CONVECTIVE FORECAST
VALID 06Z TUE 09/09 - 06Z WED 10/09 2003
ISSUED: 08/09 22:34Z
FORECASTER: HAKLANDER

General thunderstorms are forecast across the North Sea, eastern parts of the British Isles, the Benelux, northwestern Germany, the Danish west coast, and the Norwegian south coast.

General thunderstorms are forecast across southern and eastern parts of Germany, western parts of the Czech Republic.

General thunderstorms are forecast across the central and eastern Mediterranean.

There is a slight risk of severe thunderstorms forecast across southern Italy, Sicily, central parts of the Mediterranean and southern parts of the Adriatic Sea.

SYNOPSIS

The forecast period starts off with a pronounced upper air trough from the North Sea into the Alpine region. The northern part of this feature lingers over the North Sea, whereas a separate low over southern France slides southeastward and reaches central Italy by the end of the forecast period. The associated surface cold front is expected to move southeastward over Corsica and Sardinia on Tuesday afternoon and will likely reach the Adriatic Sea on Wednesday night. Surrounded by an upper ridge over the eastern Atlantic Ocean and a stationary upper high over Finland, the trough is inhibited to progress eastward at higher latitudes. During the forecast period, the mostly zonal upper flow over the Mediterranean acquires more of a meridional component at either side of the main trough axis.

DISCUSSION

...Southern Italy, Sicily, central parts of the Mediterranean Sea and southern parts of the Adriatic Sea...
At low levels ahead of the surface cold front, low to moderate CAPE is forecast by GFS 12z. On Tuesday evening into Wednesday night, a theta-e plume is expected to develop over and to the south of Sicily, creating high latent instability there with MUCAPE exceeding 2000 J/kg. In association with the cold front, 0-6km deep layer shear of 40-60 knots is expected over the SLGT area, implying favorable conditions for multicell or even supercell development. The latter convective mode is well possible, since according to GFS 12z, 1000-700 hPa SREH in the WAA regime ahead of the cold front should increase to 250-350 m²/s² by Wednesday night. In any supercell that forms, large hail, and severe wind gusts are possible. Given low LCLs and strong directional shear at low levels, a tornado is certainly not ruled out. On Wednesday 00z, the right entrance of a jet streak at 300 hPa will be situated over southern Italy, implying significant UVM over the SLGT area. This synoptic forcing will likely trigger the strongest convective development. Current thinking is, that a MCS or even MCC will develop late Tuesday or during Wednesday night over the region, moving eastward and reaching the Balkan coast at the end of the forecast period. GFS 12z, COAMPS 12z, MM5 12z and MM5 18z all show significant convective precipitation over the SLGT region. However, BOLAM 00z shows much smaller precipitation amounts. It should be mentioned, that the areas of high latent instability, high deep layer shear and high SREH are quite small. The severe weather threat depends on the amount of overlap, which is still uncertain at the moment. We have therefore refrained from issuing a MDT risk for the time being.

...North Sea countries...
Cold air filling the upper air trough over the North Sea should result in marginal CAPE above and near the relatively warm North Sea. This could result in some thundery showers and given high relative humidity at low levels, waterspouts are a possibility.

#