CONVECTIVE FORECAST
VALID 06Z MON 02/08 - 06Z TUE 03/08 2004
ISSUED: 01/08 22:58Z
FORECASTER: GROENEMEIJER

There is a slight risk of severe thunderstorms forecast across France, northern Spain

There is a slight risk of severe thunderstorms forecast across Northwestern Ukraine, Belarus, Poland, Latvia, Lithuania, Russia Kaliningrad

General thunderstorms are forecast across northern Iberia, France, the western Alps and the British Isles and across parts of eastern Europe, central and eastern Scandinavia, the northern Balkans and region around the Adriatic Sea.

SYNOPSIS

Monday at 06Z... a ridge over western Europe is flanked by a trough over the Atlantic Ocean and a trough over eastern Europe. The latter has a closed mid/upper low centre over northwestern Ukraine moving northwestward reaching central Poland Tuesday morning. A shortwave trough over the Bay of Biscay moves northeastward reaching northern France and the Channel region on Tuesday morning.

DISCUSSION

...France, northern Spain...
A deep mixed layer between 900 and 550 hPa was observed over Spain and southern France on Sunday afternoon. Within this air-mass, some elevated storms could be ongoing during the moring hours and early afternoon most likely over western France and northern Spain. In response to solar heating, high surface temperatures of 32-35 C and surface dew point values around 17C are expected, resulting in substantial CAPE over southern and central France, generally between 1000 and 2000 J/kg (MLCAPE50). Scattered surface-based storms are expected to develop during the afternoon and evening across the slight risk area as the aformentioned shortwave trough approaches the area. Around 15 m/s deep-layer (0-6 km) shear is expected across western portions of the risk area and somewhat lower shear to the east. Anticipated convective modes are multicells and perhaps a few supercells most likely in the west. The storms are capable of producing large hail locally. Across central and southern France a deep dry sub-cloud layer is expected, creating a large potential for strong and severe downbursts. A few MCS's will likely form along common outflows of the thunderstorms. As this happens, the severe threat should gradually become limited to wind gusts.

...eastern Spain...
Less forcing for upward motion and lower surface dew point temperatures are expected across this area. However, isolated storms could form in this area as well. Inverted-V type profiles favour strong downbursts with any storm that forms. Isolated large hail is possible.

... Northwestern Ukraine, Belarus, Poland, Latvia, Lithuania, Russia Kaliningrad...
A theta-e tongue is advected northwestward on the northeastern flank of the mid/upper low by a low-level jet. Convection within this tongue is expected to increase in coverage during the forecast period in response to both solar heating and increasing QG upward vertical motion. Instability should be high enough to allow for scattered large hail. Given that low-level wind shear is quite strong, there is a rather high risk of the storms being accompanied by strong winds and perhaps a few mesocyclones can form. In combination with expected low LCL height across the area, a few tornadoes cannot be ruled out.

...western Ukraine, Moldova...
More storms are expected along a stalling frontal zone over the indicated area, where the formation of one or two MCS's is possible. Shear is expected to be lower here, but a small threat of large hail and strong winds will nevertheless be present with organised convection developing here.

...Adriatic Sea region, western Balkans...
A relatively weak northwesterly jet over the area is forecast to create 15-20 m/s deep-layer shear over the region. As up to 1000 J/kg MLCAPE50 is expected over the area, storms forming here can become severe. However storm coverage should be low as a clear trigger mechanism for convective activity is absent. With the isolated storms that do form, soem large hail and strong to severe gusts are possible. If we get more confidence in convective development, an upgrade to slight risk will be issued.

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