Storm Forecast
Valid: Wed 19 Nov 2008 06:00 to Thu 20 Nov 2008 06:00 UTC
Issued: Tue 18 Nov 2008 20:48
Forecaster: TUSCHY

SYNOPSIS

Between an intense 1040hPa high pressure area over the NE Atlantic and a large-scale cyclonic vortex over extreme N-Europe, geopotential height gradients get worse and bitterly cold air from Greenland starts its move toward the SE. This period will be the prominent 'calm before the storm' for many parts of N/CNTRL and S-Europe so the upcoming 24h remain relative quiet regarding organized and deep convection. Geopotential heights remain low over the western/central Mediterranen so chances for a few thunderstorms remain. Latest soundings reveal well mixed LLs with locally enhanced lapse rates at lowest 3km and strongest convergence signals over the western Mediterranean indicate that most of the activity will be confined to this area (running from SE Spain, the Balearic Islands to Corsica). Strong wind gusts, marginal hail and an isolated waterspout could occur with stronger activity, but threat remains marginal throughout the period. There are some hints on a low-end thunderstorm risk over Latvia during the early morning hours of the 20th, but confidence is not yet high enough to go with a thunderstorm area.

DISCUSSION

... An area NE/E of Crete and coastal areas of W/SW Turkey ...

A departing and gradually filling upper trough affects the area during the forecast. Temperatures at all levels cool off constantly, resulting in steepening lapse rates at all levels keeping SSTs of 16°C (northern level area) to 21°C (extreme southern level area) in mind. A weak cold front slowly drops south/southeastwards, swamping the warm/humid air out of the area. Hence, SBCAPE values in excess of 1500 J/kg during the morning to midday hours should decrease betimes and tongue of strongest instability should exit the forecast area during the night hours. LCLs are low, atmosphere only weakly capped and 0-3km CAPE release very robust, so there will be an appreciable risk for waterspouts mainly along the W/SW coast of Turkey. 12Z soundings from Crete and W-Turkey already had nice lapse rates, which should become steeper betimes, so in combination with intense updraft strength ( large LL/mid-level CAPE release) and aforementioned, potential rapid LL vortex stretching, even an isolated strong tornado event will be possible. In addition, large hail will be also a threat in stronger storms.

The hail and tornado threat remains augmented east of Crete, where DLS increases and deep-layer updraft rotation seems likely.

Thunderstorm intensity/coverage should decrease after 21Z and exit the area of responsibility.