CONVECTIVE FORECAST - UPDATE
VALID 12Z THU 24/04 - 06Z FRI 25/04 2003
ISSUED: 24/04 10:20Z
FORECASTER: GATZEN

GENERAL THUNDERSTORMS ARE FORECAST ACROSS NORTHERN CENTRAL AND EASTERN MEDITERRANEAN, SOUTHEASTERN CENTRAL EUROPE, SOUTHWESTERN CENTRAL EUROPE, SOUTHERN GREAT BRITAIN

SYNOPSIS

SOUTHERN BRANCH OF WESTERLY JET SITUATED OVER NORTHERN AFRICA. UPPER TROUGH OVER EASTERN EUROPE EVADES SOUTHEASTWARD AS STRONG UPPER RIDGE FORMS OVER WESTERN EUROPE. WITHIN THE SOUTHWESTERLY CURRENT, SHORT-WAVE TROUGH CROSSES GREAT BRITAIN NEGATIVELY TILTED.

DISCUSSION

...SOUTHERN GREAT BRITAIN...
SHORT-WAVE TROUGH CROSSES GREAT BRITAIN DURING THE DAY. IN THE RANGE OF TROUGH AXIS, CONVECTIVELY MIXED AIRMASS SPREADS NORTHWESTWARD, EFFECTING SOUTHERN IRELAND. SOME SHOWERS HAVE FORMED UNDERNEATH THE TROUGH AXIS. DUE TO DAYTIME HEATING ... SOME THUNDERSTORMS SHOULD FORM IN THE AFTERNOON. STROMG THUNDERSTORMS ARE NOT EXPECTED DUE TO WEAK VERTICAL WIND SHEAR AND WEAK INSTABILITY.

...EASTERN CENTRAL EUROPE, BALKANS, NORTHERN CENTRAL AND EASTERN MEDITERRANEAN...
ALONG PROPAGATING TROUGH AXIS OF EASTERN EUROPEAN TROUGH INSTABILITY DUE TO DAYTIME HEATING SHOULD ALLOW CONVECTIVE ACTIVITY. MODEL OUTPUT SUGGESTS CAPE IN THE ORDER OF 700 J/KG WITHIN THE DAY OVER EASTERN POLAND/ WESTERN WHITE RUSSIA. WEAK VERTICAL SHEAR DOES NOT ARGUE FOR THUNDERSTORM ORGANISATION. SMALL HAIL IS FORECAST IN ASSOCIATION WITH THE STRONGER CELLS. HOWEVER ... INTERACTION OF SINGLE CELLS COULD LEAD TO SHORT-LIVED MESOCYCLONES AND THE ABILITY OF BIGGER HAIL. SEVERE HAIL IS EXPECTED TO BE TOO LOCALISED TO WARRANT A CATEGORICAL RISK. OVER THE MEDITERRANEAN ... DUE TO DIURNAL HEATING SHOWERS AND THUNDERSTORMS MAY FORM OVER LAND. WEAK VERTICAL SHEAR AND WEAK INSTABILITY DO NOT ARGUE FOR THUNDERSTORM ORGANISATION.

...SOUTHWESTERN AND WESTERN CENTRAL EUROPE...
IN THE RANGE OF BUILDING UPPER RIDGE ... WARM AIR ORIGINATE FROM EASTERN IBERIAN PENINSULA EFFECTS NORTHEN SPAIN AND FRANCE. SOUNDINGS INDICATE LITTLE LOW LEVEL HUMIDITY WITHIN THESE AIRMASS. HOWEVER ... MODEL OUTPUT SUGGESTS CAPE IN THE ORDER OF 300 J/KG AND SOME STORMS MAY FORM. CONVECTIVE ACTIVITY SHOULD SPREAD NORTHWARD AND DIE AFTER SUNSET.

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