CONVECTIVE FORECAST
VALID 06Z FRI 20/06 - 06Z SAT 21/06 2003
ISSUED: 19/06 21:17Z
FORECASTER: GATZEN

GENERAL THUNDERSTORMS ARE FORECAST ACROSS NORTHEASTERN AND EASTERN MEDITERRANEAN

GENERAL THUNDERSTORMS ARE FORECAST ACROSS NORTHERN CENTRAL EUROPE

SYNOPSIS

RELATIVELY INTENSE SHORT-WAVE TROUGH CROSSED CENTRAL EUROPE ON THURSDAY AND IS CUTTING OFF SOUTHEAST OF ITALY. WEST OF THIS FEATURE ... NORTHERLY FLOW DOMINATES THE MEDITERRANEAN. TO THE NORTH ... STRONG JETSTREAM POINTS FROM GREAT BRITAIN TO SOUTHERN BALTIC SEA. THIS LARGE-SCALE FLOW PATTERN IS FORECAST TO SHIFT AS UPPER LONG-WAVE TROUGH BUILDS OVER EASTERN EUROPE WHILE UPPER LONG-WAVE RIDGE AMPLIFIES OVER NORTHWESTERN EUROPE. AT LOWER LEVELS ... NORTHWESTERLY WINDS ASSOIATED WITH COLD AIR ADVECTION WILL DOMINATE EUROPE EXCEPT FOR THE EXTREMELY WEST.

DISCUSSION

...MEDITERRANEAN...
THE FOCUS OF CONVECTIVE DELEOPMENT WILL BE THE CUT OFF LOW OVER EASTERN CENTRAL MEDITERRANEAN. ON THURSDAY ... THUNDERSTORMS HAVE FORMED AHEAD OF THIS FEATURE ... AND SHOULD PROBABLY SURVIVE TILL TOMORROW. GFS MODEL OUTPUT SUGGESTS INSTABILITY SOUTH OF GREECE WHERE RELATIVELY WARM AIRMASS REMAINS EAST OF A SURFACE CONVERGENCE LINE UNDERNEATH THE TROUGH AXIS AND UPPER LEVEL TEMPERATURE SINKS. HOWEVER ... 1200 UTC SOUNDINGS DO NOT INDICATE MUCH LOW LEVEL MOISTURE, AND GFS CAPE VALUES OF MORE THAN 1000 J/KG ARE QUESTIONABLE. HOWEVER ... SYNOPTIC FORCING SHOULD BE STRONG ENOUGH TO AID CONVECTIVE DEVELOPMENT, AND THUNDERSTORMS SHOULD SPREAD OVER GREECE WITHIN THE DAY. AS DEEP LAYER SHEAR IS FORECAST TO BE QUITE LOW, PERSISTENT UPDRAFTS ARE NOT EXPECTED. HOWEVER ... IF SYNOPTIC FORCING WILL SUPPORT THE FORMATION OF A LINEAR MCS ALONG THE SURFACE CONVERGENCE, THE CHANCE OF SEVERE THREAT SHOULD BE ENHANCED SOMEWHAT ... BUT SHOULD NOT WARRANT A SLIGHT RISK THOUGH. SYNOPTIC FORCING IS FORECAST TO DECREASE IN THE AFTERNOON AS CUT OFF WILL MERGE TO THE NORTHERN FLOW REGIME. WEST OF THE CUT OFF ... UPPER SHORT-WAVE RIDGE SHOULD GENERATE SYNOPTIC SCALE SINKING AND DEEP CONVECTION SHOULD BE INHIBITED. BEHIND THIS RIDGE ... CENTRAL EUROPEAN NORTHWESTERLY FLOW SPREADS INTO NORTHERN MEDITERRANEAN AND DIFFERENTIAL CYCLONIC VORTICITY ADVECTION COULD ASSIST UPWARD VERTICAL MOTION. THUNDERSTORMS COULD DEVELOP WHERE CAPE EXISTS DUE TO DAYTIME HEATING. ISOLATED THUNDERSTORMS ARE EXPECTED OVER NORTHERN ITALY AND THE BALKANS. DEEP LAYER SHEAR IN THE ORDER OF 40 KNS COULD LEAD TO SOME PERSISTENT UPDRAFTS WITH THE CHANCE OF HAIL. GFS BACKGROUND SRH IN THE ORDER OF 100 M²/S² SUGGESTS THE CHANCE OF ISOLATED SUPERCELLS, EXSPECIALLY IF LOW LEVEL SRH WILL BE ENHANCED ALONG OUTFLOW BOUNDARIES. IF SUPERCELLS COULD DEVELOP ... ISOLATED LARGE HAIL AND STRONG WIND GUSTS SHOULD BE THE MAIN SEVERE THREAT. A TORNADO IS NOT RULED OUT AS LOW LEVEL SPREAD IS NOT FORECAST TO BE RATHER HIGH. HOWEVER ... CONVECTIVE ACTIVITY SHOULD BE ISOLATED, AND A SLIGHT RISK SHOULD NOT BE NEEDED AT THIS TIME.

...NORTHERN EUROPE...
SOUTH OF RELATIVELY STRONG AND COMPLEX LOW PRESSURE SYSTEM OVER CENTRAL SCANDINAVIA ... COLD AIRMASS SPREADS OVER THE BALTIC SEA. WITHIN THIS AIRMASS ... 1200 UTC SOUNDING FROM LERWICK (03005) SHOWS ADEQUATE LOW LEVEL MOISTURE WHAT JUSTIFIES SUGGESTED CAPE IN THE ORDER OF SEVERAL HUNDREDS J/KG, AS 500 HPA TEMPERATURE IS FORECAST BELOW -25 C. AS UPPER VORT-MAX APPROACHES ... STRONG DCVA SHOULD LEAD TO SYNOPTIC FORCING, AND THUNDERSTORMS MAY FORM UNDERNEATH THE TROUGH AXIS OVER THE BALTIC SEA, EXTREMELY NORTHEASTERN GERMANY, NORTHERN POLAND AND THE BALTIC STATES. LATEST METEOSAT-IR-IMAGE INDICATES THAT EXISTING THUNDERSTORMS COULD FORM INTO A COMMA CLOUD OVER THE NORTH SEA DURING THE NIGHT, MIGRATING EASTWARD INTO NORTHERN POLAND. SEVERE WINDGUSTS ARE EXPECTED ESPECIALLY OVER SOUTHERN SCANDINAVIA AND NORTHERN GERMANY/ POLAND WITHIN THE NIGHT. WEST OF THIS FEATURE ... DEEP LAYER SHEAR REMAINS QUITE STRONG AND LOW LEVEL SRH COULD LIKELY REACH HIGH LEVELS DUE TO STRONG LOW LEVEL SHEAR ... SHOWERS AND THUNDERSTORMS MAY ORGANISE AND SHALLOW MESOCYCLONES ARE NOT RULED OUT. IF MESOCYCLONES FORM ... SOME HAIL IS EXPECTED ... AND PROBABLY SHORT LIVING TORNADOS. HOWEVER ... STRONG TO SEVERE WIND GUSTS SHOULD BE THE MAIN SEVERE THREAT. THE MOST INTENSE ACTIVITY IS EXPECTED DURING THE NIGHT HOURS. TOMORROW, A SLIGHT RISK SEEMS NOT NECESSARY AS FORECAST UVM DECREASES AND SEVERE THUNDERSTORMS SHOULD BE RATHER ISOLATED.

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