Storm Forecast
Valid: Thu 23 Apr 2009 06:00 to Fri 24 Apr 2009 06:00 UTC
Issued: Wed 22 Apr 2009 21:38
Forecaster: GATZEN

SYNOPSIS

A subtropical high ridges into west Europe, while a cut-off low moves westward over southern Germany. At the western flank of this cut-off low, a mid-level jet ejects into the west Mediterranean, causing upward vertical motion over the central Mediterranean. Most of east Europe is dominated by relatively low geopotential pushing the main jet to northern Africa and east Mediterranean.

DISCUSSION

Central Mediterranean

Latest soundings and surface observations indicate adequate low-level moisture up to 8 g/kg low-level mixing ratio over portions of the central Mediterranean, especially the Tyrrhenian Sea. Given relatively steep mid-level lapse rates, CAPE was able to build in response of diurnal heating over Italy and Sardinia on Wednesday.

The approaching mid-level jet will likely be associated with further destabilization from the Ligurian Sea to the Tyrrhenian Sea and the surrounding portions of Italy, Sardinia, and Corsica. While surface-based instability is likely over the land during the daytime, unstable air masses may be elevated over the rather cool sea surface. At low levels, increasingly dry northerly Mistral winds are expected to spread southward from southern France, limiting the chance of convection. However, some low-level convergence is likely to the east and south-east of this dry air mass, and models expect some moisture flux convergence especially in the Tyrrhenian Sea region. As a result, deep moist convection seems to be most likely over Italy during the day. Thunderstorms are also not ruled out ahead of the dry Mistral winds from south-eastern France to Corsica and Sardinia. Potential of severe convection is limited over most places due to a lack of vertical wind shear, but large hail exceeding severe limits is possible with the stronger cells. Convection is expected to cluster along the cold pools during the day.

Shear gradually improves to the west over the Tyrrhenian Sea in the range of the mid-level jet, where instability is weak for most of the period. However, models indicate that easterly surface winds may lead to a low-level moisture increase during the night hours, and instability may build more westward underneath the flank of the mid-level jet, where some mid-level vertical wind shear may overlap with instability. The convection will likely be elevated, limiting the severe potential, but bowing lines are not excluded with a potential if strong wind gusts.

Northern Alpine region

Deep convection will likely develop in the range of the cut-off low due to surface heating, some low-level moisture, and upslope flow. Storms that form will have a potential to produce marginally severe hail given low wet-bulb zero.

Parts of east Europe and Turkey

In the range of warm low-level air mass, deep convection is relatively likely over parts of east Europe, and Turkey. Limiting factor is rather poor low-level moisture over most places, and best potential seems to exist in the Slovakia region as well as in Turkey. Storms that form are not expected to organize given weak vertical wind shear and forcing, but some stronger storms may be capable of producing large hail. Storms will rapidly weaken after sunset.