Storm Forecast
Valid: Wed 05 Dec 2007 06:00 to Thu 06 Dec 2007 06:00 UTC
Issued: Tue 04 Dec 2007 19:33
Forecaster: GATZEN

SYNOPSIS

Rather high geopotential is present over Europe east of a long-wave trough over northern Atlantic. A well-developed cut-off low is centered over Aegean Sea. During the period, a strong short-wave trough will rapidly cross the British Isles and the North Sea. In the wake of this trough, a very strong jet streak curves around the flank of central Atlantic subtropical high, spreading south-eastward into British Isles and France during the period.

DISCUSSION

Southern Turkey

At the eastern flank of the Aegean cut-off low, a tongue of rather moist air mass characterized by rather steep low-level lapse rates will remain over southern Turkey during the period, and CAPE is expected. Given up-slope flow and convergence along a frontal boundary moving eastward, thunderstorms are forecast to spread eastward along the southern coast of Turkey. Although most of the convection will already have left the forecast region, it is worth to say that strong vertical wind shear will be present in the range of the frontal boundary, and storms will likely organize into multicells and mesocyclones given favorably veering profiles and up to 150 J/kg 0-3 km SRH values. An isolated tornado as well as locally hail and severe wind gusts are not ruled out.

Aegean

Underneath the cut-off center, low-level air mass will likely be unstable given warm sea surface and cold air mass/steep lapse rates aloft. As vertical wind shear will be weak, waterspouts are forecast along the coasts.

British Isles

A cold front will cross the eastern British Isles in the morning hours. There are no indications that deep instability will develop in the warm sector air mass as QG forcing of the following trough will be further west. A mostly non-thundery narrow band with embedded convection is forecast, but will not rule out an isolated severe wind gust or tornado report along this line given strong vertical wind shear and favorably veering profiles with 0-1 km SRH values in excess of 200 J/kg. To the west, rapidly moving upper trough is forecast to spread across the British Isles, with the deepest instability over the northern regions. Showers and thunderstorms are forecast in the range of well-mixed polar air mass. Given strong low-level vertical wind shear in excess of 10 m/s and a strong background wind field, severe wind gusts are not ruled out with this convection. As low-level SRH values are also quite favorable reaching 160 J/kg, will not exclude a few mesocyclones, capable of producing large soft hail and maybe a few tornadoes. Threat seems to be not too high, but will draw a level 1 in the region mostly affected by CAPE and low-level vertical shear. Thunderstorms will rapidly weaken in the evening/night hours in the range of the following WAA regime.