Storm Forecast
Valid: Sat 09 Nov 2013 06:00 to Sun 10 Nov 2013 06:00 UTC
Issued: Sat 09 Nov 2013 00:33
Forecaster: PISTOTNIK

A level 1 was issued for parts of NE Italy, Slovenia and Croatia mainly for excessive precipitation and severe convective wind gusts.

A level 1 was issued for W France for tornadoes and severe convective wind gusts.

SYNOPSIS

Embedded in a strong Westerly background flow, a long-wave trough translates from France to Central Europe and amplifies into the Central Mediterranean. Numerous surface lows, some of them small and transient, form in response to travelling vorticity maxima in the disturbed flow and affect much of Europe. Forecast uncertainties are accordingly large, but all solutions agree that deep convection will be seasonably tied to maritime areas.
The most important features are as follows: The parent cyclone is expected to move from Denmark to the Baltic Sea, and its cold front curls across Central Europe into the Central Mediterranean, delayed by a lee cyclogenesis over Northern Italy. A subsequent, smaller frontal system will cross France, BeNeLux and Germany later on, followed by a particularly strong vorticity maximum on the back side of the main trough, which will move from England into the Netherlands, Belgium and Northern France overnight.
On the anticyclonic side of two jet streaks that flank the long-wave trough, Iberia and Southeastern Europe experience mild and calm conditions.

DISCUSSION

... Central Mediterranean ...

Ahead of the lee cyclogenesis over Northern Italy, mild and moist air is advected northward in the Adriatic region. Lift support is plentiful until 18 UTC, as long as the approaching main trough axis and the right entrance of a Southwesterly jet streak coincide over Northern Italy, then it fans out and departs towards the Northeast. A few hundred J/kg of CAPE are forecast to form under wind profiles that are adequate for organized storms, showing deep-layer shear between 20 and 30 m/s as well as patches of enhanced low-level shear and helicity.
Scattered thunderstorms are expected to form early in the day over Northern Italy and to travel eastward. They will likely peak over Friuli and Slovenia in the afternoon to evening hours and will continue to shift southeastward along the Croatian coastline overnight. Severe wind gusts and excessive precipitation are the main threat, the latter is maximized between Trieste and Zadar with prolonged onshore flow. Marginally large hail and an isolated tornado are not ruled out, either.
After the cold front passage, dry and cool downslope winds (both from the Alps and the Dinaric mountains) will quickly terminate thunderstorm activity.

Scattered thunderstorms are also expected further Southwest across the Central Mediterranean region, in particular near the Algerian and Tunisian coast in the afternoon to evening hours and again over the Ligurian Sea towards the end of the forecast period (foreshading a new and stronger cyclogenesis on Sunday). Limited instability and shear make severe weather unlikely. Isolated events of heavy precipitation, a marginally severe wind gust or a waterspout are possible, though.

... W France ...

Beneath the Atlantic jet streak, a tricky setup evolves with the new depression that runs ashore in Western France in the morning. A short-wave trough overspreads a tongue of very moist air and provides plenty of lift support until 09 UTC, resulting in neutral to slightly unstable profiles. Convection is not guaranteed and will likely stay shallow, but any shower that forms will benefit from 0-3 km shear between 15-20 m/s and helicity up to 800 (!) m^2/s^2 in coastal areas of Western France. Bowing lines or shallow supercells with severe wind gusts and tornadoes are possible. (Two F1 tornadoes occurred at the beginning of this week in a similar setup.)
Later in the day, forcing will turn neutral or even slightly negative with increasing warm air advection under an anticyclonically curved flow. The depression will quickly lose any remaining convective character while it moves further inland.

... North Sea and surroundings ...

Scattered showers are expected throughout the forecast period in the cool, weakly sheared air mass beneath the base of the main trough over the North Sea. A few of them may grow deep enough to produce a limited amount of lightning. Activity will subside over the British Isles in the second half of the forecast period, where it gets quenched by increasing warm air advection, but will increase in coastal areas of the Netherlands, Belgium and Northern France late at night with the arrival of the strongest vorticity lobe. Apart from an isolated waterspout, severe weather is unlikely.