CONVECTIVE FORECAST
VALID 06Z MON 16/06 - 06Z TUE 17/06 2003
ISSUED: 16/06 00:35Z
FORECASTER: VAN DER VELDE

THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS WESTERN FRANCE

THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS SOUTHEASTERN ROMANIA

GENERAL THUNDERSTORMS ARE FORECAST ACROSS ITALY, MEDITERRENEAN ISLES, SPAIN, SOUTHERN BALKAN INTO TURKEY

GENERAL THUNDERSTORMS ARE FORECAST ACROSS SOUTHERN SWEDEN, POLAND, BELARUS NORTHERN RUSSIA, FINLAND

SYNOPSIS

WEAK SURFACE PRESSURE GRADIENTS RESIDE OVER MUCH OF EUROPE WITH HIGH BOUNDARY LAYER WBPT STILL PRESENT OVER MOST OF SOUTHERN EUROPE. MEANDERING 500 HPA HEIGHTS WITH TROUGHS ACTIVATING CONVECTION OVER THE NORTHEASTERN HALF OF EUROPE, WHILE AN APPROACHING UPPER RIDGE AND COLD AIR ADVECTION INHIBIT CONVECTION OVER THE BENELUX AND CENTRAL EUROPE NORTH OF THE ALPS.

DISCUSSION

...WESTERN FRANCE...
EXPECTED CAPE IN 1000-2000 J/KG WILL AGAIN BE MORE THAN SUFFICIENT FOR SEVERE THUNDERSTORMS, WITH SHEAR NOT AS STRONG AS WE HAVE SEEN A FEW DAYS AGO. THE PRESENT COMBINATION OF CAPE AND DEEP LAYER SHEAR (20-30 KTS 0-6 KM) FAVOURS MULTICELLS WITH A CAPABILITY OF PRODUCING LARGE HAIL. S-R HELICITY VALUES ARE ALSO BELOW VALUES TYPICAL OF SUPERCELL ENVIRONMENTS, BUT WILL BE SOMEWHAT ENHANCED (>100 M2/S2), ESPECIALLY NEAR THE SFC CONVERGENCE LINE OVER SOUTHWESTERN FRANCE THAT APPEARS TO BE THE MAIN TRIGGER. AS LOW LEVEL SHEAR WILL BE LOW, ANY SIGNIFICANT TORNADOES ARE NOT EXPECTED. STRONG TO SEVERE OUTFLOW DUE TO EVAPORATIVE COOLING BY COMBINATION OF DRY AIR ALOFT AND STRONG VERTICAL MOISTURE FLUXES IN THE STORMS IS ANTICPITATED AS SECOND THREAT, ESPECIALLY IF A MCS COULD FORM - BUT SYNOPTIC SCALE RISING MOTIONS DO NOT APPEAR STRONG ENOUGH TO EXPECT AN MCS/WIDESPREAD SEVERE WINDS. FLASHFLOODING WILL LIKELY OCCUR DUE TO SLOW MOTION OF STORMS.

...SOUTHEASTERN ROMANIA...
0-6 KM SHEAR OVER 50 KTS UNDER THE JET COMBINED WITH GFS FORECAST CAPE VALUES UP TO 1000 J/KG SHOULD BE FAVORABLE FOR DEVELOPMENT OF ORGANISED CONVECTION ALONG THE SFC CONVERGENCE LINE, PROBABLY IN THE FORM OF A MCS/SQUALL LINE AND MAYBE AN EMBEDDED SUPERCELL. LARGE HAIL AND MARGINALLY SEVERE DOWNDRAFTS ARE LIKELY TO OCCUR LOCALLY.

...ITALY, MEDITERRENEAN ISLES, NE SPAIN, SOUTHERN BALKAN INTO TURKEY...
THESE AREAS ARE EXPECTED TO HAVE CAPE OVER 1000 J/KG AT LEAST ACCCORDING TO GFS. DEEP LAYER/LOW LEVEL SHEAR IS TOO MODEST TO ORGANISE LONG-LIVED CONVECTION, SO EXCEPT FOR SOME MARGINALLY SEVERE HAIL AND GUSTS, NO RISK IS ISSUED. HOWEVER, THE GFS MODEL INDICATES MIDLEVEL RISING MOTIONS AND STRONG SFC CONVERGENCE OVER NERN SPAIN WHICH COULD INDICATE WIDESPREAD CONVECTION, WHICH COULD ORGANISE INTO AN MCS AT THE APPROACH OF HIGHER SHEAR/HELICITY LATER IN THE EVENING, BUT WIDESPREAD EARLIER DEVELOPING CONVECTION PROBABLY LIMITS THIS CHANCE.SEA BREEZE CONVERGENCE ZONES, AND MOUNTAINS, SHOULD BE THE MAIN TRIGGERS FOR THUNDERSTORMS, LIKE THE PREVIOUS DAYS.

...SOUTHERN SWEDEN, POLAND, BELARUS NORTHERN RUSSIA, FINLAND...
POLAR AIR MASS THUNDERSTORMS MAY OCCUR, ESPECIALLY NEAR 500 HPA TROUGHS WHERE AIR ALOFT IS COOLER/HIGHER LAPSE RATES, WITH SOME VORTICITY ADVECTION GENERATED RISING MOTION. LIGHTNING FREQUENCY SHOULD GENERALLY BE LOW, MODERATE IF CLUSTERING OF CELLS OCCURS, MOST LIKELY TO HAPPEN CLOSER TO THE JET (SRN SWEDEN, POLAND).

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