CONVECTIVE FORECAST
VALID 06Z WED 04/06 - 06Z THU 05/06 2003
ISSUED: 03/06 14:51Z
FORECASTER: GROENEMEIJER

THERE IS A MODERATE RISK OF SEVERE THUNDERSTORMS ACROSS THE BENELUX, NORTHWESTERN GERMANY, NORTHWESTERN FRANCE

THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS AN AREA SURROUNDING THE SLGT RISK AREA INCLUDING MUCH OF DENMARK, NORTHWESTERN GERMANY, WESTERN FRANCE AND SOUTHEASTERN BRITAIN

GENERAL THUNDERSTORMS ARE FORECAST ACROSS MUCH OF WESTERN CONTINENTAL EUROPE, ITALY AND THE WESTERN BALKANS AND PARTS OF IRELAND AND NORTHERN IRELAND.

SYNOPSIS

AT 06Z, THE MAIN FEATURES ARE AN UPPER LOW WEST OF THE BRITISH ISLES AND A RIDGE OVER CENTRAL EUROPE. AN UPPER-LOW IS LOCATED OVER WESTERN RUSSIA. A MODERATELY STRONG SOUTHWESTERLY JET STREAK OVER THE BAY OF BISCAY IS EXPECTED TO PROPAGATE NORTHWARD DURING THE PERIOD.

DISCUSSION

...MODERATE RISK AREA...
A NNE-SSW ORIENTED FRONTAL ZONE IS LOCATED OVER WESTERN CONTINENTAL EUROPE. ON APPROACH OF A JET STREAK, A FRONTAL WAVE IS EXPECTED TO DEVELOP OVER SOUTHWESTERN FRANCE DURING THE MORNING THAT WILL MOVE NNEWARD. JUST EAST OF THE FRONTAL ZONE EXPECT DEW POINT TEMPERATURE TO RISE TO AROUND 20C, WHICH IN COMBINATION WITH SURFACE TEMPERATURES RISING TO AROUND 26C WILL YIELD 50HPA-MLCAPE VALUES OF AROUND 2000 J/KG. 0-6 KM DEEP-LAYER SHEAR IS EXPECTED TO INCREASE TO NEAR 50 KTS IN THE WARM SECTOR OF THE WAVE, WHICH WILL BE ENOUGH TO SUPPORT SUPERCELL CONVECTION. STORM-RELATIVE HELICITY IS EXPECTED TO BECOME QUITE STRONG WITHIN THE WARM SECTOR OF THE DEVELOPING WAVE AS LOW-LEVEL WINDS WILL BACK TO EAST TO SOUTHEASTERLY DIRECTIONS UNDER A SOUTHSOUTHWESTERLY MID/UPPER FLOW. LOW LEVEL SHEAR WILL BECOME QUITE HIGH AS SOUTHERLY 850 HPA WINDS INCREASE TO 25-30 KTS OVER 5-10 KNOT EASTERLY SURFACE WINDS. MOST LIKELY SCENARIO IS THAT DYNAMIC UPWARD VERTICAL MOTION NEAR THE TOP OF THE FRONTAL WAVE WILL LEAD TO THE FORMATION OF AN MCS OVER WESTERN FRANCE DURING WEDNESDAY MORNING. THE MCS WILL MOVE NORTHNORTHEASTWARD OVER WESTERN FRANCE INTO THE WESTERN BENELUX, SOUTHERN NORTH SEA AND POSSIBLY EXTREME SOUTHEASTERN ENGLAND DURING THE DAY. MORE ISOLATED CONVECTION IS EXPECTED TO FORM FURTHER EAST DURING THE AFTERNOON ALONG THE DEVELOPING WARM FRONT AND POSSIBLY WITHIN THE WARM SECTOR AS WELL. ANOTHER FOCUS FOR CONVECTION WILL BE NEAR THE COLD FRONT THAT WILL START TO MOVE SLOWLY EASTWARD AFTER THW WAVE HAS PASSED. GIVEN FAVOURABLE SHEAR CONDITIONS THE MCS MIGHT HAVE A FEW EMBEDDED SUPERCELLS. THERE WILL LIKELY BE SEVERE GUSTS AND LARGE HAIL, BOTH ASSOCIATED WITH THE CLUSTERED AND WITH THE ISOLATED CONVECTION. GIVEN THAT LCL HEIGHTS WILL BE LOW AND LOW-LEVEL SHEAR WILL BE FAIRLY STRONG, A FEW TORNADOES MAY OCCUR WHICH COULD BE SIGNIFICANT (F2 OR STRONGER). THE HIGHEST THREAT OF TORNADOES WILL BE WHERE SURFACE WINDS ARE MOST BACKED SO THAT LOW-LEVEL HELICITY WILL BE THE HIGHEST. ALSO A THREAT OF FLASH FLOODING EXISTS.

...WESTERN BALKANS...
SOLAR HEATING WILL DESTABILISE THE AIR-MASS OVER THE WESTERN BALKANS TO YIELD ABOUT 1500-2000 J/KG OF 50 HPA-MLCAPE. WIND SHEAR IS VERY LOW, BUT GIVEN THE AMOUNT OF INSTABILITY THERE MAY BE A FEW EVENTS OF MARGINALLY SEVERE HAIL AS WELL AS SOME STRONG -BUT MOSTLY NON-SEVERE- WIND GUSTS. A CATEGORICAL RISK SEEMS NOT NEEDED GIVEN THE LOW COVERAGE OF THE SEVERE EVENTS.

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