Storm Forecast
Valid: Sat 24 Aug 2013 06:00 to Sun 25 Aug 2013 06:00 UTC
Issued: Fri 23 Aug 2013 15:46
Forecaster: PISTOTNIK

A level 1 area was issued for parts of W Russia mainly for excessive precipitation.

Level 1 areas were issued for E Spain, parts of France, Switzerland and N Italy for excessive precipitation, severe wind gusts and large hail.

SYNOPSIS

A high pressure system temporarily establishes over Scandinavia, flanked by an upper-level low over Belarus and a second upper-level low that cuts off over Northern France Saturday night. This results in a transient omega-like pattern. Cool air digs southward along the Western flanks of these lows over the Baltic States, Poland and over the British Isles and France, respectively, separated by a thinning warm sector over Central Europe.
The Mediterranean region sees a mostly quiescent day in-between, but the nose of a jet streak that extends from Ireland via Southern France towards the Ligurian Sea already promotes a beginning Genoa cyclogenesis and presages more turbulent days again.

DISCUSSION

... W Russia ...

Enhanced low-level moisture and modest lapse rates create CAPE on the order of 500 to 1000 J/kg along a zonal frontal boundary East of Belarus. Deep-layer shear gradually increases to 15 m/s ahead of the upper-level low. Scattered single- and multicells are forecast in the afternoon and evening, possibly growing into clusters. The stationary boundary, persistent QG lift and solid precipitation signals for a longer time in the forecast models point to a slight risk of excessive rain. Otherwise, severe weather threat is not notably enhanced.

... Slovakia, Hungary, the Balkans, Romania ...

A short-wave trough travels eastward and supports scattered afternoon thunderstorms mainly over orographic features. The environment is characterized by limited CAPE (~500 J/kg) and deep-layer shear (10-15 m/s). Some multicells may develop with a marginal chance for large hail and severe wind gusts especially over Romania and Serbia, where the lapse rates are steepest. Further West over the Balkan states, lapse rates weaken and CAPE seems to be based more on rich low-level moisture, which may support an isolated heavy rain event. However, both threats do not seem solid enough to warrant a level 1.
CAPE and precipitation signals in most models fan out towards the North. Only ECMWF offers a solution that the convective surge which will likely cross Austria Friday night shall also reorganize over Slovakia on Saturday. However, due to serious doubts about an involvement of convection there only a 15% thunder area was drawn.

... E Spain, France, SW Germany, Switzerland, Austria, Italy ...

Model signals are somewhat unclear when it comes to the handling of the southeastward moving cold front. However, there are signals that the conditions for deep convection will come together in a rather extensive area ahead of, in the range of and also behind the front, though weak lapse rates will likely limit CAPE to around 500 J/kg over most of the inland areas.
A pronounced short-wave trough provides sufficient QG lift throughout the day, maximized over much of France and later over Switzerland and Northwestern Italy. The wind shear maximum is displaced a bit to the Southwest, where the jet streak enhances deep-layer shear to values around 20 m/s over Southwestern France and Catalonia. This belt of enhanced shear overlaps with positive CAPE areas especially where the low-level air mass exchange is delayed in the wake of the mountains (Catalonia, Northwestern Italy), and it also partly overlaps with the Southern fringes of the QG lift areas.
First thunderstorms may go on or form quite early in the day across Eastern France, and activity will gradually intensify and spread into Southwestern Germany, Switzerland, the Italian Alps and Catalonia, until during the afternoon scattered thunderstorms are expected across most of these areas. A few of them may organize into stronger multicells or isolated supercells with a risk of (marginally) large hail and severe wind gusts. With chances for upscale growth and possible repeated rounds of storms, later the dominant threat shifts to excessive precipitation.
Behind the cold front, the models agree on significant drying in the mid and upper troposphere, probably associated with a dry intrusion. Some hours of sunshine and still pronounced QG lift might suffice to create a second, postfrontal round of storms over France. Despite their small size and low cloud tops, an organization into anticyclonic multi- and supercells with a primary threat of marginally severe wind gusts is still possible due to fairly strong vertical wind shear and backing profiles in the cold air advection regime. Beginning nighttime cooling and the arrival of the core of the upper-level low will choke off this convection till sunset.
In the evening and night hours, the same short-wave trough will overspread a better air mass with CAPE around 1000 J/kg over the Ligurian Sea and Northern Italy. Deep-layer shear also increases to 20 m/s. Scattered to widespread thunderstorms are forecast with a possible growth into one or two MCSes and a threat of excessive precipitation, severe wind gusts and large hail. "Tail-end storms" in particular may turn into supercells with an even higher hail threat. Over the Ligurian Sea and coastal areas, particularly low cloud bases and slightly enhanced low-level shear and helicity may support an isolated tornado, too. The late arrival of the storms, and associated difficulties for them to become and stay surface-based, precludes a level 2 over Northern Italy.
Otherwise, storms along the cold front and isolated (if any) prefrontal activity will likely decay rather soon over Southern Germany and Austria in the evening hours.

... Ireland, Cornwall, Normandy ...

A frontal wave and another potent short-wave trough travel southward from the afternoon hours onwards. Neutral profiles and strong forcing might allow isolated thunderstorms, either along the cold front or on the cool side of the boundary. With deep-layer shear around 20 m/s, marginally severe wind gusts are not ruled out.