Storm Forecast
Valid: Sat 29 Sep 2012 06:00 to Sun 30 Sep 2012 06:00 UTC
Issued: Fri 28 Sep 2012 22:42
Forecaster: TUSCHY

A level 1 was issued for parts of NE Spain, S-France, NW and W Italy, Corsica and Sardinia mainly for heavy to locally excessive rainfall amounts, strong to severe wind gusts, isolated large hail and an isolated tornado event.

SYNOPSIS

A gradually NE ward moving upper low weakens over the W-Mediterranean but still supports scattered to widespread thunderstorm development. A strong trough crosses most of Norway and Sweden during the forecast with unsettled conditions accompanying that passage, including some offshore, non-severe thunderstorm activity.

DISCUSSION

... NE Spain, S France, far NW Italy , Corsica and Sardinia ...

Zonally elongated LL depression stalls just along the S-coast of France with a narrowing tongue of rich BL moisture present along the S coast of France. Scattered to widespread DMC is expected beneath the left exit region of an upper level jet and along a sharpening frontal boundary over far S-France. Storms will be slow moving and therefore capable of producing copious amounts of rain. Despite strong QPF signals of widespread heavy rain, there is not yet any clear signal of an area with enhanced back-building probabilities. However during the morning hours, NE Spain and SW France may see training storm activity due to still ongoing 15-20 m/s easterly flow just north of the depression, before weakening until noon. Later in the day, the focus then shifts further east, along the south-central coast of France just to the east of any ill defined shallow low pressure area, which may locally increase the LL inflow from the Mediterranean. An high-end level 1 should cover that risk, although isolated excessive rainfall amounts beneath training storm activity can't be excluded.

20 m/s 0-3 and 0-6 km bulk shear next to very strong LL directional shear support a few well organized multicells and supercells with a tornado, strong to severe wind gust and isolated large hail risk. In fact, the tornado risk may be quite robust over SE France and far NW Italy (both coastal areas), if storms manage to root into the BL. It remains still unclear, how far ashore the moist air mass advances and if any clearing (diabatic heating) materializes. Also, best shear seems to be a bit displaced towards the cooler/more stable side of the front. Due to those uncertainties, no level 2 was added for the tornado risk.

The level 1 was expanded towards Switzerland, where an active frontal boundary interacts with the unstable Mediterranean air mass, resulting in locally heavy rain. Some MUCAPE is present and therefore sporadic, embedded thunderstorm activity is possible.

During the overnight hours, the severe risk also increases further to the S/SE (e.g. Corsica and Sardinia), where abundant CAPE and strong shear overlap. SE-ward moving cold front should insert enough lift for CI and therefore expect severe thunderstorm development with large hail, severe wind gusts, heavy rain and an isolated tornado.