CONVECTIVE FORECAST
VALID 08Z FRI 21/05 - 06Z SAT 22/05 2004
ISSUED: 21/05 08:19Z
FORECASTER: DAHL

There is a slight risk of severe thunderstorms forecast across south-central Europe.

General thunderstorms are forecast across SW Europe ... south-central Europe ... NW Russia.

SYNOPSIS

Main feature today will be upper short-wave trough rounding weak/extensive upper trough over N Europe. Short-wave trough is now stretching across the North Sea ... S UK into NW France ... and will lift NE into the Baltic States late in the period ... Disturbances forming upstream will maintain/amplify N European upper trough. At low levels ... main baroclinic zone is extending from the Biscay across central France and S Germany into the N Black Sea. Weak cyclogenesis is underway along this boundary over S Germany ahead of the short-wave trough. Large high-pressure area will persist over the NW parts of the forecast area.

DISCUSSION

...France...
Air mass over central and S France should become weakly unstable during the next few hours ... soundings indicate ratively deep convective boundary layers with mean SFC dewpoints of about 13°C. Given weak capping and increasing UVV's ahead of the trough ... widespread TSTMS should initiate during the next few hours.

Wind speeds of 20 m/s are expected at the S side of the trough over N France ... decreasing to the south. Low level wind field will likely vary substantially over the complex terrain ... low-level shear modifications will likely also be realized by residual cold pools/outflow boundaries laid out by yesterday's convection.

Best kinematic and thermodynamic setup will likely materialize over E-central France. Potential problem may be rapid deterioration of the thermodynamic fields by the expected large coverage of TSTMS. Nonetheless ... chances for a few severe TSTM events exist.

Deep/dry boundary layers ... plus 20 m/s deep-layer shear will likely promote marginally severe straight-line wind gusts. Also ... aided by low-level shear modifications ... a few rotating updrafts may occur ... posing threat of large hail ... and maybe a brief tornado.

...south-central Europe...
Farther downstream ... breaks in the mid/upper clouds over Switzerland ... S Germany ... and Austria along with DCVA-related lift will likely promote early TSTM development. Deep-layer shear is expected to increase to 20+ m/s. Also ... Backed flow E of the developing SFC low ... and orographically-induced low-level flow modifications [e.g. by the Alps over NE Austria and S Slovakia as proposed by latest Meso-ETA run] will create locally favorable environments for supercells. Main threat will be large hail ... though an isolated tornado cannot be discounted. However ... majority of the storms will be non-supercellular ... but a few marginally severe wind gusts and small hail should be possible with the stronger cells. Altogether ... a SLGT is marginally warranted.

...N Balkans...
Rather high-based TSTMS should form over the N Balkans during the next few hours in weakly capped and weakly unstable environment ... weak large-scale shear suggests that organized severe TSTM threat will be low ... but a few marginally severe hail/wind events may occur given deep/dry sub-cloud layers and orographically enhanced low-level shear profiles.

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