Storm Forecast
Valid: Mon 23 Apr 2012 06:00 to Tue 24 Apr 2012 06:00 UTC
Issued: Sun 22 Apr 2012 22:59
Forecaster: TUSCHY

A level 1 was issued for Tunisia, parts of the Balkan States and S/E Hungary mainly for large hail and strong to severe wind gusts. An isolated tornado event is possible over N-Serbia,NE Croatia and SE-Hungary.

A level 1 was issued for Sicily mainly for large hail, an isolated tornado risk and severe wind gusts.

A level 1 was issued for NW-France mainly for an augmented tornado risk and strong to severe wind gust events.

A level 1 was issued for W-Ukraine mainly for isolated large hail.

SYNOPSIS

A deep cyclonic vortex still covers most parts of Europe. Embedded smaller-scale disturbances circle that vortex and result in numerous areas where enhanced showers/thunderstorms are forecast.

A strong depression enters NW France during the day with marginal weakening forecast during the night, as this feature continues to the E/NE, entering Benelux towards the end of the forecast period.

DISCUSSION

... Tunisia, Sicily, parts of the central Balkan States and Romania ...

A pronounced and progessive wave, best structured at mid to high-levels affects Tunisia/Sicily around noon before lifting out to the NE while weakening. Extensive positive tilted vort lobe evolves along that feature and offers modest forcing for the highlighted regions. Due to weak cyclonic curvature, the progressive nature and the weakening trend of that trough, only marginal advection of EML plume is forecast, which grazes Sicily around noon before moving off to the east.

SST south of Sicily at or above 16°C with synop reports showing dewpoints of roughly 13°C. Some limited time exists for this moisture to spread northwards, affecting Sicily around noon. Global and high resolution models show locally augmented MLCAPE over Sicily (~500 J/kg between 9-15 Z). All models show some low-end chances for an isolated storm or two to evolve along the N-coast of Sicily. Any DMC may rapidly gain organization due to 30 m/s DLS and 300 m^2/s^2 SRH-3 (EHI also enhanced). Therefore an isolated supercell event with large hail, severe wind gusts is forecast until ~ 18 UTC. Even a tornado can't be ruled out with some LL shear forecast. Thereafter, forcing leaves that area to the NE with strengthening NVA appraoching from the west.

Similar environmental set-up exists over Tunisia with less shear, so an isolated well organized multicell event may produce large hail/strong to severe wind gusts. As forcing exits to the NE, the activity decays (beyond 18Z).

Malta was not yet added into the level 1, despite favorable shear conditions for tornadic storms. However, initiation of DMC seems unlikely with strongest forcing diplaced to the north.

Central Balkan States are situated ahead of the aforementioned upper wave with strongest incoming lift around noon onwards. Hence, we expect rapidly developing mountain convection around noon and onwards. A 25 m/s mid-level streak grazes the area of interest just to the north but still assists in 15-20 m/s DLS. This allows mountainous convection to organize while moving off to the NE (e.g. N-Serbia, NE Croatia and SE-Hungary). A few better organized multicells may be capable of producing large hail, strong to severe wind gusts and an isolated tornado, mainly in the level 1 area. The activity decays after sunset while crossing far W-Romania and E/NE Hungary.

Areas to the east, e.g. CNTRL Romania, see daytime driven convection with marginal hail and strong wind gusts. The activity decreases after sunset.

... Parts of Ukraine ...

A marginal level 1 was added due to strong - isolated severe - pulsating storms. DLS in models varies between 10-15 m/s, so at least an isolated large hail event is possible with more discrete storms in better CAPE environment. After sunset, the activity decays.

... NW-France ...

A 990 hPa depression moves ashore during the day. A well structured occlusion crosses NW-France from west to east with wrap-around moisture sneaking in from the NW during the afternoon hours onwards. There exist two foci for probably enhanced severe:

The first one evolves along the backside of the eastward sliding occlusion, as modest dry slot feature wraps towards the center's depression with slight overlap atop better LL moisture. Also, a left exit of an intense 45 m/s mid-level streak overspreads the area of interest, so abundant lift is present. If this verifies, strong LL directional shear and LCLs around 500 m may assist in an isolated tornado event, next to strong wind gusts and marginal hail. There exists some uncertainty, how well structured the occlusion will be over the central and eastern parts of the level 1 and if infiltration of drier air from the SW in the postfrontal airmass really verifies. This may increase the potential overlap of progressively supportive LL shear (as LL shear strengthens during the daytime hours) and marginal CAPE. Hence the level 1 area was also expanded southwards.
The level 1 outruns the 15-% thunderstorm area to highlight limited DMC probabilities that far east, but still adequate LL instability/shear overlap for at least some severe.

The second focus will be the wrap-around moisture during the afternoon and evening hours mainly over the western part of the level 1. High resolution model data show an afternoon increase in SRH just ahead of and atop the incoming LL moisture plume, so again an isolated tornado event is possible. LL background flow increases rapidly in excess of 20 m/s, so any thunderstorm/shower may also assist in some downward mixing with severe wind gusts possible.

During the night, a neutral to slightly unstable air mass covers most of France. However, the activity will be sporadic in nature, despite far E-France, where some better LL moisture may offer enough fuel for isolated nighttime thunderstorms. Strong wind gusts will be the main hazard.