Storm Forecast
Valid: Thu 26 Aug 2010 06:00 to Fri 27 Aug 2010 06:00 UTC
Issued: Wed 25 Aug 2010 19:34
Forecaster: GATZEN

A level 2 was issued for North-eastern France mainly for tornadoes.

A level 1 was issued for most of France, the Benelux countries, western/central Germany mainly for tornadoes, large hail, excessive rain and severe wind gusts.

A level 1 was issued for the Alpine region mainly for large hail and severe wind gusts.

SYNOPSIS

A strong westerly flow affects most of Europe to the south of low geopotential from Iceland to northern Europe and to the north of a high present over Iberia and the Mediterranean. Several jet streaks travel eastward during the period ahead of a short-wave trough that will cross the Bay of Biscay and France during the period. At low levels, a quasi-stationary frontal boundary is expected from the Channel region to central Germany and southern Poland. A tongue of rich low-level moisture originating from the Bay of Biscay is forecast to spread eastwards to the south of this boundary, while steep lapse rates over Iberia will likely spread north-eastwards ahead of the approaching short-wave trough.

DISCUSSION

France

A broad low pressure zone is present along the frontal boundary over the Channel region that will expand eastwards during the next hours. With the westerly flow to the south of this front, latest models expect a rapid moisture return over northern France. Latest soundings confirm that rich low-level moisture is present over the Bay of Biscay exceeding 10 g/kg in the lowest 1500 metres. At the surface latest observations also show dewpoints around 20°C spreading into France.

On Thursday, hot and well-mixed air will spread into southern France ahead of the approaching short-wave trough. Over central France, the moist low-level air is expected the warm underneath the capping inversion and weak instability will likely develop given the steep lapse rates spreading northward. Latest model runs predict a surface low that travels eastwards in the range of the warm air nose ahead of the trough in the evening hours. It seems that QG forcing and low-level moisture pooling will result in increasing instability, and initiation becomes more likely in the range of low-level convergence to the east of the low.

Current thinking is that storms will develop over central France in the late afternoon and evening hours. The vertical wind shear will be around 20 m/s in the lowest 3km and 25 m/s in the lowest 6km, supporting rapid storm organization. Depending on the degree of backing surface winds, favourably veering profiles may support supercells. GFS already indicates 100 m²/s² low-level SRH and circular hodographs over north-eastern France. Given the large overlap of strong low-level vertical wind shear and instability, supercells are expected to be capable of producing strong tornadoes in the afternoon and evening hours. Large hail and severe wind gusts will be another threat. Storms will likely cluster in the range of the frontal boundary given the strong forcing, what reduces the tornado threat, and severe winds and excessive rain will be more likely.

Storms will spread eastwards during the night and are expected to decay over the southern portions where low-level moisture is expected to weak. Over the northern regions, convection is expected to continue along the cold front spreading southward from the Channel. The strong vertical wind shear supports a slight chance of brief tornadoes.

The Benelux countries, Germany and Alps

The main uncertainty over this region will be the moisture advection from the west on Thursday. The best moisture is indicated near the frontal wave that is expected over the Netherlands in the afternoon hours. A mixing ratio of 12 g/kg in the lowest 500 m is forecast by latest GFS. The lapse rates may improve in the range of the nose of warm air and models agree about instability spreading eastward along the warm front that will be supported by QG forcing. Storms that develop in the warm air advection regime and root to the moist boundary layer are expected to produce tornadoes as the low-level shear is strong. The degree of instability is unclear at this time, and stratiform precipitation will also be likely in place, reducing low-level temperatures.

Over southern Germany and the Alpine region, low-level moisture will be quite poor initially referring to latest GFS and ECMWF. Although some moisture increase is expected, the warm air advecting into the Alpine region will likely suppress initiation. The best potential seems to exist over the mountains, and the strong shear supports a large hail threat, but current thinking is that storms will not fire given the weak QG forcing and low-level moisture.

In the evening hours convection over France will likely spread into south-western Germany. While the low-level buoyancy decreases gradually, storms may become elevated. However, large hail and severe wind gusts are not ruled out given the strong vertical wind shear. Storms that root to the boundary-layer will profit from 15 m/s 0-1km vertical wind shear and may produce (strong) tornadoes, though. During the night, excessive rain may become the most significant threat across Germany.