CONVECTIVE FORECAST
VALID 06Z SAT 31/05 - 06Z SUN 01/06 2003
ISSUED: 30/05 20:40Z
FORECASTER: DAHL

GENERAL THUNDERSTORMS ARE FORECAST ACROSS SOUTHERN ... CENTRAL AND SOUTHEASTERN EUROPE.

SYNOPSIS

MOST OF CENTRAL AND SOUTHERN EUROPE REMAINING IN SYNOPTICALLY QUIESCENT CONDITIONS ... WITH MEANDERING MAIN UPPER FRONTAL ZONE STRETCHING ACROSS NRN PARTS OF EUROPE. FAIRLY INTENSE LONGWAVE TROUGH ATTM OVER THE W ATLANTIC WILL MAKE LITTLE EASTWARD PROGRESS DURING THE PERIOD. WEAK VORT MAX APPARENT ON WV IMAGERY ATTM OVER THE NORTH SEA ... IS EXPECTED TO CROSS GERMANY DURING SATURDAY AND SATURDAY NIGHT. AT LOW LEVELS ... WEAKLY UNSTABLE AND WEAKLY CAPPED AIRMASS RESIDING OVER CENTRAL AND SOUTHERN EUROPE. COLD FRONT TRAILING FROM WESTERN ATLANTIC LOW PRESSURE SYSTEM ASSOCIATED WITH THE UPPER TROUGH ... IS EXPECTED TO REACH THE W BRITISH ISLES LATE IN THE PERIOD. CONVECTIVE SCENARIO WILL ONCE MORE BE DOMINATED BY WEAK SYNOPTIC AND MESOSCALE FORCING ... WEAK CAPE AND WEAK SHEAR. GFS' CAPE FORECASTS ARE VERY OPTIMISTIC WITH 30 HPA MIXED-LAYER CAPES IN EXCESS OF 2000 J/KG OVER CENTRAL EUROPE. EXPECT ABOUT 500 TO 1000 J/KG.

DISCUSSION

...CENTRAL EUROPE...
ISOLATED/SCATTERED TSTMS SHOULD INITIATE BENEATH WEAK BAND OF DCVA ASSOCIATED WITH THE WEAK CENTRAL EUROPEAN VORT MAX STRETCHING FROM N POLAND INTO CENTRAL FRANCE BY SATURDAY AFTERNOON PER GFS 12Z ... AND ALONG OROGRAPHIC FEATURES SUCH AS THE VOSGES MOUNTAINS AND THE BLACK FORREST. MESOSCALE WINDFIELD ASSOCIATED WITH THE VORT MAX MAY YIELD DEEP-LAYER SHEAR ON THE ODER OF 20 TO 30 KNOTS ESPECIALLY OVER NE GERMANY. THIS MAY HELP THE STORMS TO BRIEFLY BECOME MULTICELLULAR BUT GIVEN WEAK AMOUNT OF INSTABILITY ... STORMS SHOULD GENERALLY REMAIN POORLY ORGANIZED. LATE IN THE DAY ... STORMS ACROSS CENTRAL EUROPE WILL LIKELY MERGE INTO SEVERAL LARGER CLUSTERS. BRIEF/ISOLATED MARGINALLY SEVERE HAIL EVENTS ALONG WITH STRONG OUTFLOW WINDS MAY OCCUR ... BUT CHANCES OF ORGANIZED SEVERE ARE TOO LOW TO WARRANT A SLIGHT RISK ATTM.

...IBERIA...

TSTMS ARE ALSO EXPECTED TO DEVELOP DURING THE AFTERNOON AHEAD OF WEAK VORT MAX AFFECTING THE IBERIAN PENINSULA. THESE STORMS WILL LIKELY BE RATHER HIGH BASED AND WILL POSE SOME DOWNBURST POTENTIAL. SINCE CAPPING SHOULD BE RATHER WEAK ... NUMEROUS STORMS MAY FORM AND MAY PRODUCE A COMMON/DEEP COLD POOL ... WHICH WOULD FURTHER ENHANCE DOWNBURST POTENTIAL. DEGREE OF UPWARD FORCING AND INSTABILITY/DCAPE DO NOT SUGGEST LONG-LIVED AND WELL ORGANIZED STORMS HOWEVER AND A CATEGORICAL RISK IS NOT NECESSARY.

... GREAT BRITAIN...
DEEP LAYER SHEAR AND CAPE IS EXPECTED TO INCREASE DURING THE DAY ... WITH AFTERNOON VALUES ON THE ORDER OF 30 TO 40 KNOTS AND 500 TO 1000 J/KG RESPECTIVELY. SFC CONVERGENCE LINE ASSOCIATED WITH PLUME OF HIGH THETA-E'S AHEAD OF THE COLD FRONT IS EXPECTED TO EXTEND ACROSS GREAT BRITAIN ... AND SHOULD PROVIDE FOCUS FOR CONVECTIVE DEVELOPMENT DURING THE AFTERNOON. SELY SURFACE WINDS BENEATH SLY 850 HPA WINDS WILL GENERALLY RISE BACKGROUND SRH SOME ... BUT UPDRAFT ROTATION WILL PROBABLY STILL REQUIRE SIGNIFICICANT AUGMENTATION OF LOW-LEVEL SRH ... AND ALLOVER CHANCE OR SUPERCELLS APPEARS RATHER SMALL. HOWEVER ... LOW PROBABILITIES EXIST FOR BRIEFLY ROTATING UPDRAFTS ... EXPECT A FEW MARGINALLY SEVERE HAIL/WIND EVENTS WITH THE STRONGEST CELLS.

...ITALY/BALKAN...
VORT MAX OVER ITALY SHOULD AGAIN PROVE SUPPORTIVE TO TSTM DEVELOPMENT ... AIDED BY OROGRAPHIC FORCING. TSTMS SHOULD ALSO FORM FARTHER E OVER THE BALKAN STATES. EXPECT GENERALLY SHORT-LIVED AND RATHER WEAK UPDRAFTS WITH ONLY SMALL CHANCES OF LARGE HAIL AND STRONG DOWNDRAFTS. HOWEVER ... ESPECIALLY OVER THE BALKAN STATES ... THE BOUNDARY LAYER APPEARS TO BE SOMEWHAT DRIER ... AND DOWNBURST POTENTIAL MAY BE SOMEWHAT ENHANCED.

#