Storm Forecast
Valid: Fri 26 Aug 2011 06:00 to Sat 27 Aug 2011 06:00 UTC
Issued: Thu 25 Aug 2011 20:40
Forecaster: GROENEMEIJER

A level 2 was issued for NE Netherlands, NW Germnay and S Denmark for damaging winds, large hail and to a lesser extent for tornadoes.

A level 1 was issued for the remainder of Denmark, much of Germany, for E, central and S France, Switzerland, and for the remainder of the Benelux countries for severe wind gusts and large hail.

SYNOPSIS

A deep trough over the western Atlantic with its North-South directed axis over the Irish Sea, moves evolves into a separate upper level low located over north-central France on Saturday morning. Ahead of the system a cold front, intially stretching from a surface low over the southern North Sea to the Massif Central, moves eastward into central parts of Europe.

DISCUSSION

... Benelux, Denmark, Germany...

Ahead of the front pronounced northward advection of warm air takes place. Friday morning, a diffuse warm frontal zone is located from the W Netherlands into N Germany.

Ahead of the front between 1000 and 2000 J/kg of CAPE will generally be available. A strong southerly jet that is particularly well-defined at 700 hPa, with wind speeds 20 - 30 m/s. This will provide wind shear that is more than sufficient for storm organization. The best overlap of wind shear and CAPE is in the morning and early afternoon across the level 2 area, where the jet crosses the warm sector of the surface low over the North Sea.

We expect convection to be ongoing near and just east of the surface low over the Benelux countries and the German Bight. Models suggest that surface-based storms will develop across the level 2 area during the late morning and early afternoon. Upon interacting with the strong wind shear, these storms may easily become severe warranting the level 2. Bowing features with severe wind gusts are the most likely mode. Some large hail, will be possible given the substantial CAPE. The development of a isolated tornado or two cannot be discounted either given the high moisture and the substantial low level shear.

Further south, forcing for upward motion during mid-day expected to be rather weak if at all present, as a slack shortwave ridge passes from the south. This, in combination with a relatively strong cap, will probably suppress convection to a large extent during the day. Following the ridge, convection is again expected to move in from eastern France during the evening hours.

The overall parallel orientation of the shear and the front is expected to limit the severe threat to a great extent. Severe wind gusts are nevertheless possible where bowing features develop, and some large hail may occur as well.

... France and Switzerland...

As the front moves eastward, it appears not to be very active across the northeastern quarter of France because of upper-level ridging. Only Isolated convection is expected in the upward branch of the frontal circulation. During the afternoon, upper-level forcing should increase from the south, and convective coverage is expected to increase across SE France. With around 1000 J/kg of MLCAPE in place and 20-30 m/s deep-layer shear, a few supercell or bowing storms with severe winds gusts and large hail will probably form. The storms will move into SW Germany during the evening.