Storm Forecast
Valid: Tue 29 Apr 2014 06:00 to Wed 30 Apr 2014 06:00 UTC
Issued: Mon 28 Apr 2014 21:26
Forecaster: PISTOTNIK

A level 1 was issued for a belt from the Netherlands towards the Balkans mainly for excessive precipitation and to a lesser extent for large hail.

A level 1 was issued for a belt from S Italy towards the Turkish West coast mainly for large hail and waterspouts.

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

SYNOPSIS

The main frontal zone is distinctly split into two branches, the Southern one running across the Mediterranean region and the Northern one across Scandinavia. In-between, rather low geopotential and weak gradients cover most of Europe. Two upper-level lows are moving from the Channel towards S France and from Central Italy to the Aegean Sea, creating an elongated stretch of unsettled and cool conditions. Warmer but equally moist air is present over Central Europe and over the Balkans, where a broad minimum of surface pressure has formed.
Weak anticyclonic influence creates dry and brighter conditions over Northeastern Europe. However, in the far North a cyclone moves from Finland into adjacent Russia, and a surge of polar air will enter most of Scandinavia in its wake.

DISCUSSION

... Central Europe into the Balkans ...

A few hundred J/kg of CAPE will form again in response to diurnal heating, and another round of scattered to widespread afternoon thunderstorms can be expected. Convective initiation is particularly likely over orographic features and along outflow boundaries over SE Europe. Across Austria, Germany and the Netherlands, it will mostly be tied to a stationary convergence zone which separates cooler air to the SW from warmer air to the NE.
With weak vertical wind shear, single cells and disorganized clusters will be the dominant mode. A couple of flash flood events are likely across the level 1 area. The strongest pulse storms might briefly attain multicellular characteristics and pose a threat of 2-3 cm sized hail. Cloud bases should be low enough to allow an isolated landspout, too, in case more vorticity can be concentrated along outflow boundaries.
The majority of thunderstorms will decay again in the evening, but a few may go on well into the night.

... S Italy to the Aegean Sea ...

Strong forcing overspreads these regions in the left exit of a jet streak that curves around the Southern flank of the upper-level low. Limited CAPE is forecast to form, though it may initially be capped over the sea. Deep-layer shear increases to moderate values of 15-20 m/s towards the South.
It is expected that the track of the upper-level low will entail isolated to scattered thunderstorms over the sea with a possibility of a few waterspouts, in particular along the coasts of S Italy near the beginning and along the Turkish West coast near the end of the forecast period. Additional storms will form in response to diurnal heating over the Italian peninsula and the S Balkan states, where the highest coverage can be expected in the afternoon to evening hours. Stronger storms may turn into multicells. Localized large hail is possible, and isolated severe wind gusts are not ruled out.

... France, S England ...

Another jet streak circles the Southern flank of the second upper-level low. A transient overlap of pronounced forcing in its left exit region with strong shear and helicity is confirmed by all forecast models for a confined region in Central France. Despite rather fragile-looking CAPE signals and low equilibrium levels, a convective line or a cluster of strongly forced multicells may develop during peak daytime heating. A small level 1 area was issued for a risk of severe wind gusts and possibly one or two tornadoes. Convection will disintegrate after sunset.
Near the center of the upper-level low, thundery afternoon showers will form over N France and S England in a weakly sheared environment. A few "cool air funnels" and an isolated landspout are possible.