Storm Forecast
Valid: Sat 31 May 2014 06:00 to Sun 01 Jun 2014 06:00 UTC
Issued: Fri 30 May 2014 23:39
Forecaster: PISTOTNIK

A level 1 was issued for W-most Russia, E Belarus and large parts of the Ukraine mainly for excessive precipitation and large hail.

A level 1 was issued for Moldova and E Romania mainly for excessive precipitation.

A level 1 was issued for the NE Aegean region mainly for excessive precipitation and waterspouts.

A level 1 was issued for parts of Italy mainly for large hail, excessive precipitation and waterspouts.

A level 1 was issued for SE France mainly for large hail and excessive precipitation.

A level 1 was issued for SE Spain, NE Algeria and N Tunisia for large hail and to a lesser extent for severe wind gusts.

SYNOPSIS

Low geopotential and rather cool air masses continue to affect most of Europe. The dominant upper-level low over Finland will re-connect with the Northern branch of the main frontal zone on Saturday. Another belt of several small upper-level lows stretches from Spain across the Southern Alpine region into Romania. In-between, weak ridging from the Atlantic Ocean extends over the British Isles into Germany.
Near the surface, fairly high pressure over the Western half of Europe is opposed by a filling low over Finland and a deepening low that moves from the Western Black Sea northward into the Ukraine. Ahead of these two cyclones, warm air advection overspreads the Ukraine, Belarus and Western Russia.
No clear focus for organized thunderstorms is present, but various regions will see convective activity with a possibility of isolated severe weather events on Saturday.

DISCUSSION

... far-Eastern Europe ...

CAPE on the order of 500 to 1000 J/kg will build in response to diurnal heating to the East of the almost stationary frontal boundary across NW Russia, Belarus and the W Ukraine. Widespread thunderstorms are expected to initiate in particular in the afternoon and evening over areas of maximized diurnal heating or along old outflow boundaries. Deep-layer shear around or below 10 m/s makes a better organization of storms unlikely. However, localized large hail is possible in their initial stage. Later on, the dominant risk quickly shifts to excessive precipitation due to rapid clustering and possible training activity (especially near the warm front). Storms may go on well into the night while gradually weakening.
Closer to the center of the cyclone, partly embedded and slow-moving thunderstorms may affect Moldova and Eastern Romania throughout the forecast period, and they are capable of producing flash floods as well.

... Mediterranean ....

The main axis of low 500 hPa geopotential slowly shifts southward, surrounded by numerous smaller vorticity maxima which provide sources of lift and foci for enhanced convective activity. While cooler and drier near-surface air gradually makes its way further South and West, the complex topography means that pockets of very rich maritime moisture will be maintained well into Saturday. Cooling upper levels on top of this maritime moisture will result in low to moderate CAPE (with localized maxima up to 1000 J/kg) under mostly weak vertical wind shear.
Friday evening's observations showed dewpoints up to 17°C in the N Aegean region, in coastal areas of W Greece, Albania and SE Spain and along the Cote d'Azur, and even dewpoints up to 21°C (!) along the W coast of Southern Italy. Widespread afternoon thunderstorms will initiate over mountains with convenient access to this maritime moisture reservoir, most notably the Appennin mountains in Italy and the Alpes Maritimes in France. Excessive precipitation and large hail are the main risks. An isolated tornado is not ruled out where sea breeze and upvalley circulations can locally enhance low-level shear or where outflow boundaries collide in a favourable way.
In general, storms and their associated risks will tend to propagate offshore around sunset and may go on overnight over open water, especially over the E Tyrrhenian Sea and the N Aegean Sea. Level 1 areas where issued where the mentioned severe weather risks seem to be highest, though very isolated severe events cannot be ruled out elsewhere, either.
Near the Southern fringes of the >15% thunder areas, deep-layer shear gradually starts to increase to 15 m/s and beyond closer to the subtropical jet stream. However, the only more robust overlap of this better shear regime with instability exists over SE Spain. Like on Friday, showers and thunderstorms are expected to initiate along a sea breeze front that propagates inland in the afternoon. Though convection may be rather low-topped, a low-end level 1 was issued for a chance of large hail and (marginally) severe downbursts with better organized storms.

... Scandinavia via Central Europe into S France ...

The polar air mass featured deep mixing and a capping inversion at a height of 700 hPa on Friday, descending even further towards the West where large-scale subsidence increases. Forecast models show consistent signals of limited CAPE and rising equilibrium levels in response to diurnal heating on Saturday. However, due to strength of the capping inversion, current thinking is that only isolated showers may grow deep enough to produce a little lightning, in particular beneath various southward slipping voriticity lobes.
Weak vertical wind shear should preclude any severe weather, though a few "cold air funnels" are well possible.