CONVECTIVE FORECAST
VALID 06Z THU 02/09 - 06Z FRI 03/09 2004
ISSUED: 01/09 17:08Z
FORECASTER: GATZEN

There is a slight risk of severe thunderstorms forecast across western Mediterranean

General thunderstorms are forecast across western and central Mediterranean, Iberian Pensinsula, France, southern Great Britain, parts of southern Balkans, Black Sea region, extreme eastern Europe, western Russia, southern Scandinavia

SYNOPSIS

While upper geopotential over Europe is increasing, upper short-wave trough over southern Scandinavia cuts of and moves eastward. West of the Iberian Peninsula ... another short-wave trough cuts of. Downstream ... warm and well-mixed airmass originating from the Sahara desert advects northward.

DISCUSSION

...Northeastern Spain, southern France, northwestern Italy, western Mediterranean ...
Latest soundings show steep lapse rates above the 750 hPa level over the western Mediterranean. East of the mentioned cut-off low ... southerly flow transports this well-mixed airmass northward into Spain, France, and northern Italy. At lower levels ... surface low pressure system over the Iberian Peninsula leads to easterly winds advecting moist Mediterranean airmass westward. During Thursday ... insolation is forecast ... and CAPE up to 1500 J/kg is expected as indicated by latest GFS model run. At the eastern flank of the upper trough ... weak vort-maxima and WAA are expected that should lead to QG UVM ... initiation is expected during the afternoon, especially over orographic features like islands and hilly terrain, where cap should be weakest. Relatively weak deep layer wind shear is forecast ... and single cells/clusters are likely. However ... models suggest increasing deep layer wind shear over the western Mediterranean during the evening ... as upper jet streak moves northward ... and the chance for organized convection will increase. It is not ruled out that a few mesocyclones and multicells will form ... capable of producing isolated large hail or severe wind gusts. However ... best vertical wind shear will remain over the Mediterranean ... where diurnal heating should be too low to break the cap ... and coverage of thunderstorms will be low.

...Northern Baltic Sea region ...
Northeast of propagating upper short-wave trough ... relatively warm airmass is present east of a cold front. This warm airmass is expected to remain as the front propagation slows down due to frontal waves. Latest ascends show rather steep lapse rates and 10 g/kg mixing ratios in the boundary layer ... leading to CAPE up to 1000 J/kg locally. On Thursday ... diurnal heating should lead to instability. Models suggest weak DCVA and moderate WAA ... leading to sufficient QG UVM to initiate thunderstorms. As deep layer wind shear is expected to reach 15 m/s ... convection may be organized ... and multicells/embedded mesocyclones are forecast. Isolated large hail or severe wind gusts are not ruled out. There is also a chance for some tornadoes due to rather strong low-level wind shear and locally low LCL heights. Convection should merge into one MCS later on. Intense precipitation and strong wind gusts should be the main threat during the early night hours.

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