Storm Forecast
Valid: Tue 11 May 2010 10:00 to Wed 12 May 2010 06:00 UTC
Issued: Tue 11 May 2010 09:55
Forecaster: SCHLENCZEK
A level 1 was issued for SE Spain and Balearic Islands mainly for large hail and severe wind gusts.
A level 1 was issued for S-central France mainly for large hail.
A level 1 was issued for E France / SW Germany mainly for large hail and tornadoes (until 18Z) respectively excessive rainfall and severe wind gusts (after 18Z).
A level 1 was issued for S Germany mainly for large hail.
A level 1 was issued for N Italy and Slovenia mainly for excessive rainfall.
A level 1 was issued for N-central Ukraine, E Belarus and W / NW Russia mainly for large hail.
SYNOPSIS
A low pressure system is located over the SW Norwegian Sea, splitting up into two upper lows during the period. One of them will translate SWward towards Scotland. An upper ridge will build over the C Mediterranean, leading to strong gradients of geopotential height over E France and SW Germany which will result in a southwesterly 40 m/s upper jet streak. During the afternoon, a shortwave trough will affect France, W Germany and the Benelux countries, translating northeastward till Wednesday morning. Warm and moist air from S France is advected into parts of SW and S Germany during the day. The cold front, initially located from SW Spain via the E Pyrenees towards E-central France, will move eastward during the period.
Another upper trough is located over the Black Sea, moving slowly northward while weakening.
DISCUSSION
...Strait of Gibraltar, S Spain, Balearic Islands...
At the southern tip of an upper trough, low-end instability is forecast near the cold front in an environment with strong (20 - 30 m/s) deep layer shear. From the late morning until late evening, some probably low-topped cells will develop in this region. Given locally more than 100 mē/sē SRH3, some storms will develop mesocyclones and may pose a threat of large hail and severe wind gusts. Rich BL moisture and locally augmented LLS may allow an isolated tornado as well.
...SW-central France...
WAA will continue in this region near the core of a low pressure system and moderate instability is expected to be available. Deep layer shear in order of 15 - 20 m/s should aid storms to organise into multicells and a few supercells, capable of producing large hail and isolated severe wind gusts. Most of the instability will depend on diurnal heating and therefore storms will likely dissipate in the late evening hours.
...SW / S Germany...
Ahead of a developing surface low, easterly / northeasterly SFC winds around 5 m/s are observed in a region between the Black Forest and Hunsrueck with 10 - 15 m/s southeasterly winds in the upper boundary layer. This will result in some spots of strongly enhanced SRH3 (300 - 500 mē/sē) in the vicinity of a SSWerly upper jet streak (20 - 25 m/s deep layer shear). Right now, most of the region is covered by stratocumulus clouds but if the low level clouds dissipate, insolation will lead to 500 - 1000 J/kg CAPE in this region. Any storm that will develop in this environment may become a supercell with a threat of large hail and tornadoes. Later in the period, storms near the cold front will tend to merge into a large MCS with an enhanced threat of heavy precipitation and severe wind gusts. If isolated storms manage to maintain for a long time, an upgrade may be issued.
Towards the southeast, SRH values are somewhat reduced but the chance of insolation is greater. Moderate values of CAPE (~ 1 kJ/kg) and 20 m/s deep layer shear should be sufficient for well-organised multicells and supercells with a threat of large hail. An isolated tornado cannot be ruled out.
...N Italy, Slovenia, SW Austria...
In an environment with 10 - 15 m/s deep layer shear, strong forcing and moderate instability will lead to thunderstorms that may form a large MCS over N Italy. Given rich moisture and strong LL convergence combined with a slow storm motion, heavy convective rainfall is expected to occur. An isolated large hail event should be possible if cells stay more discrete after convective initiation.
...W Russia, parts of Belarus and Ukraine...
Near the frontal boundary, very high LL moisture is expected in a region of warm air, leading to 1 - 2 kJ/kg CAPE. Deep layer shear has decreased to 15 m/s and less but some pulsating single cell / multicell storms may produce isolated large hail.
