CONVECTIVE FORECAST
VALID 06Z SAT 06/12 - 06Z SUN 07/12 2003
ISSUED: 05/12 21:23Z
FORECASTER: HAKLANDER

There is a slight risk of severe thunderstorms forecast across Poland, the Baltic Sea, Lithuania and Western Belarus.

General thunderstorms are forecast across Latvia, the Czech Republic, Slovakia, and Northwestern Ukraine.

General thunderstorms are forecast across coastal parts of the Iberian Peninsula and the Balearic Sea.

General thunderstorms are forecast across Central and Eastern parts of the Mediterranean and the Black Sea.

SYNOPSIS

Upper H-over-L blocking pattern near 10W at SAT/06Z transforms into omega block with a strong upper H core across the British Isles at SUN/06Z. At its ERN periphery, a strong NWLY jetstream plunges into Poland, slowly veering to the NNE during the FCST period. An intense surface low (about 975 hPa SLP) moves SEWD across Belarus. Across the NRN coasts of Africa, a strong WLY flow persists.

DISCUSSION

...SLGT area...
AFWA MM5 model charts show the cold front of the Belarusian low as a strong meridional 1000-500hPa thickness gradient that crosses Poland between SAT/03Z and SAT/12Z, along with a deepening surface trough. The back-bent occlusion, identifiable as a second surface trough from the north, should cross Poland between SAT/12Z and SAT/18Z.

Strong surface pressure gradients and isallobaric effects near the surface troughs could yield severe non-convective windgusts of 50+ knots. Considering low CAPE and winds of 50-65 kts at 500-600 m AGL, convective downward momentum transport will possibly yield 50+ kts windgusts at several locations. With the high low-level shear and low LCL’s, one or two tornadoes cannot be ruled out either. For these reasons, a SLGT risk has been issued for the area.

...Southwestern Europe...
MUCAPE of generally several hundreds of J/kg and the presence of latent instability up to 850 hPa (Showalter Index of about zero) could result in scattered thunderstorm activity. The largest deep layer shear (50+ knots) will probably remain outside the positive CAPE area, but storms could form in a 40-50 kts deep shear environment, which might be sufficient for one or two convective wind events. However, CAPE appears too low and the situation is too conditional to warrant a SLGT risk.

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