Storm Forecast
Valid: Sun 06 Oct 2013 06:00 to Mon 07 Oct 2013 06:00 UTC
Issued: Sun 06 Oct 2013 07:39
Forecaster: VAN DER VELDE

A level 2 was issued for S Italy, Malta, N Tunesia mainly for excessive rainfall and large hail.

A level 1 was issued for parts of Italy, Corsica, Adriatic Sea mainly for tornadoes and excessive rainfall.

SYNOPSIS and DISCUSSION

Encapsulated by high pressure over the European mainland, low pressure occupies the central Mediterranean Sea area. Warm air is advected northward from Tunesia into southern Italy and the Adriatic Sea, while cold advection dominates the western Mediterranean including Corsica, Sardinia and northwestern Italy. The highest CAPE values of 1000-1500 J/kg (GFS model) reside in the level 2 area. The onshore flow of moist air lifts over orography providing focus points for persistent convective development, mainly the west side of southern Italy, the south of Sicily and northeastern Italy. The first two are additionally affected by linear convergence zones associated with the cold front. The SFC-300 hPa precipitable water - integrated saturation deficit difference amounts to over 40 kg/m² (mm) between Tunesia and Sicily suggesting high precipitation efficiency.

Deep layer shear is around 15 m/s, SREH up to 200 m²/s² sufficient for a mix of multicells/MCS and supercells in which rotating updrafts pose a threat of large hail. Deeper into Tunesia and Algeria the SREH values are even around 300. Isolated severe wind gusts are possible as well, in particular in the south of the level 2 where storm motion is faster and steeper lapse rates and drier mid level air are more effective in evaporational cooling of downdrafts.
Low level shear is not great for mesocyclonic tornadogenesis, but local vorticity zones can spin up waterspout type tornadoes, helped by slow storm motion vectors across the northern half of the level 1 area. This slow storm motion also aids local excessive rainfall.