected to develop ahead of the trough over the Benelux countries during the former half of the (06-06 UTC) forecast period. Model guidance shows that part of the strong mid-level winds will overlap with the surface-based instability. It must be noted, though, that model guidance has not been consistent regarding the extent of this overlap.

The main question that determines the extent of the shear/instability overlap is the development of the baroclinic wave/surface low ahead of the trough (right entance of the preceding jet streak). In case this low develops aggressively, stronger shear and more storm-relative helicity are expected to develop.

Regardless, the setup looks sufficiently potent to warrant a level 2, given the very strong forcing. It is expected that surface-based convective storms will develop around noon, or shortly after, over west-central and southwestern Germany, eastern France and Switzerland. Some storms may be supercellular but will quickly organize into a linear mode given the strong forcing and small angle between the convergence and the deep-layer shear vector. These convective systems are expected to translate rapidly northeastward, producing swaths of damaging winds during the afternoon and evening. Current thinking is that a broad corridor along an line from Karlsruhe to Cottbus will be most at risk.

In case the surface low deepens strongly, and lower tropospheric wind fields become as strong as forecast in the most recent GFS (12 UTC) and ECWMF (12 UTC) runs, a few tornadoes could occur. In this scenario, the most likely location for tornadoes would be across Thuringia, Saxony, Saxony-Anhalt, Brandenburg and Berlin.

... NE Switzerland, S Germany...

Storms are expected to develop here during the afternoon. Moderate CAPE and about 20-25 m/s deep-layer shear should support large hail. As models forecast rather high surface temperatures of close to 30 C, strong evaporational cooling may occur, suggesting severe gusts will be a distinct threat here. The storms will likely organize and move eastward during the late afternoon and evening.

... to the south and east across S Poland, Czech Republic, Austria, Hungary, NW Balkans, N Italy ...

More isolated supercell and multicell storms are expected to develop here during the afternoon, that will eventually also cluster but more gradually. Their primary threat should be large hail, but a few severe wind gusts may occur as well.

... E France, W Switzerland ...

Models agree on developing some instability across much of the French level 1 zone and W Switzerland.Elevated convective activity may however already be ongoing across these areas and insolation may become a problem in creating sizeable amounts of CAPE. Hence, the threat for severe storms is accordingly lower, marginally warranting a level 1 mainly for hail with storms that should develop rather early in the day.

... Spain ...

Moderate CAPE and moderate shear should allow a few multicell, perhaps weakly rotating storms to produce some large hail during the afternoon in addition to some strong wind gusts.