Storm Forecast
Valid: Sat 17 May 2008 06:00 to Sun 18 May 2008 06:00 UTC
Issued: Fri 16 May 2008 22:41
Forecaster: SCHLENCZEK

SYNOPSIS

Upper trough over the Bay of Biscay will slowly move eastward. A broad area with deep moist convection is expected ahead of the trough from western France to the eastern Ukraine and parts of Russia in a region with weak pressure gradients. A weak upper ridge is present over parts of the eastern Mediterranean. Over parts of Italy and the northern Balkans, geopotential height is too low to suppress convection in the hot and moist airmass.

DISCUSSION

...Eastern Germany, Poland, Czech Republic and Slovakia...

During the afternoon, about 500 to 1000 J/kg weakly capped MUCAPE may be created due to diurnal heating. As the upper level vort max approaches, QG forcing may initiate a few organized multicells and maybe a MCS over eastern Germany / Czech Rep. Isolated large hail is not ruled out but overall threat will likely stay below the level-one threshold.

Further east, GFS predicts strong (> 15 m/s) low level shear and about 25 m/s deep layer shear over parts of Czech Rep. / Slovakia. Some storms may become supercellular as SRH3 is somewhat enhanced and may produce severe hail. Best conditions for an isolated tornado threat exist over eastern Czech / Slovakia in the late afternoon as LL wind field strengthens and LCL heights stay quite low. There are some uncertainties whether storms will manage to move into Slovakia during the evening or not. Convective activity will rapidly decrease after sunset. A level-one threat is marginally warranted as convective initiation is expected before 15 UTC according to the recent 18 UTC GFS run.

...Corsica, Sardinia and northern/central Italy...

Ahead of the upper trough, warm and moist air is advected northeastward. Around 12 UTC, a small jet streak / left exit region at 500hPa is centered above the region. During the afternoon, some hundred J/kg of CAPE should be released and organized thunderstorms will develop in an environment with more than 20 m/s deep layer shear. Current thinking is that some storms likely become supercellular, given more than 300 J/kg SRH3 over Corsica and northern Italy. Main threat will be large hail and severe gusts due to a strong (> 20 m/s) low level wind field. In the evening, storms will spread northeastward towards northern and central Italy. According to GFS 12 UTC, boundary layer moisture should increase during the morning hours and LCL heights will stay quite low. About 15 m/s low level shear and steep lapse rates suggest that an isolated tornado is not ruled out.

Strong low-level moisture convergence may maintain a large convective system over northern Italy. Some regions may experience torrential rainfall and therefore an enhanced flash-flood risk.

...Iberian Peninsula, Balearic Islands and France...

In the wake of the trough, diurnally-driven showers and thunderstorms will likely evolve. A few hundred J/kg MLCAPE should be available but the kinematic environment is not supportive for severe convection as deep layer shear stays below 15 m/s in most places. Best chances for organized multicells exist over northern Spain and northeastern France. Isolated large hail is not ruled out but a level-one threat does not seem to be warranted at the moment.
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