CONVECTIVE FORECAST
VALID 06Z TUE 06/05 - 06Z WED 07/05 2003
ISSUED: 05/05 20:07Z
FORECASTER: DAHL

THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS NE EUROPE.

THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS SW MEDITERRANEAN REGIONS.

THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS CENTRAL AND SOUTHERN EUROPE.

SYNOPSIS

WESTERN EUROPEAN UPPER LONG-WAVE TROUGH IS IN THE PROCESS OF CLOSING OFF INTO AN UPPER CUT-OFF LOW ... RESIDUAL NORTHERN-STREAM SHORT-WAVE TROUGH WILL ACCELERATE EAST ACROSS NORTHERN AND NORTH-CENTRAL EUROPE INTO THE BALTIC STATES BY WEDNESDAY 06 Z. THE DEVELOPING CUT-OFF LOW WILL REMAIN QUIASI-STATIONARY OVER THE IBERIAN PENINSULA DURING THE FCST PERIOD. NORTHERN PORTIONS OF THE MAIN FRONTAL BOUNDARY ATTM STRETCHING FROM CENTRAL IBERIA NORTHEASTWARDS ACROSS WESTERN GERMANY INTO SCANDINAVIA ... WILL QUICKLY SHIFT EAST ON TUESDAY IN ADVANCE OF THE ACCELERATING UPPER TROUGH. SFC CYCLOGENESIS AHEAD OF THE CUT-OFF LOW OVER WESTERN SPAIN WILL PUSH MOIST AND UNSTABLE AIRMASS BACK INTO NORTHERN SPAIN AND FRANCE TOWARDS THE END OF THE FCST PERIOD.

DISCUSSION

...CENTRAL AND EASTERN EUROPE...
AIRMASS AHEAD OF THE COLD FRONT IS STRONGLY CAPPED BY EXTENSIVE ELEVATED MIXED LAYER WHICH HAS BEEN ADVECTED INTO THE PREFRONTAL ENVIRONMENT OVER CENTRAL EUROPE. TODAY'S SOUNDINGS SUGGEST THAT BOUNDARY LAYER IS QUITE SHALLOW AND RELATIVELY DRY ... AND MAY BE MIXED OUT WITH STRONG INSOLATION DURING THE DAY ... RESULTING IN A SFC BASED WELL-MIXED LAYER REACHING UP TO 600 TO 500 HPA. HOWEVER ... SHALLOW AND STRONGLY CAPPED LOW-LEVEL AIRMASS IS EXPECTED REMAIN IN PLACE OVER MOST PARTS OF THE WARM-SECTOR ... YIELDING VERY HIGH CINH VALUES OF SEVERAL 100 J/KG AND ONLY MINIMAL CAPE.

POTENTIAL FOCUS FOR SURFACE-BASED INITIATION WILL BE PRE-FRONTAL LOW-PRESSURE AREA ASSOCIATED WITH HIGHEST LOW/MID-LEVEL THETA-E VALUES E OF THE COLD FRONT ... AND THE COLD FRONT ITSELF WHICH SHOULD EXTEND ACROSS CENTRAL EUROPE DURING THE AFTERNOON.

PRESENT THINKING IS THAT ISOLATED HIGH-BASED TSTMS WILL FORM DURING THE AFTERNOON AHEAD OF THE COLD FRONT WHERE LOW-LEVEL INVERSION MAY LOCALLY BE MIXED OUT ... ALLOWING PARCELS TO REACH THEIR LCL'S/LFC'S NEAR 700 HPA OR HIGHER IN NEARLY NEUTRAL ENVIRONMENT. OTHER BRIEF CB'S MAY FORM WHERE STRONGER LOW-LEVEL MOISTURE CAN BE TRAPPED BENEATH THE CAP ... WHICH COULD BE BROKEN IN WEAKLY CONVERGENT LOW-LEVEL FLOW REGIME E OF THE COLD FRONT ... AIDED BY DIFFERNTIAL HEATING OVER THE COMPLEX/HILLY TERRAIN. THE SURFACE COLD FRONT MAY PROVIDE ANOTHER FOCUS FOR CONVECTIVE DEVELOPMENT. THESE STORMS SHOULD POSE THE THREAT OF VERY GUSTY WINDS ALONG WITH SMALL HAIL ... OVER NE GERMANY THE SEVERE THREAT MAY BE SOMEWHAT GREATER SINCE DEEP-LAYER SHEAR WILL BE STRONGER. ALLOVER THREAT WITH THIS ACTIVITY DOES NOT JUSTIFY A CATEGORICAL RISK THOUGH AN ISOLATED SEVERE TSTM EVENT OR TWO ARE EXPECTED. TSTMS WILL LIKELY DIMINISH WITH THE LOSS OF DAYTIME HEATING.

