Storm Forecast
Valid: Tue 11 Nov 2008 06:00 to Wed 12 Nov 2008 06:00 UTC
Issued: Tue 11 Nov 2008 02:21
Forecaster: VAN DER VELDE

SYNOPSIS

A low pressure area on the North Sea and cold upper levels suggest convective tops can reach -50°C and produce thunder. The climatology shows the highest chances for the southern part of the North Sea, which seems likely as a vorticity maximum affects the area. Later during the night a back-bent occlusion drags southward just east of the UK.

The cold front of the low affects Spain, France and Germany, and remains stationary over Poland and the Baltic states.
Renewed destabilization occurs over the northwestern Mediterranean as the upper trough approaches, and over the eastern Mediterranean. The Ionean Sea stabilizes during the day under the influence of a ridge.


DISCUSSION

...Denmark...

A mid level PV anomaly moves through the southern North Sea heading for Denmark. In advance of this feature, in the area of greatest lift, GFS forecasts instability, leading to clustered or linearly organized thunderstorms (EL at -20 to -40°C), probably as a comma cloud. The wind field in which they occur suggests gusts around 20 m/s at the west coast. Since low level and deep layer shear are enhanced to over 12 m/s and 20-25 m/s respectively, and some SREH > 150 m2/s2, there is chance of an isolated tornado as well.


...SW Germany...

A line-echo wave pattern (squall line) will enter Germany at the start of the period, associated with the cold front.
The development of this line corresponds with some signals of most-unstable (or vertically integrated) parcel instability in GFS. Large 0-2 line-normal component of 12-15 m/s is in support of bow echoes. Bowing segments pose a threat of isolated severe gusts (>25 m/s) and in the moderate-strong 0-3 km SREH and 0-1 km shear environment (250 m2/s2 and >15 m/s respectively) and low LCL heights, embedded cells in the line can obtain rotation and produce a tornado. The probability of lightning however seems low and QG vertical motion and SREH are weakening, and flow becomes more parallel to the front.


...Mediterranean Sea...

Waterspouts can occur, especially where soundings indicate good low level buoyancy and near surface lapse rates in a weak convergent wind environment. GFS suggests roughly the best conditions for Ionian Sea in the morning, southern Turkey later in the day, and the northwestern Mediterranean the next morning.