Storm Forecast
Valid: Wed 21 Mar 2007 06:00 to Thu 22 Mar 2007 06:00 UTC
Issued: Tue 20 Mar 2007 21:52
Forecaster: TUSCHY

SYNOPSIS

A strong upper-level trough over south / central Europe will start a transformation towards a closed upper-level low.
Numerous small-scale impulses will rotate around this broad cyclonic vortex, resulting in unsettled weather conditions over a broad area.
Merging polar front / subtropical jet will produce an impressive at or above 160kt jet core over N-Africa, causing a slow shift of the main upper-level trough axis towards the east.

Exceptionally warm and stable conditions prevail for the northeastern parts of Europe.

DISCUSSION

...Extreme western Turkey / eastern Greece and southern Bulgaria...

Conditions become favorable for a small area to experience severe thunderstorms mainly between a 12Z-18Z time-frame !

At upper-levels, a rapidly northeastward racing vorticity maximum will cross the area of interest between aforementioned time-frame and accompanying UVV field looks pretty compact and intense.
Main support will be warranted by a coupled upper-level jet configuration with outstanding UL divergence spreading rapidly towards the north over Bulgaria and southern Romania.

At lower levels, the focus for evolving / ongoing thunderstorms will be a very slowly eastward shifting frontal boundary ( cold front ), becoming more and more diffuse during the later afternoon / evening hours.

So dynamics look favorable for intense thunderstorm development. The main uncertainty will be the degree of instability release.
Yesterday's 19Z synop datas showed dewpoints of 11-13°C over / along the Aegean Sea, which will be advected towards the north, given the strong southerly flow.
Expect nice conditions for surface based thunderstorms mainly in the level-2 area , given increasing influx of moisture and a cooling atmosphere at lower - / mid-levels.
LLS of 15-20m/s and DLS of about 35m/s will be present and although veering at lower levels is not that strong, tornadoes and severe wind gusts will be a distinct possibility...as well as an isolated large hail report.

Threat will decrease rapidly towards the north, but a risk for a few storms with mainly an marginal - isolated large hail risk will stretch up to southeastern Romania.

Only slowly eastward shifting north-south aligned frontal boundary, the intense LLJ pointing into the level areas and impressive upper-level dynamics favor the risk of possible training thunderstorm development with an attendant flash flood threat !

... Belgium, France and parts of Spain...

A borderline thunderstorm area was issued for those areas.
Yesterday's 12Z soundings already indicated that despite the steep lapse rates the boundary layer moisture was pretty shallow and rapid mixing was possible during the afternoon hours.
Same for this forecast period with a few modifications becoming necessary.
Models indicate a constant but faint warming of the mid-levels from the north with an attendant rapid decrease of the lapse rates' slope.
This would bring down TSTM chances significantly, but in contrast surface dewpoints will rise 2-3°C compared to yesterday over parts of central / northeastern France .
In addition, GFS indicates a weak wave, crossing Belgium and NE France during the peak heating from the north...UVV not that impressive, but still enough to issue a large TSTM line that far towards the north.

Further towards the south, conditions will be the same like yesterday and hence isolated thunderstorms can be expected.

In all areas, shear will be too weak for expecting any organized thunderstorm activity.

...Central Mediterranean...

Scattered to widespread thunderstorm development can be expected under the base of this huge upper-level cyclonic vortex. Shear won't support any storm organisation.
Weak shear and steep lapse rates in all levels favor an enhanced waterspout risk, stretching from the Balearic Islands to Italy to the Adriatic Sea.