CONVECTIVE FORECAST
VALID 06Z SUN 13/04 - 06Z MON 14/04 2003
ISSUED: 13/04 01:07Z
FORECASTER: VAN DER VELDE

THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PORTUGAL, WESTERN SPAIN

GENERAL THUNDERSTORMS ARE FORECAST ACROSS SE ITALY, SOUTHERN BALKAN, AEGEAN SEA, WESTERN TURKEY

SYNOPSIS

TWO LOW PRESSURE AREAS WILL GENERATE UNSTABLE CONDITIONS IN EUROPE TODAY. A LOW APPROACHING THE IBERIAN PENINSULA HAS ALMOST MERIDIONAL STRONG 500 HPA FLOW AND ASSOCIATED STRONG DEEP AND LOW LEVEL SHEAR CONDITIONS. THE COLD FRONT ASSOCIATED WITH THIS LOW WILL PROGRESS INTO PORTUGAL AND SPAIN BY SUN AFTERNOON, AND WILL BE THE MAIN TRIGGER FOR CONVECTION. THE COLD FRONT SEEMS TO BE INDICATING MOSTLY COLD AIR ALOFT, AS SFC THETA-W DOES NOT DECREASE BEHIND THE FRONT. ANOTHER LOW WITH COLD POOL OVERHEAD HAS GENERATED THUNDERY CONDITIONS THE FORMER DAY AND WILL LINGER OVER THE GREECE AREA ON SUNDAY, WITH UNSTABLE CONDITIONS MOST EVERYWHERE UNDER THE UPPER LOW. CONVERGENT SFC FLOW SHOULD TRIGGER DEEP CONVECTION.

DISCUSSION

...PORTUGAL, WESTERN SPAIN...
AS THE COLD FRONT MIGRATES INTO THE IBERIAN PENINSULA BY ABOUT 12Z, STORMS ARE LIKELY TO BE TRIGGERED DURING THE AFTERNOON AS GFS MODEL INDICATED CAPE RISES OVER 250 J/KG. INITIALLY LOW LCLS AND LFC HEIGHTS ALONG WITH 0-1 KM SHEAR OVER 20 KTS AND A DEEP LAYER SHEAR OF 30-60 KTS WILL SUPPORT A FEW SUPERCELLS CAPABLE OF SOME WEAK TORNADOES. HELICITY VALUES OVER 200 INDICATED BY THE 12Z GFS MODEL REMAIN MOSTLY AHEAD OF THE COLD FRONT... ANY CELLS FORMING AHEAD OF THE FRONT COULD PROFIT FROM IT TO FORM ROTATING UPDRAFTS, BUT CHANCES OF CONVECTION AHEAD OF THE FRONT SEEM TO BE MUCH LOWER THAN AT THE FRONT AND AIR BEHIND BECAUSE OF DRYNESS OF PREFRONTAL SFC AIR. THE MAIN CONVECTIVE MODE GIVEN THE LINEAR TRIGGERING COULD BE A SQUALL LINE WITH BOW ECHOES... AND GUSTS OF AT LEAST 50 KTS ARE ANTICIPATED OVER PORTUGAL AT AND BEHIND THE FRONT... THIS IS EXPECTED TO BE THE MAIN THREAT AND BY ITSELF WOULD JUSTIFY A SLIGHT RISK ALREADY. THE COLD FRONT SHOULD REACH MID SPAIN BY SUN EVENING, AND INSTABILITY WILL DIMINISH AS SFC HEATING CEASES AND SFC CONVERGENCE IS NOT EXPECTED TO BE STRONG DURING MON NIGHT.

...AEGEAN SEA, WESTERN TURKEY, SOUTHERN BALKAN...
GENERAL THUNDERSTORMS SLOULD LINGER IN THE SE ADRIATIC AROUND 06Z, NEAR ALBANY AND GREECE, AS INDICATED BY GFS MODEL CAPE AND CURRENT IR SAT AND SFLOCS ACTIVITY UPSTREAM. THE MAIN ACTIVITY WILL THEN AFFECT GREECE AND ADVECT OVER THE AEGEAN SEA WITH CAPE INCREASING TOWARDS 600 J/KG AROUND 18Z (GFS MODEL) AND LATER NEAR SW TURKEY. DEEP LAYER SHEAR CONDITIONS COULD BE JUST ENOUGH TO SUPPORT ANY LOW TOPPED SUPERCELLS AT THE SOUTHERN PARTS OF THE AREA, BUT LIMITED LOW LEVEL SHEAR AND RELATIVELY HIGH LFC OVER 1500M ARE NOT ENOUGH TO MAKE THIS WORTHY OF ISSUING A SLIGHT RISK AT THIS MOMENT. AFTERNOON TSTMS ARE POSSIBLE OVER THE INLANDS OF BULAGRIA AND TURKEY AS INSOLATION AND SFC CONVERGENCE AND SYNOPTIC ASCENDING MOTION COOPERATE UNDER THE COLD UPPER LOW.

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