Storm Forecast
Valid: Sat 18 Apr 2009 06:00 to Sun 19 Apr 2009 06:00 UTC
Issued: Fri 17 Apr 2009 20:43
Forecaster: SCHLENCZEK

SYNOPSIS

A high over low blocking pattern over western Europe will continue throughout the forecast period, leading to rather weak temperature and pressure gradients over central Europe. In the vicinity of the associated upper trough over France and northern Iberia, showers and thunderstorms will develop in an environment with little CAPE, most of them should be diurnally driven. Another upper trough over western Russia moves eastward, but there is almost no CAPE near the cold front which reduces the chances for electrified convection. Stable and quiescent conditions are expected for the eastern Mediterranean and also for northwestern Europe on the eastern flank of an upper ridge.

DISCUSSION

...N Italy...

In the morning / early afternoon, diurnal heating will create a few hundred J/kg of CAPE over N-central Italy. Even though QG forcing is rather weak, some organized multicells may develop in this weakly capped environment with 15 to 20 m/s deep layer shear. Steep LL lapse rates, weak BL winds and LCL heights only around 500m may allow an isolated funnel / brief tornado. Some marginally severe hail is also not ruled out as the freezing level is relatively low. As there is almost no elevated CAPE, thunderstorm activity will rapidly decrease after sunset.

...NW Iberia...

In the vicinity of a northwesterly 35 m/s jet streak over Portugal, about 300 J/kg CAPE are forecast to be created during the afternoon. An upper shortwave trough which moves from NW to SE will provide strong QG forcing and about 15 m/s deep layer shear may allow some briefly organized multicells that may produce small to marginally severe hail and gusty winds. Limited instability will preclude any threat level for that region.

...Balearic Islands, SW Mediterranean...

The aforementioned shortwave trough will cross the SW Mediterranean in the late evening / night hours and may initiate some organized storms over SE Spain and the SW Mediterranean. Ahead of a small surface low near the Strait of Gibraltar, LL winds may back to the east which increases directional shear and allows some short-lived mesocyclones with the stronger updrafts. The severe threat should be low but a marginally severe hail event as well as a waterspout are not ruled out.

...Other thunderstorm regions...

Severe thunderstorms are not expected in the other areas as there is a lack of instability or shear to support organized convection. Eastern Germany was excluded from the thunderstorm area as there is some weak instability but also strong subsidence at upper levels.