CONVECTIVE FORECAST
VALID 06Z MON 19/05 - 06Z TUE 20/05 2003
ISSUED: 18/05 21:21Z
FORECASTER: GROENEMEIJER

THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS SOUTHERN AND EASTERN FRANCE, SWITZERLAND, SOUTHERN GERMANY, NORTHERN AUSTRIA AND THE SOUTHERN CZECH REPUBLIC

GENERAL THUNDERSTORMS ARE FORECAST ACROSS THE BRITISH ISLES, THE BENELUX COUNTRIES, SOUTHERN SCANDINAVIA, MUCH OF FRANCE, IBERIA AND CENTRAL EUROPE. SOUTHEASTERN FINLAND, WESTERN RUSSIA, THE BALTIC STATES, THE UKRAINE AND BELARUS AS WELL AS THE CENTRAL AND EASTERN BALKANS.

SYNOPSIS

DISCUSSION

...SLIGHT RISK AREA...
AT THE BEGINNING OF THE FCST PERIOD A FRONTAL ZONE IS PRESENT FROM SOUTHWESTERN FRANCE OVER CENTRAL GERMANY EASTWARD INTO SOUTHERN POLAND. INITIALLY...FRONTAL ZONE WILL BE ALMOST STATIONARY, AS A FEW WAVES WORK THERE WAY TO THE NE. HOWEVER IT WILL GRADUALLY ACCELERATE SOUTHEASTWARD MONDAY AFTERNOON. THE AIR-MASS SOUTH OF FRONT WILL LIKELY BECOME QUITE UNSTABLE AS 30 HPA MLCAPE IS FORECAST TO RISE TO 400-800 J/KG OVER SOUTHERN AND SOUTHEASTERN FRANCE. MLCAPE MAY AS WELL BE OF THIS MAGNITUDE OVER SOUTHERN GERMANY AND NORTHERN AUSTRIA. IT IS STRONGLY DEPENDENT ON THE AMOUNT OF SOLAR HEATING THAT WILL BE REALISED, WHICH IS HARD TO FORECAST AT THIS MOMENT. QUITE STRONG DEEP-LAYER SHEAR WIL BE IN PLACE UNDER 60 MID-LEVEL WINDS, AND LOW-LEVEL SHEAR WILL BE MODERATE /ON THE ORDER OF 20KTS 0-1KM SHEAR/, THOUGH S.R.-HELICITY WILL BE RATHER LOW. MOST LIKELY SCENARIO SEEMS TO BE THAT CONVECTION WILL INITIATE DURING THE EARLY- OR MID-AFTERNOON OVER SOUTHERN AND EASTERN FRANCE. GIVEN AMPLE DEEP-LAYER SHEAR SOME OF THESE STORMS WILL POSSIBLY BE SUPERCELLS. HOWEVER, THE ACTIVITY WILL ORGANISE QUITE QUICKLY INTO ONE OR MORE LINEAR CONVECTIVE SYSTEMS. DURING THE LATE AFTERNOON AND EVENING, MORE CONVECTIVE ACTIVITY WILL INITIATE ON THE NORTHERN FLANKS OF THE ALPS. GIVEN STRONG SHEAR PROFILES, SEVERE WIND GUSTS AND LARGE HAIL WILL ACCOMPANY SOME OF THESE STORMS.

...BENELUX, NORTHWESTERN GERMANY...
AS THE FRONTAL ZONE WITH ITS EMBEDDED WAVES MOVES INTO THE CONTINENT, A POLAR AIR-MASS WILL OVERSPREAD THE AREA. GFS MODEL PREDICTS SOME UPPER-LEVEL COOLING DUE TO BOTH COLD ADVECTION AND RISING MOTIONS DUE TO DCVA. DURING THE SECOND HALF OF THE AFTERNOON 30 HPA MLCAPE WILL LIKELY BE IN THE 200 - 500 J/KG RANGE. STRONG DEEP-LAYER SHEAR AND MODERATE LOW-LEVEL SHEAR IN COMBINATION WITH LINEAR SOURCE OF FORCING FOR UPWARD MOTION WILL CAUSE THE THUNDERSTORMS TO ORGANISE INTO ONE OR TWO LINEAR SYSTEMS. PRESENT INDICATIONS ARE THAT LOW-LEVEL DIRECTIONAL SHEAR WILL BE QUITE MODEST AND TORNADO THREAT RELATIVELY LOW DESPITE LOW LCL'S AND LFC'S. IF SURFACE WINDS MANAGE TO BACK SUBSTANTIALLY AHEAD OF THE SYSTEM, A TORNADO THREAT WOULD BE PRESENT. THERE IS A CHANCE OF SOME HAIL, POSSIBLY NEAR THE SEVERE LIMIT OF 2 CM. HOWEVER, GIVEN THAT WIND GUSTS ARE EXPECTED TO BE GENERALLY BELOW THE SEVERE LIMIT OF 50 KTS AND TORNADO THREAT SEEMS SMALL WE REFRAIN FROM ISSUING A CATEGORICAL RISK AT THIS TIME.

...BELARUS...
MLCAPE ON THE ORDER OF 1000 J/KG WILL BE PRESENT IN THE AREA. MODERATE SHEAR PROFILES SUGGEST MAIN CONVECTIVE MODE WILL BE MULTICELLS. THERE WILL BE SOME LOCALISED LARGE HAIL AND POSSIBLY SOME STRONG / BUT MOSTLY NON-SEVERE/ GUSTS WITH THE STORMS THAT WILL FORM STARTING EARLY AFTERNOON. IF SHEAR PROFILES WILL TURN OUT TO BE SOMEWHAT STRONGER AN UPGRADE TO SLGT RISK MAY ME NEEDED.

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