HOWEVER ... DCVA-RELATED UVV'S WILL OVERSPREAD THE WARM-SECTOR AIRMASS DURING THE AFTERNOON/EVENING ... WHICH SHOULD PROVIDE SUFFICIENT DEEP-LAYER LIFTING TO INITIATE TSTMS ALONG AND EAST OF THE FRONT. A FEW ELEVATED TSTMS MAY ALSO OCCUR WEST OF THE FRONT WHERE RESIDUAL MID-LEVEL MOISTURE SHOULD STILL BE PRESENT.

DEEP-LAYER SHEAR IS EXPECTED TO BE ON THE ORDER OF 60 TO 70 KNOTS ... AND LOW-LEVEL VEERING WILL LIKELY BE MAXIMIZED AHEAD OF PREFRONTAL SFC PRESSURE TROUGH ... SUGGESTING THAT POTENTIAL FOR A FEW HIGH-BASED SUPERCELLS EXISTS ... ESPECIALLY OVER NORTHERN POLAND AND THE BALTIC STATES. PRIMARY THREAT WILL BE STRONG/DAMAGING WIND GUSTS THOUGH A FEW LARGE-HAIL EVENTS MAY ALSO OCCUR ESPECIALLY IF SUPERCELLS CAN FORM. TSTMS SHOULD DIMINISH AFTER WEDNESDAY 00 Z.

...SE EUROPE...
ANOTHER ROUND OF DIURNALLY DRIVEN HIGH-BASED TSTMS IS EXPECTED OVER THE S BALKAN AND TURKEY. EXPECT ISOLATED BRIEF MARGINALLY SEVERE WIND GUSTS AND HAIL ... BUT COVERAGE IS EXPECTED TO BE TOO LOW FOR A CATEGORICAL RISK. GAP OF TSTMS SHOULD BE EVIDENT OVER CENTRAL GREECE AND THE AEGEAN SEA WHERE TONGUE OF COOL/STABLE LOW-LEVEL AIRMASS IS EXPECTED.

...SW EUROPE ...
STRONG VORT MAX/ASSOCIATED COLD FRONT WILL CROSS THE SW MEDITERRANEAN DURING THE EVENING AT THE PERIPHERY OF THE IBERIAN UPPER LOW ... THOUGH LOW-LEVEL MOISTURE WILL BE SOMEWHAT LIMITED ... THERE WILL BE VERY STEEP LAPSE RATES ASSOCIATED WITH A DEEP ELEVATED MIXED LAYER ... WHICH SHOULD YIELD AT LEAST WEAK CAPE. DEGREE OF SYNOPTIC- AND MESOSCALE UPWARD FORCING ... AS WELL AS EXCEPTIONALLY STRONG VERTICAL SHEAR PROFILES SUGGEST THAT TSTMS WILL LIKELY INITIATE AND QUICKLY BECOME SEVERE ... WITH THE PRIMARY THREATS BEING SEVERE WIND GUSTS AND HAIL. 0-3 KM SRH IN EXCESS OF 300 M2/S2 TOWARDS LATE EVENING SHOULD SUPPORT A FEW MESOCYCLONES ALONG AND AHEAD OF THE COLD FRONT... WHICH WOULD ENHANCE THE THREAT OF ISOLATED VERY LARGE HAIL. HOWEVER ... DEGREE OF INSTABILITY IS QUESTIONABLE ... AND STABLE BOUNDARY LAYER OVER THE RELATIVELY COOL SEA WILL LIKELY PRECLUDE SFC-BASED INITIATION. THIS WOULD LIMIT THE SEVERE-WIND THREAT SOMEWHAT.

ELEVATED TSTMS SHOULD ACCOMPANY ISENTROPIC LIFT ALONG THE WARM-FRONTAL BOUNDARY OVER FRANCE INTO THE BISCAY ... BUT SEVERE THREAT SHOULD BE MINIMAL GIVEN WEAK INSTABILITY AND WEAK SHEAR.

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