CONVECTIVE FORECAST
VALID 06Z FRI 30/05 - 06Z SAT 31/05 2003
ISSUED: 29/05 22:37Z
FORECASTER: GATZEN

THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS NORTHEASTERN FRANCE, BENELUX, NORTHWESTERN GERMANY

GENERAL THUNDERSTORMS ARE FORECAST ACROSS CENTRAL BRITISH ISLES, EASTERN FRANCE, ALPINE REGION, NORTHERN AND NORTHEASTERN MEDITERRANEAN, WESTERN IBERIAN PENINSULA

SYNOPSIS

BROAD RIDGE PLACED OVER EUROPE ... AND WEAK SHORT-WAVE TROUGHS WEST OF IBERIAN PENINSULA AND OVER EXTREMELY EASTERN EUROPE COMPLETE A STABLE OMEGA FLOW PATTERN. WARM AIRMASS AFFECTS SOUTHERN AND WESTERN EUROPE.

DISCUSSION

...WESTERN CENTRAL EUROPE...
OVER SOUTHERN AND WESTERN CENTRAL EUROPE AIRMASS WITHIN THE BOUNDARY LAYER IS QUITE WARM AND MOIST ... WITH DEWPOINTS IN THE ORDER OF 10C AT 850 HPA. SOUTHWEST OF THE SURFACE PRESSURE HIGH THIS AIRMASS SPREADS NORTHWESTWARD SLOWLY. ABOVE THE BOUNDARY LAYER ... WELL MIXED AIRMASS IS SITUATED IN THE ALPINE REGION AND WEST OF IT ... AND SHOULD SPREAD NORTHWARD. AS A CONSEQUENCE, HIGH CAPE VALUES ARE FORECAST OVER WESTERN CENTRAL EUROPE TOMORROW. DIURNAL HEATING SHOULD ALLOW FOR THUNDERSTORM DEVELOPMENT OVER NORTHEASTERN FRANCE, BENELUX, AND WESTERN GERMANY ... AND ISOLATED LARGE HAIL, STRONG WIND GUSTS /ESPECIALLY TO THE SOUTH, WHERE SPREAD SHOULD BE HIGHEST/ AND FLASH FLOODING IS EXPECTED. OVER MOST OF THE AFFECTED REGION ... VERTICAL WIND SHEAR WILL BE RATHER WEAK, BUT SHOULD INCREASE JUST SOUTH OF THE NORTH SEA DUE TO NORTHERLY SEA-BREEZE WINDS, AND A CHANCE OF MESOCYCLONES EXISTS IN THIS REGION. LARGE HAIL, DAMAGING WIND GUSTS AND HEAVY RAINFALL WITH A CHANCE OF FLASH FLOODING SHOULD BE THE MAIN SEVERE THREAT, A TORNADO IS NOT RULED OUT. SYNOPTIC-SCALE FORCING SHOULD STRENGTHEN IN THE AFTERNOON AS POOR DEVELOPED SHORT-WAVE TROUGH REACHES BENELUX, AND THUNDERSTORMS COULD POSSIBLY GO ON THROUGH THE NIGHT.

...PARTS OF BRITISH ISLES...
EAST OF EASTERN ATLANTIC TROUGH ... WARM AIR ADVECTION TAKES PLACE OVER THE BRITISH ISLES. GFS MODEL OUTPUT SUGGESTS CAPE IN THE ORDER OF SEVERAL HUNDREDS J/KG. SYNOPTICAL FORCING WILL NOT SUPPORT CONVECTION AS UPPER SHORT-WAVE RIDGE ENTERS THE BRITISH ISLES WITHIN THE AFTERNOON. OROGRAPHICALLY INDUCED BOUNDARY LAYER MOISTURE CONVERGENCE /SEA-BREEZE CONVERGENCE, HILLY TERRAIN/ SHOULD BE THE FOCUS OF THUNDERSTORM DEVELOPMENT. AS VERTICAL WIND SHEAR IS RELATIVELY WEAK, SEVERE THUNDERSTORMS ARE NOT LIKELY. ISOLATED BIG HAIL AND STRONG WIND GUSTS SHOULD BE THE MAIN THREAT.

...NORTHERN MEDITERRANEAN...
WEAK UPPER GEOPOTENTIAL SPREADS OVER NORTHERN MEDITERRANEAN, AND THUNDERSTORMS SHOULD DEVELOP DUE TO DIURNAL HEATING OVER LAND. SHEAR AND SYNOPTICAL FORCING REMAINS QUITE LOW AND ORGANISED THUNDERSTORMS ARE NOT FORECAST. ISOLATED HAIL AND WIND GUSTS ARE NOT RULED OUT.

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