CONVECTIVE FORECAST - UPDATE
VALID 12Z SUN 27/04 - 06Z MON 28/04 2003
ISSUED: 27/04 13:36Z
FORECASTER: VAN DER VELDE

THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS THE BENELUX, NRN FRANCE, WRN GERMANY, SERN GREAT BRITAIN

GENERAL THUNDERSTORMS ARE FORECAST ACROSS IRELAND, GREAT BRITAIN

GENERAL THUNDERSTORMS ARE FORECAST ACROSS GERMANY, POLAND, CZECH REPUBLIC SOUTHWARD THROUGH FORMER YUGOSLAVIA

SYNOPSIS

WAVE THAT DEVELOPED YESTERDAY OVER BENELUX AREA IS MOVING TO SRN SWEDEN. A COLD FRONT EXTENDS NORTH-SOUTH OVER ERN EUROPE. VERY LOW LCLS ASSOCIATED WITH THE CF BUT SHEAR AND INSTABILITY CONDITIONS ARE GENERALLY LESS THAN YESTERDAY. ....VARIOUS MODELS INDICATE LOW AMPLITUDE SHORT WAVES IN THE SURFACE FLOW UNDER THE JET OVER NRN FRANCE AND THE BENELUX AREA INTO GERMANY, FUNCTIONING AS THE MAIN TRIGGERS FOR CONVECTION, EVEN THOUGH INSTABILITY IS ONLY MARGINAL. UNSTABLE POLAR AIRMASS ALSO CIRCULATES OVER THE IRELAND AND GREAT BRITAIN AROUND THE SFC LOW NORTHWEST OF SCOTLAND. ....HIGHER PRESSURE IS SITUATED OVER THE MEDITERRENEAN. THERE IS ALSO A FRONTAL WAVE IN THE COLD FRONT OVER THE ADRIATIC.

DISCUSSION

...THE BENELUX, NRN FRANCE, WRN GERMANY, SERN GREAT BRITAIN...
WIDER AREA CHOSEN BECAUSE OF ONGOING ENHANCED CONVECTION AS VISIBLE ON SAT IMAGERY... SUN 00Z GFS INDICATES CAPE VALUES UP TO 100 J/KG SUGGESTING SHOWERY CONDITIONS IN THE POLAR AIRMASS. INSTABILITY IS PROGGED TO BE HIGHEST TOWARDS THE NORTH ...WHILE DEEP LAYER & LOW-LEVEL SHEAR SHOULD BE HIGHER TOWARDS THE SOUTH AND INCREASING TOWARDS 18Z. UPWARD MOTIONS BY UPPER AIR VORTICITY ADVECTION ARE FORECAST THROUGHOUT THE RISK AREA. ...UPPER AIR DISTURBANCE CURRENTLY INDICATED BY ENHANCED CIRRUS SHOULD PROVIDE LIFT AND A TENDENCY TO FORM MORE INTENSE SHOWERS, WITH A CHANCE OF ELECTRIFICATION AND HIGHER DEGREE OF ORGANISATION. VERY LOW LCLS IN COMBINATION WITH 0-1 KM SHEAR OF >20 KTS ARE ONCE AGAIN IN PLACE AND POSE A THREAT OF A FEW TORNADOES AS WAS THE CASE YESTERDAY UNDER SIMILAR LCL/SHEAR CONDITIONS WITH CONSIDERABLE LOW LEVEL BUOYANCY. 00Z SOUNDINGS IN THE AREA SHOW AGAIN VERY MOIST LAYERS UP TO 700 HPA AND VERIFY LOW LCLS AND POTENTIAL INSTABILITY ...CURRENT METEOSAT VIS IMAGES SHOW ENHANCED CUMULI OVER A WIDE AREA... SUGGESTING THAT THE LOW MODEL CAPES GIVE BAD GUIDANCE... THERE IS A RISK OF STRONG OUTFLOW UP TO 45 KTS ASSOCIATED WITH THE MORE INTENSE AND MORE ORGANISED CONVECTION, POSSIBLY IN THE FORM OF LINES/BOW ECHOES ASSOCIATED WITH THE UPPER LEVEL DISTURBANCE .... CURRENT GERMAN RADAR COMPOSITE SHOWS AT LEAST CLUSTERING OF STORMS. SMALL HAIL IS POSSIBLE WITH THE STRONGER CELLS. ........JUST RECEIVED 12Z SOUNDINGS SHOW VERY LOW LCLS INDEED, LOW LEVEL BUOYANCY MORE THAN MODEL INDICATED... AND 30 KTS OF 0-1 KM SHEAR IS PRESENT ON ESSEN SOUNDING.... NOTTINGHAM AND HERSTMONCEUX SOUNDINGS SHOW CONSIDERABLE INSTABILITY, VERY HUMID LOW TO MIDLEVELS AND VERY STRONG 0-1 KM SHEAR >25 KTS SUGGESTING TORNADO POTENTIAL, BEING ADVECTED TOWARDS THE NETHERLANDS.

...GERMANY, POLAND, CZECH REPUBLIC SOUTHWARD THROUGH FORMER YUGOSLAVIA...
CURRENT VIS SAT IMAGERY SHOWS ENHANCED CUMULI AND DETECTION SYSTEMS HAVE ALREADY DETECTED OCCASIONAL ENHANCED LIGHTNING ACTIVITY IN STORMS OVER GERMANY/POLAND, AND AUSTRIA. GIVEN BELOW OPTIMAL SHEAR CONDITIONS AND HIGHER LCLS, NO RISK IS GIVEN....HOWEVER IN FORMER YUGOSLAVIA LOW LCLS ..ARE.. IN PLACE NEAR A FRONTAL WAVE WHERE CURRENT SAT IMAGES SUGGEST CONVECTIVE CLOUDS. ALTHOUGH LOW LEVEL SHEAR IS MINIMAL, A COUPLE OF SPOUTS COULD FORM IN THIS AREA.... HELICITY COULD POSSIBLY ENHANCE BY LOCAL OROGRAPHY.

...IRELAND, GREAT BRITAIN...
CAPES OVER 100 J/KG IN A CYCLONIC FLOW AROUND THE LOW NORTHWEST OF SCOTLAND SHOULD GENERATE NUMEROUS SHOWERS WITH SOME THUNDER, POSSIBLY OROGRAPHICALLY ENHANCED. ....12Z NOTTINGHAM AND HERSTMONCEUX SOUNDINGS SHOW CONSIDERABLE INSTBILITY, VERY HUMID LOW TO MIDLEVELS AND VERY STRONG 0-1 KM SHEAR >25 KTS SUGGESTING TORNADO POTENTIAL.

#