Storm Forecast
Valid: Sat 15 Nov 2014 06:00 to Sun 16 Nov 2014 06:00 UTC
Issued: Fri 14 Nov 2014 16:38
Forecaster: PISTOTNIK

A level 1 and level 2 were issued for NW Italy, the Ligurian Sea and Corsica mainly for excessive precipitation and to a lesser degree for tornadoes and severe wind gusts.

A level 1 and level 2 were issued for N Italy and the S Alpine region mainly for excessive precipitation.

SYNOPSIS

A pronounced long-wave trough covers Western Europe. It is surrounded by a mid-level jet streak, which runs across Iberia and the western Mediterranean before it sharply turns northward over Germany and the North Sea. Two strong short-wave troughs are embedded in the flow and move from eastern Spain into Austria and from the Atlantic into northern Portugal and Spain, respectively, during this forecast period.
Anticyclonic influence dominates further east. The 500 hPa ridge axis stretches from the Balkans into central Scandinavia, whereas the surface low is centered over Finland and northwestern Russia, where very cold air is present.

DISCUSSION

... west-central Mediterranean into Italy, Adriatic and S Alpine region ...

In response to the strong forcing of the short-wave trough aloft, a surface cyclone travels eastward from the Lion Gulf into northern Italy. Ahead of it, southerly low-level winds around 15 m/s establish a return of warmer and moister air over the Tyrrhenian, Ligurian and Adriatic Sea.
Friday's 12 UTC soundings from these regions uniformly showed very stable layers or even inversions near 700 hPa on top of neutrally stratified, not too moist low levels (precipitable water around 20 mm, surface dewpoints between 11 and 15 °C). This indicates that further moistening and/or considerable lift will be needed to allow convection to grow deep enough to become thundery, and that a considerable part of the beginning precipitation event may be of stratiform nature.
However, surface dewpoints from Sardegna, the Baleares and the French Mediterranean coast were already as high as 16 bis 19°C on Friday afternoon (often surpassing the model forecasts), and the trough's appearance in the forecast model leaves little doubt about the presence of strong large-scale lift Friday night and Saturday morning. A few hundred J/kg of CAPE should develop over the sea surfaces, partly overlapped by 0-3 km shear up to 20 m/s especially in the first half of the forecast period.

Chances for thunderstorms are highest along the Ligurian coast, where both large-scale and orographic lift come together best in the 06 to 15 UTC time frame. Backbuilding storms, embedded in the large-scale cloud shield, can easily produce more than 100 mm of rain within a few hours and are able to create another situation of life-threatening flash floods and mudslides. In addition, one or two tornadoes may spin up along offshore convergence zones.
A particularly tricky situation evolves in the wake of the Appennin mountains over northern Italy due to interactions between the strong upper-level flow and the complex topography. Despite somewhat diverging signals, limited area models mostly agree on a compact lee cyclone in the Piedmonte area. Such a development could create sustained inflow of moist Adriatic air from the southeast into the Po Valley, limiting Foehn subsidence in the wake of the Appennin and keeping chances for heavy rain and thunderstorms high also further north, where the models still agree on a little (elevated) CAPE. Certainly another focus for very heavy precipitation will evolve along the southern Alpine rim over northern Italy and the Swiss Ticino.
A southeasterly low-level jet over the Po Valley, if it forms, would also be associated with very high 0-1 km shear (>15 m/s) and storm-relative helicity (>300 m^2/s^2). The possible decoupling of the cool near-surface air and the embedded nature of storms make a better organization of storms unlikely, but the level 2 area was extended over much of northern Italy mainly for the excessive rain risk.
In the late evening hours, rain and embedded thunderstorm debris will continue to move into southern Austria, Slovenia and Croatia while weakening.

Further south, the cold front of the cyclone will make a comparably swift eastward progress across Corsica and Sardegna into Italy and the Tyrrhenian Sea. The risk of flooding is particularly high in the Tuscany region, which is forecast to experience sustained onshore and upslope flow until the evening. Tail-end storms over the open sea might also turn into supercells with a secondary risk of tornadoes and severe wind gusts. In general, any of the mentioned risks - and the overall chances for thunderstorms - decrease from north to south, though isolated storms can spread as far as southern Italy and the west coast of the Balkans overnight.

... Ireland, England, Bay of Biscay, France, N Spain, N Portugal ...

Another day of widespread showers and weakly electrified thunderstorms is in store in the deeply mixed polar air. Convection will go on throughout the forecast period over the sea, while it will be mostly confined to daytime hours in inland areas. Due to low CAPE and mostly weak vertical wind shear, the severe weather risk is low.
Vertical wind shear gradually increases towards the south into Spain and Portugal, but the depth of the convection will be limited. Marginally severe wind gusts or heavy rain are possible in northwestern and northern Spain especially in the late afternoon and evening, when the second trough will foster shower development. However, involvement of thunderstorms is questionable and hence no level 1 was issued.

... S Turkey ...

Some leftover showers and thunderstorms will go on beneath and old cut-off low. Heavy rain and an isolated waterspout are possible in coastal regions, but mostly outside our forecast domain.