Storm Forecast
Valid: Tue 27 Jun 2006 06:00 to Wed 28 Jun 2006 06:00 UTC
Issued: Tue 27 Jun 2006 03:21
Forecaster: VAN DER VELDE

SYNOPSIS

At the eastern flank of an mid/upper level trough, winds over 15-20 m/s at levels of 700 hPa and above persist over an area that stretches from northern Spain into France, Switzerland, Germany and the Czech Republic.
At levels 850 hPa and below, geopotential height gradients are fairly weak over most parts. The entire zone between northern Spain and Czech Republic will see some low level convergence and midlevel rising motions induced by small vorticity maxima in the flow. GFS model tends to put emphasis on southern France into northern Switzerland, and Germany in particular (latter also emphasized in INM Hirlam), apparently associated with a wave in the front, as 850 hPa temperature advection shows a consistent area of warm air advection collocated with a maximum of midlevel vorticity advection. This increases also the low level wind speeds to around 7 m/s at 850 hPa and hence, the shear conditions.

00Z soundings show an abundance of instability throughout the level 1 area. The western half may see 500-1500 J/kg 50-hPa MLCAPE, whereas the eastern half is even more unstable with values over 1500 or 2000 J/kg being common today. CAPE over the Mediterranean areas of Spain, Italy and former Yugoslavia is capped.


DISCUSSION

...(general) western Europe level 1 area: ...

Moderate to strong deep layer shear and in places enhanced SREH may induce development of supercells. The main threat will be large hail and damaging winds. Weak low level wind shear seems insufficient to promote tornado chances. Although extremely large hail cannot be excluded, lack of more SREH is likely to prevent well-organised, long-lived supercells - but elevated, complex terrain may still enhance SREH in some cases.
A few MCS'ses are expected to develop over night and may locally produce a severe gust or large hail, possibly flash flooding as well.


...(general) central/eastern Europe level 1 area: ...

With 1500-2000 J/kg MLCAPE, expect large hail to be a possibility, especially in the bigger storms. Shear will be weak to moderate and allow organised clustered storms. 00Z soundings have also shown rather dry midlevels along with the steep lapse rates, and by evaporative cooling of precipitation-filled downdraft air, these circumstances allow severe downburst gusts. Over much of the area there will be only little (local orographic) forcing, except possibly along the center of the theta-e max, from Austria into the Ukraine.


...southern/eastern Germany...

With the described convective environment in place and a wave developing in this area, thinking is that convection is likely to organize into a large system during the evening, possibly a squall line with a main threat of severe widespread winds. Initially, supercells may develop as response to the moderate to strong deep layer shear, bearing a threat of large hail. Precipitation amounts predicted by GFS yield several tens of mm's.
The evolution (or not) of the wave may require an update later today.