Storm Forecast
Valid: Tue 04 Nov 2008 06:00 to Wed 05 Nov 2008 06:00 UTC
Issued: Tue 04 Nov 2008 04:55
Forecaster: VAN DER VELDE

SYNOPSIS

A large upper low is situated over the Iberian Peninsula and western Mediterranean Sea and surface low moves from the Balearic Islands to southern France. Strong mid level height gradients create a strong steering flow from the south over the central Mediterranean and Italy, favorable for the development of severe storms in the very unstable airmass. GFS model forecasts a cold front at 700 hPa moving eastward from Sardegna, with associated zone of strong convergence and moisture near the surface.

The entire region of southern France, northern Italy and Slovenia should be aware of the threat of flash floods and land slides posed by forecast large precipitation sums by persisting storms and upslope moisture advection.

DISCUSSION

...Italy...

Strong mid and low level winds, backed near the surface, create an environment in which storms can easily become severe. Cagliari 00Z already shows such environment, with 900 J/kg CAPE and incredible shear. Ajaccio similar with less CAPE. Trapani 00Z throws also some low/upper level dry air in the mix for enhanced gusts.

GFS indicates deep layer shear (DLS) can reach 25 m/s in places, 0-3 km storm-relative helicity (SREH) ranges 150-300 m2/s2, supporting storm rotation, even soaring to values of 400-1000 m2/s2 over central and northeastern Italy during the late afternoon and evening. Combined with 10-15 m/s 0-1 km level shear vectors (LLS) in a large area, to possibly more than 20 m/s over central Italy, and low LCL heights, there is a significant potential for tornadoes, which may be violent. Large hail is possible as well, and during downbursts gusts of >20 m/s can occur, most likely in the level 2 areas.

...southern France...

Large SREH of 300-600 m2/s2 and significant LLS are calculated by GFS for the morning and afternoon through a convergence zone. This is supportive of supercells with large hail, gusts and an isolated tornado. Note that the current storm activity on the Mediterranean is complex, the GFS convective scheme reacts explosively (out of realistic bounds) and so the situation may have changed in the next model forecasts. Both the GFS and AFWA WRF precipitation look reliable so far and the shear and instability are supported by Nimes 00Z sounding.

Several waterspouts will likely occur throughout the western Mediterranean Sea.