CONVECTIVE FORECAST - UPDATE
VALID 00Z SUN 28/12 - 06Z SUN 28/12 2003
ISSUED: 27/12 23:57Z
FORECASTER: GROENEMEIJER

There is a slight risk of severe thunderstorms forecast across parts of western and northwestern France

General thunderstorms are forecast across parts of France and northern Spain

SYNOPSIS

DISCUSSION

...Slight risk area...
An intense shortwave trough over the Bay of Biscay is expected to migrate eastward. Rapid cyclogenesis is taking place over Bretagne. A cold front at 00Z located along the 3deg east meridian (or from central Bretagne southward) is expected to move eastnortheastward during the night. Shallow linear convection is expected to take place near cold front in the north, while deeper thundery convection is currently occurring over the southern Bay of Biscay. The AFWA-MM5 18Z model run seems to be performing quite well though the surface pressure is slightly lower in reality. This model has a 50 to 65 kt low-level jet ahead of the cold front, leading to strong low level shear. Dew point temperatures of 9 - 10 C in combination with temperatures of 11 - 13 C should be yielding low LCL heights along the cold fronts. This should favour the development of small embedded mesocyclones in the frontal convection. The development of a few tornadoes is possible and scattered severe wind gusts in excess of 50 knots are expected. The focus of the severe threat should be along the cold front. The evolution of the system is quite uncertain as models diverge quite strongly. Most likely scenario seems that the front will encounter somewhat drier air in the early morning hours and the low level jet will weaken as well, so that the threat of severe weather will gradually reduce.

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