CONVECTIVE FORECAST
VALID 06Z FRI 13/06 - 06Z SAT 14/06 2003
ISSUED: 13/06 00:13Z
FORECASTER: GATZEN

THERE IS A MODERATE RISK OF SEVERE THUNDERSTORMS ACROSS WESTERN AND CENTRAL FRANCE

THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS IBERIAN PENINSULA, FRANCE, NORTHEASTERN ITALY, THE ALPINE REGION

GENERAL THUNDERSTORMS ARE FORECAST ACROSS SOUTHERN CENTRAL EUROPE

SYNOPSIS

WEST OF AN ATLANTIC TROUGH, DEEP SSW-ELY FLOW AFFECTS WESTERN EUROPE, WHILE WESTERLY JETSTREAM LIES OVER NORTHERN EUROPE NORTH OF BROAD MEDITERRANEAN RIDGE. SHORT-WAVE TROUGH WILL MOVE ENE-WARD REACHING NORTHEASTERN FRANCE AT THE END OF THE FORECAST PERIODE.

DISCUSSION

...WESTERN AND CENTRAL EUROPE...
WEST OF THE ATLANTIC TROUGH, WARM AIRMASS SPREADS NORTHWARD OVER SW-ERN EUROPE, BUILDING A NEW RIDGE AXIS. ABOVE THE BOUNDARY LAYER ... WELL-MIXED AIRMASS IS SITUATED OVER IBERIAN PENINSULA, PORTIONS OF FRANCE, THE ALPINE REGION AND SOUTHWESTERN GERMANY. UNDERNEATH OF THE CAP ... WARM AIRMASS REMAINS SOUTH OF A FRONTAL BOUNDARY EXTENDING FROM THE GULF OF BISKAYA OVER CENTRAL FRANCE AND CENTRAL GERMANY INTO THE CZECH REPUBLIC. ON THURSDAY ... THUNDERSTORMS HAVE FORMED ALONG AND SOUTH OF THIS FRONTAL BOUNDARY AND SHOULD PERSIST TILL EARLY FRIDAY MORNING. AFTER SUN RISE ... REMAINING STRATIFORM CLOUDS AND WEAK THUNDERSTORMS SHOULD AFFECT NORTHERN AND EASTERN FRANCE, CENTRAL GERMANY, SOUTHERN POLAND AND CZECH REPUBLIC, REDUCING INSOLATION IN THIS REGIONS. TO THE SOUTH AND TO THE WEST, TEMPERATURES SHOULD QUICKLY RISE INTO THE 30S C. LOW LEVEL MOISTURE WILL BE ENHANCED ALONG OUTFLOW BOUNDARIES OF YESTERDAY'S STORMS AS WELL AS ALONG THE FRONTAL BOUNDARY OVER CENTRAL EUROPE. GFS SUGGESTS CAPE VALUES OF MORE THAN 3000 J/KG IN THE AFTERNOON OVER WESTERN FRANCE. AS SOUTHWESTERN SHORT-WAVE APPROACHES ... WEAK SYNOPTIC SCALE LIFT IS FORECAST DUE TO WARM AIR ADVECTION OVER WESTERN AND CENTRAL FRANCE AND NORTHERN IBERIAN PENINSULA, HELPING THUNDERSTORMS TO DEVELOP ALONG SOME OUTFLOW BOUNDARIES AND ALONG THE WARM FRONT OVER NORTHERN FRANCE. EAST OF THE ATLANTIC TROUGH ... VERTICAL WIND SHEAR IS FORECAST TO BE ADEQUATE FOR SUPERCELL DEVELOPMENT ESPECIALLY OVER CENTRAL AND WESTERN FRANCE AND NW-ERN IBERIAN PENINSULA. IN ASSOCIATION WITH SUPERCELLS ... VERY LARGE HAIL /PROBABLY MORE THAN 6 CM IN DIAMETER/ SHOULD BE POSSIBLE DUE TO HIGH CAPE VALUES AND DRY MID-TROPOSPHERIC LAYERS. STRONG DOWNBURSTS UP TO 65 KN ARE EXPECTED AS THE BOUNDARY LAYER SHOULD BE WELL MIXED IN THE AFTERNOON. TORNADO THREAT SEEMS NOT TO BE PARTICULARLY HIGH SINCE LOW LEVEL SHEAR WILL BE LOW TO MODERATE AND LCL HEIGHTS ARE NOT EXPECTED TO BE PARTICULARLY LOW. HOWEVER ... A FEW TORNADOS ARE NOT RULED OUT ESPECIALLY ALONG THE OUTFLOW BOUNDARIES WHERE LOW LEVEL MOISTURE POOLING SHOUD BE ENHANCED. IN THE EVENING ... SYNOPTIC FORCING SHOULD INCREASE AS SHORT-WAVE TROUGH REACHES NORTHERN FRANCE. THUNDERSTORMS COULD LIKELY MERGE INTO ONE OR TWO MCS OVER CENTRAL FRANCE AND MOVE NE-WARD, WITH A SLIGHT CHANCE OF DAMAGING WIND GUSTS, LARGE HAIL, FLASH FLOODING AND POSSIBLY A FEW TORNADOS IN THE EVENING HOURS. OVER THE ALPINE REGION ... NE-ELY WINDS LEAD TO UPSLOPE FLOW OVER THE NORTHERN ALPS. THUNDERSTORMS ARE FORECAST TO DEVELOP AFTER SUN RISE. VERTICAL WIND SHEAR WILL BE QUITE WEAK AND ORGANISED CONVECTION IS NOT EXPECTED. HOWEVER ... SOME SUPERCELLS ARE NOT RULED OUT AT THIS TIME ... WITH THE CHANCE OF ISOLATED LARGE HAIL AND STRONG WIND GUSTS. OVER THE WESTERN IBERIAN PENINSULA ... THUNDERSTORMS SHOULD FORM DURING THE DAY WITHIN THE WARM AIR ADVECTION REGIME EAST OF THE SHORT-WAVE TROUGH. VERTICAL WIND SHEAR SHOULD SUPPORT SUPERCELL DEVELOPMENT. HIGH LCLS AND A WELL MIXED BOUNDARY LAYER SHOULD YIELD TO STRONG DOWNBURSTS IN ASSOCIATION WITH STRONGER CELLS. ISOLATED LARGE HAIL IS ALSO EXPECTED. TORNADO THREAT SEEMS TO BE QUITE LOW DUE TO EXPECTED HIGH SURFACE SPREADS.

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