Storm Forecast
Valid: Thu 20 Aug 2009 06:00 to Fri 21 Aug 2009 06:00 UTC
Issued: Wed 19 Aug 2009 20:32
Forecaster: GROENEMEIJER

A level 2 was issued for the Netherlands, eastern Belgium, northwestern and northern Germany, and western Denmark, mainly for damaging winds and large hail.

A level 1 was issued for eastern Denmark, southern Norway and southwestern Sweden, mainly for damaging winds, large hail and excessive precipitation.

A level 1 was issued for central and SW Germany, and for northern and eastern France, mainly for large hail.

SYNOPSIS

Thursday at 06:00 UTC... a very warm air-mass, characterized by a deep elevated mixed layer, flows north-northeastward across western parts of the European continent. An intense trough initially located just to the west of Ireland is expected to move rapidly northeastward and reach Scotland by 18:00 UTC. Ahead of the trough, a strong southwesterly jet is present with speeds in excess of 40 m/s at 500 hPa.

DISCUSSION

Netherlands, eastern Belgium, northwestern and northern Germany, and southwestern Denmark...

The western limit of the elevated mixed layer, is expected to move eastward during the day, moving into the Benelux countries during the afternoon. On the warm side of this temperature gradient upward vertical motion is expected which should lead to convective initiation across Belgium, the Netherlands and the south-central the North Sea during the early to mid-afternoon.

Between 1000 and 2000 J/kg of MLCAPE are expected to be in place across the level 2 area by mid-afternoon. Such values combined with 0-3 km bulk shear of 15-20 m/s should be more than sufficient for the development of supercells, capable of producing large to very large hail. Additionally, the deep and dry boundary layer and 15 m/s winds at 850 hPa suggest that damaging winds are likely.

During the late afternoon and evening storm coverage should gradually increase. The activity should then spread eastward and northward into northern Germany and later into Denmark. The speed with shich this will happen is a bit unclear and differs among models.

The adequate shear in the 0-3 km layer and the potential for strong evaporative cooling due to the deep boundary layer suggests that strong cold pools will form. This will probably allow a couple of bowing elements to form, despite the fact that the initiating boundary and the shear vector are almost parallel. Indeed, some bowing elements, accompanied by damaging winds, may move move from the Netherlands across NW Germany and the German Bight to Denmark during the evening.

In general, high LCL heights and moderate vertical wind shear will limit the tornado threat during the forecast period. When the boundary layer cools Thursday evening and LCL heights become lower, a small threat of tornados may develop most likely over northwestern and northern Germany and Denmark where 0-1 km shear should exceed 10 m/s. Overall, the tornado threat is not the biggest issue in this forecast period.

Eastern Denmark, southern Norway and southwestern Sweden...

Eastern Denmark, Sweden and southern Norway will be affected by elevated storms around and after midnight. In those areas, a small risk of large hail and severe winds will still be present despite the elevated nature of the storms. Additionally, some places may see excessive rainfall.

France, central and southwest Germany, Switzerland...

GFS and ECMWF both simulate some storms that develop further south along the frontal zone across France and central/southwestern Germany during the evening and overnight. However, both CAPE and shear should become smaller further to the southwest along the zone, so that the level 1 cover only SW Germany and NE France, where a few large hail events will be possible.