Storm Forecast
Valid: Mon 30 Nov 2009 06:00 to Tue 01 Dec 2009 06:00 UTC
Issued: Sun 29 Nov 2009 18:37
Forecaster: TUSCHY

A level 1 was issued for NE-Italy, NW Slovenia and the east-coast of the Adriatic Sea mainly for excessive rainfall, severe wind gusts, large hail and tornadoes.

A level 1 was issued for extreme SW-France/N-Spain mainly for severe wind gusts and to a lesser extent for large hail and tornadoes.

SYNOPSIS

A significant trough enters the Mediterranean from the west. Widespread convection is expected in the range of this trough, affecting most of west/central and south Europe. WAA downstream of this trough keeps the atmsophere warm but stable over SE/E-Europe.

DISCUSSION

... Central Mediterranean ...

The forecast is pretty much straightforward with marginal discrepancies in the model data. A potent trough swings into the Mediterranean area during the forecast with intense height falls spreading eastwards. Embedded in a large-scale surface pressure drop, numerous centers evolve during the forecast, one over N-Italy, sliding northeastwards and another one over the Ionian Sea during the night hours.

Main story will be heavy orographic rain event along the S-Alps. The main activity gradually shifts eastwards during the day and weakens until midnight as surface depression moves off to the east. Models give different signals in respect of warning criteria. Overall guidance indicates a slow weakening of QPF maxima over NE-Italy and NW-Slovenia during the forecast as consolidating surface depression increases its forward speed. However, GFS forecasts an augmented risk for embedded convection, especially over NE-Italy during the afternoon hours, which could boost final rain amounts locally considerably. We decided to stick with an high-end level 1 for heavy rainfall with the chance for sporadic amounts, matching our level 2 criterion.
The level 1 was also issued for NE-Italy to reflect the severe thunderstorm risk, which is forecast. Moist onshore flow sustains some marginal CAPE well inland until sunset in a sheared environment. Large hail, severe wind gusts and isolated tornadoes accompany storms, especially more discrete ones. The risk abates until sunset from west to east.

Further south, covering the Tyrrhenian Sea, Italy and the Adriatic Sea, isolated thunderstorms are possible in the strong branch of the highly baroclinic zone, running from SSW to NNE. Models differ somewhat how unstable the atmosphere will be during the day, but all models indicate at least a chance for embedded thunderstorms, so we went with a broad thunderstorm area. Activity will be probably maximized along the west/southwest facing coastlines. Another heavy rain event, fostered by training thunderstorms, evolves over the SE-Adriatic Sea at the end of the forecast as trajectories beneath 700hPa unanimously show a straight southerly path with a rapidly increasing fetch over the warm south-central Mediterranean (Ionian Sea). A LLJ is in progress and a cluster of storms is about to evolve over the
S-Adriatic Sea well after midnight. Due to the proximity of the end of the forecast period, only a level 1 was issued, mainly for excessive rainfall, severe wind gusts and tornadoes. The level 1 also highlights the rest of the east-Adriatic coast for the same risks, although more marginal compared to the SE-part.

Finally, very cold mid-levels overspread the western Mediterranean with scattered showers and thunderstorms. Dry NW-erly winds from France and Spain keep thunderstorm probabilities low over the far western Mediterranean, but a few isolated events can't be ruled out. Maximized coverage will be between the Balearic Islands and west/southwest Sardinia. LL lapse rates increase with a low-end waterspout risk, but the main risk will be strong wind gusts and marginal hail.


... SE-Bay of Biscay, extreme SW-France and N-Spain ...

Same story here, as coldest mid-level airmass traverses the highlighted area. Strong to severe wind gusts and isolated large hail will be the main risk. In the level 1, onshore, rapidly increasing ageostrophic deflection of the BL wind causes enhanced SRH values and therefore an isolated tornado event is possible. Thunderstorms decrease after sunset as ridging builds in from the north.

... English Channel ...

A cold-core vortex over the Channel moves onshore over N-France until noon. A few thunderstorms are possible until the late afternoon hours. Some LL CAPE and LL shear overlap, so an isolated funnel/tornado event is possible next to marginal hail and strong wind gusts. Expected coverage is too low for a level 1. Convection weakens onshore as LL moisture support decays.

A very isolated thunerstorm is possible over south-central France during the peak-time heating, but very slim instability preclude any thunderstorm probabilities for now.