Storm Forecast
Valid: Thu 14 Dec 2006 06:00 to Fri 15 Dec 2006 06:00 UTC
Issued: Wed 13 Dec 2006 20:54
Forecaster: GATZEN

SYNOPSIS

Impressive ridge covers western and central Europe, with an axis from eastern Atlantic to Bay of Biscay, Alps, and further to southern Adriatic. At the north-western flank, up to 100 m/s wind speed are forecast at the 300 hPa level in the range of a strong and rather long jet streak. This jet is directed towards northern Scotland and southern Scandinavian mountains during the period. To the south of the ridge, a well-developed but weakening cut-off low moves slowly northward in the range of the Algerian coast. In the range of the ridge, stable air mass is present. To the north, a tongue of moist and relatively warm maritime air mass advects into British Isles and southern Scandinavia. This air mass will be stable as expected by latest model output, and GFS indicates up to 45..70 m/s 0-6 km vertical wind shear in the warm sector. To the north of the surface cold front, models suggest only weak low-level CAPE underneath the anticyclonic flank of the jet streak, while deep instability is forecast at the cyclonic flank. While the nearly flow-parallel front will move south-ward pretty slow, stratiform precipitation is expected to be most likely in the range and north of the cold front. Showers and thunderstorms are forecast further north, from Scotland to Norway. Another region of convective development is forecast over south-western Mediterranean, where upper cut-off low and low-level moist maritime air mass will be present.

South-western Mediterranean

Weakening African cut-off low moves northward into south-western Mediterranean Sea. Temperatures of around -25°C at the 500 hPa level and low-level moisture advection from central Mediterranean are expected to increase the instability. Showers and thunderstorms are forecast to go on during the period, spreading northward into Balearic Islands and Corsica. Given moderate vertical wind shear with north-easterly surface winds and south-easterly winds at the 500 hPa level, reaching around 15 m/s 0-6 km shear, a few thunderstorms may organize. Especially near the coasts, there is also rather strong low-level vertical wind shear. So at least a marginal tornado threat is forecast. A level 1 is choosen because of rather widespread convective activity, making some severe events more likely. Severe wind gusts and large hail are not expected to be a significant threat.

Northern Scotland, western Norway region

Along the cyclonic flank of the strong jet streak, a few showers and thunderstorms are forecast to form in the cold air mass. Both 0-6 km and 0-1 km vertical wind shear will likely be rather high in the range of expected convection, especially near the coasts. Thunderstorms are forecast to form along small vort-maxima travelling eastward and may produce bowing segments, producing high winds. Shallow mesocyclones may develop, capable of producing a few tornadoes as well given rather high helicity values south of the surface low, where very strong synoptic winds are also likely.