CONVECTIVE FORECAST
VALID 06Z SAT 07/06 - 06Z SUN 08/06 2003
ISSUED: 06/06 18:53Z
FORECASTER: DAHL
GENERAL THUNDERSTORMS ARE FORECAST ACROSS CENTRAL AND SOUTHERN PARTS OF EUROPE.
SYNOPSIS
MAIN FEATURE THIS PERIOD WILL BE A SHORT-WAVE TROUGH ... WHICH IS ROTATING ABOUT EXTENSIVE UPPER LOW CENTERED OVER THE CENTRAL NORTHERN ATLANTIC. THIS TROUGH/ASSOCIATED COLD FRONT IS PROGGED TO REACH THE BRITISH ISLES ... W FRANCE AND W IBERIA TOWARDS THE END OF THE PERIOD EARLY SUNDAY MORNING. OTHERWISE ... SYNOPTICALLY QUIESCENT CONDITIONS ACROSS MOST OF CENTRAL AND SOUTHER EUROPE.
DISCUSSION
...FRANCE...IBERIA...
SOUTHERLY LOW-LEVEL FLOW WILL EVOLVE ACROSS FRANCE AND PARTS OF SPAIN DURING THE DAY. MAIN CHALLANGE APPEARS TO BE THE CAPE FORECAST ACROSS FRANCE AHEAD OF THE COLD FRONT. GFS MUCAPE FORECASTS USING A 30 HPA SURFACE-BASED MIXED LAYER PARCEL ... HAVE SHOWN TO REPRESENT THE ACTUAL CAPE VALUES RATHER POORLY OVER THE PAST FEW DAYS. TODAY'S 12 Z SOUNDINGS SHOW RELATIVELY MEAGER LOW-LEVEL MOISTURE ACROSS THE MEDITERRANEAN ... AND CURRENTLY IT IS NOT BELIEVED THAT MOISTURE WILL SIGNIFICANTLY INCREASE AS THIS AIR ADVECTS INTO FRANCE. EVAPOTRANSPIRATION OVER FRANCE THE WILL LIKELY SUPPORT A VERY SHALLOW SFC-BASED LAYER OF HIGH DEWPOINTS ... BUT ALLOVER MOISTURE CONTENT OF THE BOUNDARY LAYER SHOULD NOT BE TOO IMPRESSIVE. ALTOGETHER IT IS BELIEVED THAT LCL HEIGHTS WILL GENERALLY BE QUITE HIGH ... AND THAT MLCAPE WILL GENERALLY BE ON THE ORDER OF 500 J/KG. SLIGHTLY HIGHER VALUES MAY BE REALIZED WHERE MOISTURE CAN BE TRAPPED E.G. ALONG MESOCSALE CONVERGENCE LINES E OF THE COLD FRONT.
CURRENT THINKING IS THAT A FEW HIGH-BASED TSTMS WILL FORM ACROSS SPAIN AND FRANCE DURING THE AFTERNOON ALONG PREFRONTAL WIND SHIFT ... THOUGH SHEAR WILL LIKELY BE VERY WEAK ... A FEW STRONG DOWNDRAFTS ARE LIKELY GIVEN DEEP ADIABATIC LAYER BENEATH THE LCL ... LOCALLY CAUSING STRONG OUTFLOW WINDS WHICH MAY BRIEFLY APPROACH SEVERE LEVELS.
AS STRONG Q-VECTOR CONVERGENCE ALONG THE COLD FRONT INTERCEPTS MARGINALLY UNSTABLE AIR OVER W FRANCE LATE SATURDAY NIGHT/EARLY SUNDAY MORNING ... MORE WIDESPREAD CONVECTIVE DEVELOPMENT IS ANTICIPATED ALONG THE COLD FRONT. PROBLEM MAY BE THE BAD TIMING OF THIS FEATURE ... AND INITIATION MAY HOLD OFF UNTIL SUNDAY MID DAY. HOWEVER ... GIVEN DEGREE OF QG UPWARD FORCING ... CONFIDENCE IS RATHER HIGH THAT TSTMS WILL INITIATE. IF SO ... STORMS WILL GROW IN A FAIRLY STRONGLY SHEARED ENVIRONMENT ... POSSIBLY AROUND 50+ KNOTS IN THE LOWEST 6 KM. IN ADDITION TO THE DRY SUBCLOUD LAYERS ... THIS WOULD BE A VERY FAVORABLE SETUP FOR THE DEVELOPMENT OF A LEWP AND WIDESPREAD DAMAGING WIND GUSTS. LATER OBSERVATIONS OF THE THERMODYNAMIC AND KINEMATIC FIELDS WILL BE NECESSARY TO SUPPORT THIS SCENARIO ... BUT AN UPGRADE TO SLIGHT RISK MAY BE NECESSARY WITH TOMORROW'S UPDATE.
...BALKAN...
SOUNDINGS AVAILABLE ACROSS SE EUROPE ALSO SHOW A VERY SHALLOW SURFACE-BASED MOIST LAYER ... BUT QUALITATIVELY INVERTED-VEE PROFILES ... WITH CAPES OF A FEW HUNDRED J/KG. AGAIN ... LOCAL MOISTURE POOLING MAY OCCUR AND MLCAPES OF 1000 TO 2000 J/KG MAY BE POSSIBLE ... BUT MAJORITY OF THE STORMS WILL LIKELY NOT PROFIT FROM SUCH HIGH CAPE VALUES. OUTFLOW BOUNDARIES LAID OUT BY TODAY'S CONVECTION MAY SERVE AS FOCI FOR INITIATION ... AND COULD ALSO LOCALLY PROVIDE SUBSTANTIAL SRH.
PRIND THAT MAJORITY OF THE STORMS WILL BE SUBSEVERE BUT PRODUCE VERY GUSTY WINDS AND SMALL HAIL ... BUT ONE OR TWO LARGE-HAIL HAIL EVENTS CANNOT BE EXCLUDED IN CASE ROTATING UPDRAFTS CAN FORM.
...S GERMANY/ALPINE REGIONS...
WARM/MOIST ADVECTION MAINLY FROM THE EAST SHOULD HELP MLCAPES TO RISE TO SEVERAL HUNDRED J/KG DURING THE AFTERNOON. DEEP- AND LARGE-SCALE LOW-LEVEL SHEAR IS COHERENTLY PROGGED TO BE VERY WEAK. NONETHELESS ... OUTFLOW BOUNDARIES FROM TODAY'S AND TOMORROW'S CONVECTION ALONG WITH THE COMPLEX OROGRAPHY ... SHOULD LOCALLY PERTURB THE LOW-LEVEL FLOW SUFFICIENTLY TO YIELD STRONG SRH ... AND YET ONCE MORE ... A FEW SUPERCELLS PRODUCING SEVERE HAIL ARE EXPECTED. ALSO ... STRONG WIND GUSTS SHOULD ACCOMPANY ANY STORMS GIVEN EXPECTED HIGH LCL HEIGHTS.
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