Storm Forecast
Valid: Fri 23 Feb 2007 06:00 to Sat 24 Feb 2007 06:00 UTC
Issued: Thu 22 Feb 2007 18:30
Forecaster: TUSCHY

SYNOPSIS

A high over low constellation at upper-levels over the central Mediterranean supports a sustained blocking over Europe with bitterly cold conditions over NE Europe and warm conditions over SW Europe.
The main focus for some convective activity will be a NE-ward moving upper-level low over the SE Mediterranean and a broad trough over NW Europe.


DISCUSSION

...Crete...

A well developed upper-level low can be seen on latest WV images moving offshore from the NE-African coast.
An area of significant instability release will evolve under this system after PBL modification over the Mediterranean will be finished assisted by very steep mid-level lapse rates and warm SSTs. A field of 500-1000 J/kg SBCAPE will cover an area between Malta and Crete.
GFS continues to indicate a decoupling of the LL depression from the upper-level low pressure area during the forecast period.
Main concern for thunderstorm activity in the area of responsibility can be found under the base of the upper-level low. Although instability will be fine, shear should be well displaced towards the east.

Parameters for a few organized storms could come together south and east of Crete and a few severe wind gusts can be expected. Storms will continue to develop towards the east and will leave the area of responsibility betimes.

... Ireland, United Kingdom and France...

Two rounds of isolated to scattered thunderstorm development will be possible over parts of UK and France:

The first round will occur during the morning hours.
A few thunderstorms will develop over parts of S-UK and NW-France as a result of a few, weak impulses, which will cross those regions from the SW.
Shear will stay weak and no significant storm organisation can be expected.
Main concern right now will be the pretty humid airmass, which is characterized by very low LCLs, a well mixed boundary layer and some marginal veering at the lowest 2 km so don't want to exclude an isolated tornado report.

Anorther and more significant round of thunderstorm activity will evolve during the evening and night hours over SW-UK and mainly over parts of France.
A southward developing upper-level jet over the Bay of Biscay will place N-CNTRL France in a favorable left-exit region and upper-level divergence will be present mainly between 23Z and 06Z.
Mid-levels will cool-down significantly but PBL will undergo no significant modification and hence still low T-Td spreads can be expected over the area of interest.
GFS indicates a wide area of low-end instability release and up to 200 J/kg SBCAPE look reasonable.
A lot will depend of the amount of boundary moisture but current thinking is that scattered storms will manage to evolve due to a nearly uncapped environment and strong lift.

LCLs will stay low and GFS indicates an area of enhanced tornado potential over W / NW and W- central France, where dewpoints will stay high.
GFS has a 25m/s 850hPa speed max which will cross the region from the west between 00 and 06 Z and will strengthen the LL shear.... additionally,some nice veering mainly at lower levels will be present.
The main negative fact will be the radiative cooling during the night hours ( although limited by cloudy conditions ) and decoupling of the boundary layer could be the result, especially if temperatures at 850hPa cool down not as strong as latest model runs indicate.

Uncertainties about the exact region and the thunderstorm coverage that far out are still high and will therefore wait for further model runs before issuing any higher probabilities.