Storm Forecast
Valid: Sat 26 Sep 2009 06:00 to Sun 27 Sep 2009 06:00 UTC
Issued: Fri 25 Sep 2009 17:34
Forecaster: KOROSEC
A level 1 was issued for coastal areas of N Marocco, NW Algeria, S Spain and Gibraltar mainly for excessive rainfall, large hail and lesser extent for tornadoes.
A level 1 was issued for parts of S Italy and Sicily, Peloponnese and Ionian sea towards Crete mainly for excessive rainfall.
SYNOPSIS
A strong zonal flow prevails over northern Europe with several disturbances embedded in this flow, affecting Sweden and Norway during the forecast period. Over southern Europe, two upper level cut off lows support convective activity. One filling out cut off low over Ionian sea and another over extreme S Mediterranean sea and southern Iberian peninsula. A large high pressure area persists over most parts of Europe with stable conditions.
DISCUSSION
... parts of S Italy with Sicily, Peloponnese and Ionian sea towards Crete ...
Slowly weakening upper low finally fills out during the forecast period, but severe storms will still be on going through much of the forecast period. Impressive moisture content throughout the troposphere, moderately high instability with values near or above 1000 J/kg of MLCAPE will support numerous storms with excessive rainfalls threat, as storms will be rather stationary/very slow moving. This rainfall threat will be enhanced even more where orographic effects onshore will take place as well. A level 1 was issued from eastern Sicily towards Crete where conditions seem favorable for excessive rainfall threat. Given the high 0-3km CAPE release and LL lapse rates, a couple of waterspouts can occur as well. Areas from SE Ionian sea across Peloponnese islands towards Crete will also be under moderate shear and better organized storms should be able form there. Therefore a few supercells with threat for large hail will exist as well. A minor tornado threat will be possible as well, as models suggest about 100-200 m^2/s^2 of SREH and 5-10m/s LL shear will be available ahead of the coming short-wave trough there.
... coastal areas of N Marocco, NW Algeria, S Spain and Gibraltar ...
With a new upper low approaching southern Iberian Peninsula severe weather potential will increase. Only small progress of this low towards NE is expected until Sunday morning. Moderate jet streak wraps around it and about 10-15m/s of DLS will be available. At low levels, high 0-1km mixing ratios (10-12g/kg) suggest that moderate CAPE will likely result with steepening mid-level lapse rates aloft. Ascent/forcing ahead of the trough should result in scattered thunderstorm initiation. Models suggest that a few organized storm clusters will form. Given the overlaping instability and shear, a couple of supercells could form as well. As storms will be rather slow moving in the highly moistured environment, excessive rainfall seems to be the main threat. A large hail can occur as well, especially more towards the African coast where instability/shear seem slightly higher.
... Western Alps towards SE France ...
A few daytime driven showers and thunderstorms are expected over the western Alps towards Massif central in SE France. Slightly cooler airmass at mid-levels with steep lapse rates will result in marginal instability. No severe threat is expected, but marginal hail can occur within stronger cells. Storms will be slow moving and will quickly diminish after sunset.
