Storm Forecast
Valid: Fri 29 Apr 2011 06:00 to Sat 30 Apr 2011 06:00 UTC
Issued: Thu 28 Apr 2011 14:19
Forecaster: TUSCHY

A level 1 was issued for NE-Algeria and S-Sardinia mainly for large to very large hail and severe wind gusts. A tornado risk may arise over S-Sardinia during the night hours.

A level 1 was issued for parts of the UK, Belgium, the Netherlands, Luxembourg, France and parts of W-Germany mainly for an isolated tornado and large hail/strong to severe wind gusts.

SYNOPSIS

Extensive cyclonic gyre covers Europe with cold mid-levels overspreading most of Europe. This kind of mid tropospheric air mass atop modest BL moisture and seasonal diabatic heating all assist in widespread thunderstrom development.

An high pressure area over N-Europe strengthens during the forecast and forces drier and cooler air further to the south.

At the surface, the main feature of interest is an intensifying depression over far N-Africa/SW Mediterranean, which pushes eastwards. Enhanced shear allow rapid thunderstorm organization with a risk for severe thunderstorms.

DISCUSSION

... N-Algeria to Sardinia ...

Broad surface low pressure just east of Spain shifts gradually to the east with some strengthening forecast during the start of the forecast period. Ahead of this depression, a very hot and dry air mass advects northeastwards over NE-Algeria with mid-level lapse rates approaching 9 K/km atop 5 g/kg BL mixing ratio. As this air mass moves offshore, a gradual BL modification is forecast, but abundant CIN remains present. Main initiation awaits strong vort. lobe, which arrives during the afternoon from the west, reaching Sicily during the morning hours of Saturday. We expect highest severe probabilities along west-east aligned moisture convergence zone just offshore with moisture pooling and therefore enhanced MLCAPE forecast (roughly 800 J/kg). Despite very favorable veering at all levels, expect low-levels capped most of the times and hence limited tornado probabilities. Hence, large to very large hail will be the main hazard given up to 400 m^2/s^2 SRH-3 next to severe wind gusts as thunderstorms rapidly grow into supercells ( 25 m/s DLS). We can't exlucde more surface based storms along the S-coast of Sardinia during the morning hours. Tornadoes would be a distinct chance, given strong LL shear magntiude.

A level-1 was issued due to ongoing model fluctuations in final cap strength, QPF maxima and strength of the LL depression. If the thunderstorm risk will be more widespread than a level 2 may become necessary.

... E-UK, Benelux, parts of France and W-Germany...

Gradient flow increases during the daytime hours as high pressure to the north strengthens. Modest BL moisture content beneath T<-20°C at 500 hPa results in modest mid-level lapse rates and SB/MLCAPE of ~800/500 J/kg. Numerous, weak vorticity maxima, embedded in this strenghtening easterly flow affect the highlighted area during a prolonged period, resulting in scattered thunderstorm activity over a broad area. Main uncertainty... if wind field ramps up fast enough, before instability diminishes due to BL stabilisation. Also, weak low-tropospheric WAA is anticipated during the forecast, which could limit thunderstorm coverage mainly over the Netherlands and NW-Germany. We went with a level 1, where low-end to modest CAPE overlap with up to 12 m/s speed shear and roughly 200 m^2/s^2 SRH-1. LCLs not too low with 1 km, but still an isolated tornado and strong to severe wind gust event is well possible, probably maximized over Belgium. A somewhat enhanced large hail risk exists not only in the aforementioned areas, but also further south over N-central France with low WBZ levels and DLS of 10 - 15 m/s. Hence, the level area was expanded well to the south.

... Rest of the highlighted areas ...

A few large hail events also occur with thunderstorm activity over south-central France, Italy and the Alpine region due to the pretty cold temperature profiles and locally modest MLCAPE of 500 J/kg. Shear is very weak, so expect pulsating and rapidly clustering thunderstorms, which gradually decay after sunset. This large hail risk is mainly driven by mesoscale features, so no level 1 was issued to account for the limited coverage and unpredictability.

Scattered thunderstorms are also forecast over east-central Europe. Weak CAPE/shear fields preclude anything more widespread severe.