Storm Forecast
Valid: Thu 20 Nov 2008 06:00 to Fri 21 Nov 2008 06:00 UTC
Issued: Wed 19 Nov 2008 23:34
Forecaster: GATZEN

SYNOPSIS

To the east of strengthening high over the northern Atlantic, an intense outbreak of arctic air sets in over Europe. During the period, very low geopotential and unseasonably cold mid-level air mass will spread across Scandinavia, North Sea, and Baltic Sea region in the wake of a strong surface low pressure system moving eastward into western Russia. The cold front of this low will be located from south-western British Isles to southern North Sea and Baltic Sea and further to the Baltic States. It will spread south-eastward rapidly. At low levels, another cold front will spread southward across the North Sea at the end of the period.

To the south of the large north-European long-wave trough, rather low geopotential covers most of the Mediterranean, where rather cool air masses are present.

DISCUSSION

Western Russia region

Ahead of the approaching cold front, models indicate a strong westerly low-level jet pointing towards Belarus and western Russia on early Thursday. Relatively moist air mass is expected in the range of this low-level jet. A strong midlevel jet streak will be placed from Poland to Belarus in the morning hours, and quite strong QG forcing can be expected especially from northern Belarus to western Russia, where warm air advection and differential cyclonic vorticity advection are forecast.

Affected air mass will likely be characterized by neutral lapse rates due to low-level south-westerly moisture advection and lift. Weak instability is expected to develop underneath the cyclonic flank of the mid-level jet streak, where rather cold mid-levels and rather warm/moist low-levels will overlap. However, limiting factor is cold boundary-layer actually situated over western Russia, and instability will likely be elevated. Nevertheless, current thinking is that at deep convection will be present along the convective line ahead of the cold front that may produce some isolated thunder from Latvia to western Russia in the morning hours. With strong vertical wind shear, severe wind gusts and small hail are not ruled out, as well as isolated tornadoes. Threat seems to be quite low given weak low-level buoyancy. Weakening instability will limit chance for deep moist convection later on.

Southern North Sea

Late in the period, instability over the North Sea will increase as cold mid-level air mass spreads across the warm sea surface. Expect neutral to slightly unstable lapse rates just to the north-east of very strong mid-level jet streak situated over British Isles and Belgium at the end of the forecast period that will also be associated with some lift over the North Sea, and deep convection will likely develop ahead of the arctic cold front moving southward in the wake of a surface pressure trough. Given quite strong synoptic wind field, moderate to strong low-level vertical wind shear is expected near the coasts of Benelux and western Germany. Thunderstorms may produce isolated severe wind gusts and soft hail, and isolated tornadoes are not ruled out. Severe potential is rather low, though.