CONVECTIVE FORECAST
VALID Thu 25 Aug 10:00 - Fri 26 Aug 06:00 2005 (UTC)
ISSUED: 25 Aug 09:35 (UTC)
FORECASTER: GATZEN

There is a slight risk of severe thunderstorms forecast across southern Balkans, southern Black Sea region

There is a slight risk of severe thunderstorms forecast across northern France, Benelux, northwestern Germany

SYNOPSIS

Well-developed blocking high is present over northeastern Europe. To the west ... an unseasonably intense long-wave trough affects northwestern Europe. At the edge of this feature ... a strong upper jet stretches from northern Atlantic to southern British Isles ... northern France and Germany ... and further to northwestern Scandinavia today. A very strong upper jet streak/vort-max will rotate around the base of this trough ... leading to strong DCVA over Benelux, northern Germany, and Denmark today. To the south ... weakening upper trough travels into Black Sea region. At lower levels ... cold airmass spreads into western Europe in the wake of the upper trough ... while WAA is forecast to the east from southern France to Alpine region and further to eastern Scandinavia.

DISCUSSION

...Southeastern Europe ...
Weakening upper trough moves northeastward over east-central Mediterranean. At its eastern flank ... a rather strong upper jet affects southern Balkans, Turkey, and Black Sea region. Affected airmass is unstable as indicated by latest Istanbul sounding ... showing low-level mixing ratio of 14 g/kg and steep mid-level lapse rates. On Thursday ... weak QG forcing is expected from Greece to western Turkey and northern Black Sea region ... and thunderstorms are forecast. Thunderstorms that form may be severe given strong (20+ m/s) DLS over Greece, Turkey, and Black Sea region. A few mesocyclones are forecast ... capable of producing large hail, severe wind gusts and probably a tornado. Allover threat is limited due to weak forcing.

...Southern British Isles, southern North Sea region, northern France, Benelux, northern Germany, southern Baltic Sea region ...
A very strong upper jet streak/vort-max visible on latest WV image travels from southern British Isles eastward ... crossing NErn Germany on late Thursday. Strong DCVA is expected ahead of this feature. At lower levels ... a cold front is forecast to move eastward over Germany ... and CAA is forecast to spread eastward from southern North Sea to western Poland during the period ... while warm airmass is advected northeastward ahead of the cold front into Alpine region and east-central Europe. Warm airmass is stable as indicated by latest ascends ... and warm mid-levels create a strong inversion above the boundary layer. Thunderstorms seem to be unlikely ahead and along the cold front over most of Germany, and Poland. In the range of the trough axis ... cold airmass is characterized by quite steep low-level lapse rates ... leading to weak low-level CAPE. Today ... this airmass will spread into Benelux and northwestern Germany .. where diurnal heating is expected. As upper vort-max approaches ... mid-level height falls should lead to increasing equilibrium level ... and deep convection should develop as strong DCVA will lead to QG forcing. Thunderstorms that form over southern British Isles, northern France, Benelux, northwestern Germany, and Denmark will likely be well-organized given strong vertical wind shear at the cyclonic flank of the upper jet streak. DLS of 30 m/s and LLS of 10+ m/s should overlap with significant instability over parts of Benelux during the afternoon. Bow echoes and mesocyclones are forecast ... capable of producing high winds and large hail. Strong LLS may be favorable for tornadic supercells. However ... low-level moisture is questionable ... as cold air advection is forecast ... and chance for tornadoes should be not too high. A few tornadoes ... including probably a strong tornado ... are forecast. Convective activity is expected to spread northeastward over northern Germany and southern Baltic Sea during the evening ... and strong vertical wind shear and QG forcing should remain. A few severe reports are expected over northeastern Germany/southern Baltic Sea during the night hours.

...Alpine region ...
To the south ... frontal boundary will be more flow-parallel ... and CAA should be weak. As the upper trough axis approaches ... DCVA/height falls should lead to significant destabilization ... and showers and thunderstorms are forecast to form along the cold front over Alpine region. Given strong DLS ... it is not completely ruled out that a mesocyclone or two will form ... capable of producing large hail and severe wind gusts. Allover threat seems to be low as instability will be limited.

...Finland...
Steep low-level lapse rates and some low-level moisture are present over Finland ... and weak CAPE is expected during the day. A few thunderstorms may form due to diurnal insolation. Thunderstorms that form may be well-organized given strong LLS west of high pressure system. Severe wind gusts and a tornado or two are not ruled out. However ... weak instability and weak QG forcing should limit chance for deep convection.

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