CONVECTIVE FORECAST
VALID 06Z MON 28/04 - 06Z TUE 29/04 2003
ISSUED: 27/04 23:22Z
FORECASTER: GROENEMEIJER

THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF FRANCE

GENERAL THUNDERSTORMS ARE FORECAST ACROSS PARTS OF THE BRITISH ISLES, NORTHWESTERN AND NORTHERN IBERIA, FRANCE, THE BENELUX, PARTS OF DENMARK, MUCH OF GERMANY AND NORTHWESTERN SWITZERLAND.

GENERAL THUNDERSTORMS ARE FORECAST ACROSS PARTS OF POLAND, LITHUANIA, BELARUS, UKRAINE, SLOVAKIA, MOLDOVA, ROMANIA AND BULGARIA.

SYNOPSIS

A LONGWAVE IS APPROACHING WESTERN EUROPE FROM THE ATLANTIC. A SURFACE CYCLONE OVER THE BAY OF BISCAY WILL MOVE NORTHWARD AND BE SITUATED OVER SCOTLAND AROUND 18Z. A WEAKLY AMPLIFIED UPPER TROUGH IS PROPAGATING EASTWARD OVER EASTERN EUROPE.

DISCUSSION

...SOUTHERN, CENTRAL AND EASTERN FRANCE....
A STRONG THETA-E ADVECTION REGIME WILL FORM OVERNIGHT OVER SOUTHERN FRANCE EXPANDING NORTHWARD DURING THE DAY IN THE WAKE OF A WARM FRONT REACHING THE BENELUX COUNTRIES IN THE EVENING. SOME MARGINAL LATENT INSTABILITY WILL FORM OVER SOUTHERN AND CENTRAL FRANCE AND POSSIBLY NORTHWARD INTO NORTHERN FRANCE AND THE SOUTHERN BENELUX COUNTRIES. ON APPROACH OF UPPER TROUGH...CONVECTION WILL INITIATE OVER SOUTH-CENTRAL AND CENTRAL FRANCE IN THE EARLY AFTERNOON, AHEAD OF THE COLD FRONT. GIVEN HIGH /~50-60 KT SFC-6KM/ BULK SHEAR AND RELATIVELY HIGH S.R.-HELICITY NORTH OF THE PYRENEES LEE-LOW...SOME STORMS WILL LIKELY BECOME SUPERCELLS. THE STORMS WILL BE ACCOMPANIED BY DAMAGING GUSTS AND LARGE HAIL. THE THREAT OF TORNADOES SEEMS RELATIVELY LOW GIVEN RATHER HIGH LCL HEIGHTS. THE CONVECTION WILL LIKELY CLUSTER INTO A LINEAR MCS NEAR THE COLD FRONT MOVING NORTHEASTWARD OVER CENTRAL FRANCE DURING THE EVENING AND INTO NORTHEASTERN FRANCE OVERNIGHT. DURING THIS STAGE, DAMAGING WINDS WILL BE THE LARGEST THREAT.

...NORTHWESTERN FRANCE AND THE BENELUX COUNTRIES....
BOTH GFS AND MM5 FORCAST MID/UPPER-LEVEL JET TO SPLIT AS A STABLE BAROCLINIC WAVE FORMS OVER FRANCE. NORTHWARD BRANCH OF THE JET, WILL BE SSW-LY OVER NORTHWESTERN FRANCE. THIS WILL YIELD LARGE /~60-70 KT SFC-6KM/ BULK SHEAR OVER THE ANA-COLD FRONT. STRONG WIND GUSTS APPROACHING SEVERE LEVELS WILL BE POSSIBLE. IF SURFACE WIND BACKS STRONGLY AHEAD OF THE FRONT, A QUITE HELICAL INFLOW FORMS WHICH /IN COMBINATION WITH FORECAST LOW LCL-HEIGHTS/ WILL GIVE A CHANCE OF TORNADOES WITH THE FRONTAL CONVECTION. IF EVOLUTION TOWARDS THIS SCENARIO BECOMES EVIDENT, A RISK CATEGORY WILL BE NEEDED FOR THIS AREA AS WELL.

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