Storm Forecast
Valid: Tue 21 Oct 2008 06:00 to Wed 22 Oct 2008 06:00 UTC
Issued: Tue 21 Oct 2008 03:58
Forecaster: VAN DER VELDE

SYNOPSIS

A long cold front belonging to a large depression over the north Atlantic Ocean is the main feature on the map today. Cool unstable air with showers and isolated thunder is found over the northwestern half of Europe, and is combined with strong winds especially north of 55°N. A strong jetstream is present behind the surface cold front all the way from Spain to Finland. As warm air is advected northward via France and Germany the theta-e gradient tightens considerably. The front slows down early Wednesday over southern Germany, Czech Republic and Poland and will be mostly parallel to the steering flow there.
Over eastern Spain and southern France warm moist Mediterranean air will advect inland creating conditional instability which will be released orographically and in the zone of strong low level convergence at the cold front in the form of thunderstorms.


DISCUSSION

...southern Scandinavia...

Surface to 700 hPa average winds suggest gusts of 17-22 m/s (35-45 kts) should be possible in case of strong convective downdrafts. Over land, 0-1 km shear is enhanced due to friction to values over 12 or even 15 m/s, and deep layer shear is very strong near the jet. GFS suggests marginal CAPE will be present north of the jet axis even over land areas of Sweden and Finland. It is chosen to reflect the (marginal) threat of some tornadoes and severe gusts in a level 1, but found chances for thunder too marginal/isolated for a TSTM area.

...SE North Sea - Channel...

As the wind field will be weaker here, 0-500m lapse rates steepen and 0-3 km CAPE increases, chances of waterspouts increase in the coastal convergence zone and upper trough axis.

...France, Germany...

Deep layer shear is not really strong in the warm sector. However, increase of SREH to over 300 m2/s2 at the front after 00Z in combination with 0-1 km shear of 7-10 m/s suggests a marginal level 1 threat for tornadoes. Positioning in GFS keeps the SREH lagging the 0-1 km shear, and no real MLCAPE seems present, but some slightly elevated CAPE (as in uncapped parcel layer depth parameter) may be sufficient. Area is extended into Germany where sufficient frontal forced ascent could be present.