UPDATE
VALID 12Z SUN 18/05 - 06Z MON 19/05 2003
ISSUED: 18/05 12:25Z
FORECASTER: HUGO/GROENEMEIJER

THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF THE BRITISH ISLES

GENERAL THUNDERSTORMS ARE FORECAST ACROSS GERMANY, POLAND, THE CZECH REPUBLIC, SLOVAKIA, EASTERN AUSTRIA, HUNGARY, ROMANIA, YUGOSLAVIA AND PARTS OF TURKEY

SYNOPSIS

LARGE LONG WAVE CLOSED MID/UPPER LEVEL TROUGH LOCATED OVER NW=ERN EUROPE IS BRINGING AN UNSTABLE, CYCLONIC FLOW OVER THE BRITISH ISLES...A SLACK 500MB PATTERN IS PRESENT OVER THE CENTRAL/SW MED IN ASSOCIATION WITH A RELATIVELY WEAK SURFACE HIGH PRESSURE...ANOTHER SURFACE HIGH PRESSURE IS PRESENT OVER THE UKRAINE & BELARUS...A WEAK COL FEATURE IS AFFECTING PARTS OF CENTRAL EUROPE.

DISCUSSION

...WALES, CENTRAL & NORTHERN ENGLAND...
THE UNSTABLE CYCLONIC W/SW=LY FLOW WILL BRING A SLGT RISK OF SEVERE THUNDERSTORM DEVELOPMENT DURING SUNDAY AFTERNOON AND EVENING...FORECAST SOUNDINGS SHOW MLCAPE VALUES ACROSS THE SLGT RISK REGION OF BETWEEN 200 AND 400J/KG WITH SBCAPE VALUES IN EXCESS OF 700J/KG...AN E=WARD MOVING JET WILL AID DEEP LAYER SHEAR WITH 0-6KM SHEAR VALUES OF 50 & 70KTS WITH SOME REASONABLE 0-1KM SHEAR IN EXCESS OF 25KT...DIRECTIONAL SHEAR IS NOT EXPECTED BUT WITH LOW LCL'S AND LFC'S THERE IS THE RISK OF AN ISOLATED TORNADO AND FUNNEL CLOUDS...AN E=WARD MOVING 500MB SHORT WAVE TROUGH IN ASSOCIATION WITH A SURFACE TROUGH WILL BRING A FOCUS POINT FOR POSSIBLE LINE CONVECTION & SQUALLS ALONG THIS FEATURE WITH A GENERAL E=WARD PROGRESSION DURING THE AFTERNOON AND INTO THE EVENING. FORECAST SURFACE GUST VALUES OF 30 TO 50MPH ARE EXPECTED WITH SOME SMALL TO MEDIUAM SIZED HAIL IN ASSOCIATION WITH WBZ LEVELS OF 5000 TO 6000FT & THE DEEP LAYER SPEED SHEAR. ONCE SOLAR INPUT REDUCES LATER THIS EVENING, ANY CONVECTION WILL SLOWLY REDUCE TO WINDWARD COASTS.

...BRITISH ISLES...
THE UK (EXLCUDING SLGT RISK REGION) IS AT A GEN RISK OF SEEING SOME THUNDERSTORM DEVELOPMENT THROUGH SUNDAY AFTERNOON IN ASSOCIATION WITH THE UNSTABLE CYCLONIC W TO SW=LY FLOW...LOCATIONS OUTSIDE THE SLGT RISK REGION HAVE LIMITED DIRECTIONAL AND SPEED SHEAR AND ONLY LOW CAPE VALES...MAIN CONVECTIVE ACTIVITY WILL LEAD TO HEAVY SHOWERS WITH THE RISK OF SOME SMALL HAIL, WITH ONLY AN ISOL TO SCT RISK OF GEN TSTSM.

...CENTRAL/EASTERN EUROPE...
A WEAK COL FEATURE IS PRESENT WITHIN CENTRAL/EASTERN REGIONS OF EUROPE...FORECAST SOUNDINGS SHOW LIMITED OR NO DIRECTIONAL OR SPEED SHEAR AND MLCAPE VALUES OF BETWEEN 200 AND 500J/KG...SOLAR, AIRMASS RELIANT ISOL TO SCT TSTSM ARE EXPECTED WITHIN THE GEN RISK REGION THIS AFTERNOON AND FOR A TIME THIS EVENING...SEVERE TSTSM ARE NOT EXPECTED BUT SOME HEAVY RAINFALL & SMALL HAIL IS POSSIBLE IN CELLS THAT ARE ENCOUNTERED JUST BEFORE THE MATURING STAGE BY MID TO LATE AFTERNOON...CONVECTIVE DEVELOPMENT WILL SLOWLY DECAY ONCE SOLAR INPUT REDUCES LATER ON TONIGHT.

#