Storm Forecast
Valid: Mon 07 Dec 2009 06:00 to Tue 08 Dec 2009 06:00 UTC
Issued: Mon 07 Dec 2009 04:47
Forecaster: GATZEN

A level 1 was issued to the south of the British Isles mainly for severe wind gusts.

SYNOPSIS

A broad delta flow is present across Europe, blocking the westerly jet to the west. On Monday, an intense short-wave trough travelling around the main north-eastern Atlantic long-wave trough cuts off over the Benelux countries and France. In the range of the trough axis, a convectively mixed air mass will spread across western Europe. To the south, a tongue of moist and warm air advects into south-western Europe, but weak QG forcing and rather stable low-levels will likely suppress deep moist convection. Further east, northerly winds and cold air advection are expected over the east Mediterranean. Stable conditions are likely in the range of weak winds over most of Europe.

DISCUSSION

British Isles, northern France, Benelux

Ahead of the approaching trough axis, a tongue of rather moist maritime air mass spreads into France, the British Isles and the Benelux countries. As the trough cuts off over France, a cold front will spread rapidly south-eastward into France. Along the front, a narrow convective line has developed west of Ireland that is expected to spread south-eastward today. Along The Channel and the surroundings, this line may be still active given some low-level convergence and strong QG forcing in the range of a jet streak curving around the cut-off trough. As low-level vertical wind shear is strong (10-15 m/s in the lowest kilometer) and low-level winds will reach more than 25 m/s at the 850 hPa level, isolated severe wind gusts are not ruled out, especially when bowing segments may form along the leading gust front. Limting factor is rather weak instability and weak low-level buoyancy that also leads to a quite low chance of tornadoes. Furth south, low-level convergence and QG forcing will be quite weak as well, and deep moist convection is not expected along the cold front.

In the wake of the cold front, a convectively mixed air mass will spread across the British Isles and France. However, latest models indicate that the low levels will be quite dry, and deep moist convection seems to be not very likely. However, some showers and isolated thunderstorms are not completely ruled out.

Southern France, northern Italy, western Switzerland

Along the cold front, some showers are likely as the low-level air is forecast to be slightly unstable due to the rather high moisture content. The best potential of a few thunderstorms seems to exist over the Mediterranean Sea given the warm water surface as well as along the western Alps, where upslope flow is present.