VALID 00Z FRI 01/11 - 06Z SAT 02/11 2002
ISSUED: 01/11 02:19Z
FORECASTER: HAVEN

THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS WESTERN ITALY, THYRRENIAN SEA

GENERAL THUNDERSTORMS ARE FORECAST ACROSS THE AREA SURROUNDING THE SLIGHT RISK AREA

GENERAL THUNDERSTORMS ARE FORECAST ACROSS SOUTHERN BRITISH ISLES, NORTHERN FRANCE, BENELUX

SYNOPSIS

UPPER LOW AND ASSOCIATED SURFACE LOW OVER WESTERN MEDITERRANEAN IS MOVING TOWARDS THE VICINITY OF TUNESIA TODAY. STRONG ZONAL JETSTREAM OVER ATLANTIC IS PROGRESSING EASTWARD WITH FIRST VORTICITY MAXIMUM REACHING THE BRITISH ISLES AND NORTHWESTERN FRANCE LATER TODAY. A COLDFRONT OCCLUSION WILL MOVE ACROSS ENGLAND IN THE AFTERNOON AND REACH THE SOUTHERN NORTHSEA AROUND MIDNIGHT.

DISCUSSION

...WESTERN ITALY, THYRRENIAN SEA...
UNSTABLE TEMPERATURE PROFILES UNDER UPPER LEVEL LOW, TOGETHER WITH LOW LEVEL FORCING NEAR SURFACE LOW AND TROUGHS, AS CAN BE SEEN ON UKMO ASXX, HAS LEAD TO SEVERAL AREAS OF THUNDERSTORMS OVER TOSCANA AND BETWEEN ALGERIA AND BALLEARICS. 00Z SOUNDING OF LIEE (SARDINIA) AND 18Z SOUNDING OF LIRE (ROMA) ARE BOTH SHOWING UNSTABLE PROFILE WITH LI OF -3 TO -5, ALTHOUGH THE ROMA SOUNDING IS CAPPED. LIEE SHOWS ALSO MODERATE SHEAR IN THE LOWEST 3 KM OF ABOUT 40 KTS. THIS SEEMS SUFFICIENT TO SUPPORT STRONG UPDRAFTS WITH LARGE HAIL POSSIBLE. THE AREA EAST OF LIEE IS THEREFORE PLACED UNDER A SLIGHT RISK. WITH LOW WIND AND LOW VERTICAL SHEAR BENEATH THE UPPER LOW, CONDITIONS ARE FAVOURABLE FOR WATERSPOUTS NEAR SURFACE LOW/TROUGHS, AS LOW LEVEL VORTICITY COULD BE STRETCHED IN UPDRAFTS.

...SOUTHERN BRITISH ISLES, NORTHERN FRANCE, BENELUX...
DCVA DUE TO APPRACHING VORTICITY MAXIMUM AS WELL AS WAA IN SMALL WARM SECTOR EAST OF OCCLUSION, WILL CONTRIBUTE TO STRONG UVM OVER BRITISH ISLES, NORTHERN FRANCE AND LATER BENELUX. INSTABILITY IS EXPECTED TO BE MARGINAL, HOWEVER WITH CAPE BELOW 150 J/KG. A FEW (EMBEDDED) THUNDERSTORMS MIGHT FORM IN THE VICINITY OF THE OCCLUSION WHERE LOW LEVEL CONVERGENCE IS ENHANCED. GIVEN THE EXPECTED STRONG, 0-6 KM SHEAR AROUND 50 KTS SOUTH OF 52N...THERE IS A POSSIBILITY THAT STORMS BECOME WELL ORGANIZED WITH WINDGUSTS OF 40-50 KTS. AN ISOLATED TORNADO CANNOT BE RULED OUT, SINCE 0-3 KM SRH WILL BE IN THE 200-300 M2S-2 RANGE, THEREBY FAVOURING UPDRAFT ROTATION. THE MARGINAL INSTABILITY WILL STRONGLY LIMIT SEVERE POTENTIAL AND WILL ALSO BE TOO LOW FOR STORMSPLITTING, SO THERE IS NOT ENOUGH CONFIDENCE FOR ISSUING A SLIGHT RISK AT THE MOMENT.

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