CONVECTIVE FORECAST
VALID Thu 18 Aug 09:00 - Fri 19 Aug 06:00 2005 (UTC)
ISSUED: 18 Aug 08:55 (UTC)
FORECASTER: GATZEN

There is a moderate risk of severe thunderstorms forecast across northwestern Black Sea region

There is a slight risk of severe thunderstorms forecast across northeastern Hungary, northern Romania, southwestern Ukraine

SYNOPSIS

Intense trough over Scandinavia moves eastward and cuts off over western Russia, while central European high ridges into Scandinavia and slowly shifts eastward. To the west ... northern Atlantic long-wave trough has entered British Isles and amplifies over western Europe. As a consequence ... weak upper trough over Bay of Biscay will disappear leaving a short-wave trough that travels eastward over western Mediterranean. Cut-off low present over central Balkans slowly propagates eastward into Black Sea region. At lower levels ... warm and unstable airmass advects into France, Benelux, eastern England, southern North Sea, western Germany, and Alpine region east of west-European trough. Cold airmass spreads into Balkans west of propagating cut-off low.

DISCUSSION

...Romania and southwestern Ukraine ...
Impressive EML has spread northwestward over Black Sea and was present over southern Ukraine as indicated by latest ascends ... showing steep lapse rates up to the 500 hPa level at Odesa. Below the inversion ... rich boundary layer moisture with mixing ratios up to 16 g/kg creates CAPE of more than 2000 J/kg. Today ... this airmass is expected to spread westward at the northern flank of propagating cut-off low ... and should affect northeastern Romania. To the north and west ... moist and convectively mixed airmass is present over Romania and western Ukraine ... where large MCS has formed during the night hours. On Thursday ... insolation is expected over northern and northwestern Black Sea region ... where latest satellite image shows clear sky ... and strong instability ... probably 2500+ J/kg ... is expected to develop. Quite strong capping inversion may inhibit initiation over southern Ukraine. To the west ... upper jet streak/ vort max is forecast to rotate around the cut-off low ... and DCVA should affect western and northern Romania as well as southwestern Ukraine. Thunderstorms are forecast to develop in the afternoon/evening hours and should spread northeastward. Given DLS around 20 m/s ... mesocyclones and multicells are forecast .. capable of producing large or very large hail and severe wind gusts. Chance for tornadoes is also enhanced given quite strong (10+ m/s) LLS and rather low LCL heights. A few tornadoes ... some of them may be strong ... are not ruled out. Strongest QG forcing is expected further west in the range of the nose of high THETA-E plume from northern Romania to Hungary. Thunderstorms are expected to persist as weak to moderate instability will build in the response of diurnal heating. Although vertical wind shear will be lower than over northwestern Black Sea region ... mesocyclones are not ruled out ... capable of producing large hail, severe wind gusts, and a few tornadoes. Intense precipitation associated with local flash flooding may be possible. Thunderstorms may merge into mesoscale systems later on over southwestern Ukraine/northern Romania.

...Western Mediterranean ...
EML is present over western Mediterranean ... and rich low-level moisture creates CAPE up to 1000+ J/kg. Strong capping inversion is expected during the day that should inhibit deep convection over most places. During the day ... upper trough migrates eastward over western Mediterranean ... and DCVA should lead to QG forcing. As a consequence ... chance for thunderstorms should increase over islands and over northern/western Iberian Peninsula ... where orography may support initiation. Thunderstorms that form may be severe given 15-20 m/s DLS ... and mesocyclones are not ruled out capable of producing large hail and severe wind gusts. Over southwestern Mediterranean ... DLS will be much stronger underneath a jet streak at the base of propagating trough. However ... initiation is very questionable over the sea. Severe thunderstorms that form over northern Africa may affect this region later on ... allover threat may warrant a SLGT later on.

...Northern Benelux and northern France, northwestern Germany ...
Thunderstorms are expected during the day in the range of warm and unstable airmass spreading northward. In the range of the THETA-E-nose over southern North Sea region ... models show increasing LLS during the evening ... and chance for organized convection will be slightly enhanced. Isolated hail/wind events are expected ... and brief tornadoes are not completely ruled out. Allover threat will be marginal.

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