Storm Forecast
Valid: Sat 12 Jan 2008 09:00 to Sun 13 Jan 2008 06:00 UTC
Issued: Sat 12 Jan 2008 09:09
Forecaster: DAHL

SYNOPSIS

Main feature this period is an upper cut-off cyclone which is developing over the NW Mediterranean early in the period ... slowly moving towards central Italy by early Sunday morning. At the surface, this activity is accompanied by cyclogenesis over the Gulf of Genoa, though the low is expected to start filling late on Saturday already. The SFC cold front associated with this system -- now just west of Corsica and Sardinia -- will cross the Tyrrhenian Sea and most of Italy during the period. Otherwise ... meandering zonal upper frontal zone is stretching across the Atlantic and the northern portions of Europe.

DISCUSSION

... Gulf of Lion/Genoa ... Tyrrhenian Sea ... Italian west coast ...

Model guidance suggests that regions of 20 m/s DLS will remain spatially separated from regions of deep instability through the period. Appreciable, frictionally-enhanced LLS will be present over N Italy, so that storms reaching the Italian W coast may benefit from this shear. CAPE does not look utterly impressive, though a few hundred J/kg have been present in the pre-frontal air mass on Saturday 00Z. GFS maintains about 6 K/km lapse rates over this region, which should suffice for MLCAPEs on the order of 500 J/kg. more interestingly, the boundary layer is practically saturated, with little capping, and appreciable buoyancy close to the ground. All this suggests that some potential for tornadoes exists, especially along the Italian west coast where favorable LL kinematic and thermodynamic environments will coincide.

In the wake of the cold front, convection is likely to persist, and rejuvenate, as it overspreads the warm sea waters. LL lapse rates are simulated to increase to about 15 K/km from the Gulf of Lion to the N Tyrrhenian Sea , which is not impressive but suggests that some buoyancy-driven low-level vortex stretching may occur ... once a misoscale vortex is encountered/developing underneath the updraft. However, background flow is rather strong in response to the N Mediterranean cyclogenesis, and climatology suggets that waterspouts become unlikely as the large-scale wind field strengthens. Given this, along with the somewhat marginal low-level lapse rates, a categorical risk for waterspouts does not seem to be warranted at the moment.