Storm Forecast
Valid: Sat 05 Oct 2013 07:00 to Sun 06 Oct 2013 06:00 UTC
Issued: Sat 05 Oct 2013 07:06
Forecaster: PISTOTNIK

A level 1 was issued for Central Italy and the Tyrrhenian Sea mainly for excessive precipitation and severe wind gusts.

A level 1 was issued for S Italy, Tunisia and N Algeria mainly for large hail, severe wind gusts and tornadoes.

SYNOPSIS

A large upper-level low slowly exits the forecast domain to the East. It has flooded the Eastern half of Europe with cold and dry air and leaves a cold surface anticyclone over these regions.
Further upstream, another anticyclone - this time a subtropical, warm one - extends into Southwestern Europe, whereas a strong zonal flow with progressive waves establishes further North. The Northern part of a first upper-level trough moves from the North Sea across Scandinavia, whereas its Southern part cuts off over France and slowly continues into the Central Mediterranean. A second trough approaches the British Isles from the West late in the forecast period.
Near the surface, these troughs are accompanied by two powerful cyclones over the Northern Atlantic Ocean. The Eastern one extends an occluding frontal system across Scandinavia into Central Europe, gradually decelerating and fanning out towards the South. The Western one sends a pronounced warm front over the British Isles, followed by the arrival of an ana cold front in Scotland and Ireland overnight. A secondary cyclogenesis will develop at its occlusion point over the Norwegian Sea overnight and will bring non-convective severe wind gusts off the Central Norwegian coast towards the end of the forecast period.

DISCUSSION

... Central and S Italy, Tyrrhenian Sea, Tunisia, N Algeria ...

Despite the dynamic European weather pattern, the regions with deep convection are tied to the Mediterranen cut-off low. Low-level moisture is rich (dewpoints around 20°C) as long as it is not mixed out by convection, but steep lapse rates will stay confined to Algeria, Tunisia and Sicily, which get also overspread by a 500 hPa wind speed maximum. This creates the quite typical bimodal setup of moderate CAPE (500-1500 J/kg), moderate deep-layer shear (10-15 m/s) and fairly certain instability release (cooling atmosphere on top of a warm sea surface) towards the North, and of increasing CAPE (up to 2500 J/kg), increasing deep-layer shear (up to 25 m/s) and increasing uncertainty about convective initiation (stronger capping and weaker forcing) further South.
At the beginning of the forecast period, a healthy-looking convective line is stretching from Central Italy into the Tyrrhenian Sea near the East coast of Sardegna, supported by patchy vorticity maxima aloft. Localized cases of excessive precipitation and severe wind gusts seem to be the primary risks with this line while it continues to move into mainland Italy, covered by a level 1. In case of more discrete storms (which is unlikely here), marginally large hail and an isolated tornado may occur as well. Convection will get elevated and decay as soon as it encounters the fringe of the Eastern European cold air body over the Adriatic Sea.
Further South, storms will stay more isolated but convective initiation is still forecast around Sicily in the afternoon and along the Algerian and Tunisian coast in the evening to night hours. With substantial shear and helicity even down to lower levels, multi- and supercells will be the dominant mode. They may bring the full spectrum of severe weather: large hail, severe wind gusts, tornadoes and also excessive precipitation in case of upscale growth, which seems to be possible overnight. Due to remaining uncertainties about convective initiation and coverage, it was decided to stay with a high-end level 1.

... Corsica, Sardegna, Western Mediterranean, Catalonia ...

Cold air advection (Mistral and Tramontana winds) on top of the still warm Western Mediterranean Sea will result in maritime convection that may grow deep enough to produce showers and low-topped thunderstorms where the fetch is sufficient. An enhanced risk of waterspouts exists between Corsica and North-Central Italy and off the Catalonian coast. In these regions, low-level buoyancy will be maximized and the background wind is weak enough to allow a spin-up.

... Western Europe ...

Large parts of Western Europe get flooded by rich Atlantic moisture (dew points in the 10 to 15°C range), but in general the lapse rates are too weak to allow deep convection. Isolated afternoon thunderstorms are possible across the Southern half of France, though, aided by the lift support of a vorticity lobe which swings around the back side of the cut-off low. Weak vertical wind shear and marginal instability should preclude any severe weather.