Storm Forecast
Valid: Sat 05 Nov 2011 06:00 to Sun 06 Nov 2011 06:00 UTC
Issued: Sat 05 Nov 2011 04:45
Forecaster: VAN DER VELDE
A level 2 was issued for southern France, western Italy, Monaco, Corsica and Sardinia mainly for excessive convective rainfall and tornadoes.
A level 1 was issued for parts of southern France, Italy and the Mediterranean Sea mainly for more isolated excessive convective precipitation and tornadoes.
SYNOPSIS
The massive low pressure area which has severely affected the northern shores of the Mediterranean with extreme rain sums during the past two days remains in place and will continue unabated during this period. In fact, new energy is supplied from the west, with a second surge of potential vorticity and colder mid levels digging into the Mediterranean. A PV nose will start pushing into the Sardinia area around 12-15Z and develop a second cold front moving northeastward, providing focus for vigorous thunderstorms which will drift along the front to the northwest, causing large local rain sums upon landfall.
DISCUSSION
...southern France, western Italy area...
With respect to yesterday, the storm-carrying flow should gradually turn from southerly to east-southeasterly. It impinges then on the SE France, NW Italy area but toward the late hours of the period, may become parallel to the coast, with coastal convergence and cold front keeping the storms more at sea but will likely still affect land. Vertical wind shear remains strong. Storm-relative helicity and low-level shear initially should decrease somewhat compared to yesterday, but at the front at night will increase again. Supercells are likely, and some tornadoes/waterspouts are not ruled out.
Hodograph configurations as indicated by GFS and HiRLAM models should become more favorable yet for backbuilding storms, with inflow vector resulting in decreased storm-complex motion as opposed to accelerated. Scattered reports of two hundred mm of rain (or more), can be expected in the level 2 area.
...Sardinia, Corsica and central Italy, Sicily...
Some 250-300 mē/sē of SREH and 0-1 km shear values over 17 m/s could spawn tornados in some places as the cold front passes. The fast southeasterly transport of moisture and convergence along the front and upslope lifting will cause very intense and persistent storms dumping large amounts of rain.
1-3 km AGL mean wind speed reaches 50-60 kts in GFS and HiRLAM models, so damaging convective downdraft winds are possible. Also somewhat drier midlevels from Africa are picked up which could enhance this.
Over Sicily the helicity and shear picture looks as good or better but models have more difficulty initiating storms there.
Waterspouts can occur throughout the thunder area but may be preferred in weaker flow convergence/vorticity regions.
