CONVECTIVE FORECAST
VALID Tue 13 Sep 06:00 - Wed 14 Sep 06:00 2005 (UTC)
ISSUED: 13 Sep 00:31 (UTC)
FORECASTER: VAN DER VELDE

SYNOPSIS

High pressure is building over Europe in a zone from western France to southern Finland. The core of an old upper low remains over Italy. This is also where 12Z soundings have indicated the most unstable airmass, up to 1900 J/kg MLCAPE50. Weaker instability is present over southeastern Spain (several hundred J/kg), and Austria/Slovakia (several hundred J/kg) into the northern Balkan and Ukraine/Russia ahead of the cold front that pushes southeastward. Strong activity of frontal disturbances are seen over Scotland and Norway, where some isolated thunder may happen from elevated convection.

DISCUSSION

...southern Spain area...
Slow-moving storms can be expected to flash flood some areas, and GFS predicts strong LL buoyancy which may easily induce waterspouts/tornadoes. DL shear is moderate and may act to sustain storm clusters. However total instability and dynamics are weaker than past week, translating into lesser storm coverage, this is certainly also reflected in the weak GFS precip rates.

...Italy, Adriatic, Balkan and eastwards...
Latent instability in weak flow can induce slow-moving storms with a chance of isolated flash floods and tornadoes of vorticity spin-up type, especially over sea where virtual temperature lapse rates are very steep. NMM precip is less than GFS even, so that expected storm coverage isn't very significant.

...southern Italy, Sicily...
Stronger shear to over 20 m/s 0-6 km is indicated by GFS. Not much CAPE would be present, but steep midlevel lapse rates may induce elevated storms. In fact GFS and NMM do produce precip in this area much more than in the others. The area touches storm-relative helicity >200 m2/s2 and low-level shear values >10 m/s both supportive of supercells, but if convection is indeed elevated and rather rainy stratiform, this isn't effective and would not worry much about tornadoes and other severe weather.

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