Storm Forecast
Valid: Sun 30 Nov 2014 06:00 to Mon 01 Dec 2014 06:00 UTC
Issued: Sun 30 Nov 2014 02:27
Forecaster: VAN DER VELDE

A level 2 was issued for Sardinia, Corsica, western Italy and northern Adriatic mainly for tornadoes, large hail, and severe winds gusts.

A level 1 was issued for the Balearic islands mainly for spout-type tornadoes.

A level 1 was issued for S France/NE Spain mainly for excessive convective precipitation.

SYNOPSIS

A low pressure area shifts from southern Spain to the western Mediterranean, where it will deepen to below 992 hPa by early Monday. Tight pressure gradients develop during the morning with the low situated over the Balearic islands. Between 09Z-12Z the low pulls in dry air with steep lapse rates from the Sahara which will meet Mediterranean moist air with theta-e near 15°C and rising motions due to a PV intrusion and create 300-800 J/kg MLCAPE, mainly along and behind the mid level cold front. The airmass is humid along the occluded western and northern sides of the low, while it is relatively dry between Algeria and Italy with cloud bases of 1000-1500m.
North and east of the front, where CAPE is present, vertical wind shear and storm-relative helicity will be strong, creating a setup conducive to thunderstorms producing severe weather.

DISCUSSION

...Sardinia, Corsica, western Italy, Croatia...

A negatively tilted shortwave trough ejecting from Algeria is associated with a deep PV intrusion in the GFS model. This feature will rapidly move into Sardinia and Corsica during the afternoon and Italy during the evening and night. Strong lift, CAPE and high shear parameter values will coincide over a wide area. With 0-6 km shear upwards of 30 m/s and 0-3 km SREH of 300-450 m²/s² supercells are highly likely, apart from linear clusters. Together with steep mid level lapse rates and moderate LCL heights, large to very large hail is possible. Given 0-1 km shear over 15 m/s with veering in low levels and supercells, chances for tornadogenesis are significant. Expect a couple of tornadoes within the level 2 area, possibly strong ones (EF2 or higher). Severe straight-line wind gusts are likely as well, given the low/mid level winds of 25 m/s in the Sardinia-central Italy corridor near the apex of the PV lobe.
The threats shift to Sicily and the northern Adriatic Sea in the Monday 00-06Z time frame. The southeastern Adriatic appears to remain capped.

...Balearic islands...

Slower storm motion and concentrated low-level vorticity combined with strong low-level lapse rates and buoyancy are favorable for development of waterspout type tornadoes.

...eastern Pyrenees...

The continuous lifting of a strong easterly flow of unstable warm, moist airmass by the terrain should lead to excessive convective rainfall sums in a limited region. The threat is ended by the tramontane coming in after 00Z according to GFS-18Z.

...south side of the Alps...

Significant rain sums are predicted at the wind-facing slopes of the Alps, partially with a convective precipitation component.