Storm Forecast
Valid: Thu 06 Dec 2007 06:00 to Fri 07 Dec 2007 06:00 UTC
Issued: Wed 05 Dec 2007 19:15
Forecaster: GATZEN

SYNOPSIS

Atlantic long-wave trough evades into Europe, while east European high moves eastward. During the period, a sharp short-wave trough moves into central and northern Europe, but indications for instability are very weak given rather cold boundary layer. To the west, a strong upper jet streak enters southern British Isles, and intense cyclogenesis is forecast with a surface low pressure center moving across Scotland. In the wake of the low, an unstable polar air mass spreads into British Isles and Benelux until the end of the period. Over south-eastern Europe, a cut-off low moves eastward.

DISCUSSION

North Sea, British Isles, Benelux

Intense cyclogenesis is forecast over this region, with a strong upper jet reaching southern British Isles in the period, and a tongue of moist warm air mass spreading north-eastward ahead of the cold front. Latest indications are that strong QG forcing will lead to continuous lift in the warm sector air mass, leading to weak instability especially over Ireland and the central North Sea near the occlusion. However, all regions crossed by the cold front will have the potential for weak instability in the warm air mass.

This will likely lead to a partly convective line along the cold front with embedded thunderstorms given 15 m/s 0-1 km vertical wind shear. Given strong low-level wind field, isolated severe wind gusts are forecast. As low-level helicity is laso rather high exceeding 200 J/kg SRH, mesoclones are not ruled out, with a slight chance for tornadoes. Allover threat seems to warrant a level 1.

A convectively mixed polar air mass enters the region in the wake of the strong surface low pressure system in the evening/night hours. Expect that this air mass is characterized by neutral lapse rates and rather rich boundary-layer moisture, forcing in the range of the trough center as well as underneath the cyclonic flank of propagating upper jet streak will likely be sufficient for showers and thunderstorms to develop. Given strong vertical wind shear around 10 m/s 0-1 km LLS and more than 20 m/s DLS especially in the southern regions, thunderstorms may organize into bowing lines capable of producing severe wind gusts. A tornado along the flanks of such bowing lines is not ruled out. Overall threat seems to be rather low, though, as QG forcing of the trough will be rather limited due to cold air advection.

Southern Turkey

In the range of the weakening cut-off low, weak vertical wind shear and rather strong low-level buoyancy are forecast southern of Turkey, where showers and thunderstorms will develop. A few waterspouts are expected to develop during