Storm Forecast
Valid: Sun 25 Jun 2006 06:00 to Mon 26 Jun 2006 06:00 UTC
Issued: Sat 24 Jun 2006 23:45
Forecaster: GROENEMEIJER

SYNOPSIS

Sunday at 06:00 UTC... a ridge stretches from Tunisia to Poland and a longwave trough is located over the eastern Atlantic. A moderate southwesterly flow is present over western continental Europe in between. A shortwave trough extends from the Bay of Biscay over northeastern Spain.

DISCUSSION

northwestern and north-central France, Benelux...

A deepening surface low is expected ahead of the aforementioned shortwave centered over centrl France. An area of elevated storms and stratiform precipitation should be ongoing over Wrn France. As the surface low deepens, the low-level wind field is expected to intensify, leading to strong low-level shear around its centre.

Heating of the boundary layer should allow for surface-based convective development on the Nrn and Ern flanks of the low as 500-1000 J/kg MLCAPE50 should be able to develop.
High storm-relative helicity values are expected to develop on the low's Nrn flank, partially within the zone that should allow for surface-based storms. Especially in this area, supercells should be able to develop quite easily and an enhanced threat of large hail and -given the strong low-level shear- tornadoes should be present. A negative for supercell development, however, is the rather limited deep-layer shear with 500 hPa winds on the order of only 15-20 m/s, that will decrease further into the evening. The fact that the shear will be present mostly in a shallow layer and the strength of the forcing suggest that many storms will be organized linearly into one or more small bow echoes. These should be able to bring winds in excess of the severe limit of 25 m/s in places. Additionally, a few tornadoes may occur with the bowing system as well.

Severe weather will likely develop with the first surface-based convection over north-central France in the early afternoon and shift into Belgium and the Rheinland and later the SErn Netherlands as it get increasingly linearly organized.

Large areas of stratiform and elevated convective precipitation are expected on the low's Wrn flank. It is here that flash flooding will probably be a primary threat. Mind that the issued threat levels do not incorporate flash flood threat.

eastern France, Switzerland, extreme northwestern Italy...

Ahead of the cold front of the low pressure system, MLCAPE of 500-1000 J/kg will likely develop in an environment of 15-20 m/s deep-layer (0-6 km bulk) shear and somewhat higher towards the SE. Scattered storms are expected to initiate primarily during the afternoon. These are likely to organize into multicells and later a few linear, bowing systems. Isolated large hail is expected with the storms and some severe wind gust threat is likely to develop with maturing linear systems.