Storm Forecast
Valid: Thu 11 Dec 2008 06:00 to Fri 12 Dec 2008 06:00 UTC
Issued: Wed 10 Dec 2008 18:36
Forecaster: GATZEN

SYNOPSIS

A sharp, amplified trough is situated over western Europe. Initially, it will reach well into northern Africa, while it becomes negatively tilted and turns to the north-east until Friday morning. This will be associated with cyclogenesis over the central Mediterranean. Warm air mass ahead of the deepening low is expected to be unstable and thunderstorms are forecast. To the north and east, widespread high pressure covers most of Europe, where stable air mass is present. Over western Europe, cold and relatively well-mixed air mass is present in the range of the trough axis and advects into the western Mediterranean.

DISCUSSION

Central Mediterranean to Aegean

Latest soundings from the central Mediterranean indicate that warm air mass ahead of the trough axis is characterized by rich low-level moisture, weak CIN, and also steep low- to mid-level lapse rates, associated with MLCAPE up to 1000 J/kg. This air mass will likely advect further north ahead of the surface trough that is expected west of Italy on Thursday noon. During the period, strong lift is forecast over the central Mediterranean as the trough axis will turn to the north-east, and models indicate that the surface low will deepen.

Latest Models indicate that the cold front will be rather far to the west initially and will accelerate during the period. Given expected strong lift in the range of the trough axis overspreading the warm sector, warm air mass will likely be unstable, and widespread deep moist convection is forecast that may be embedded in stratiform clouds.

In detail there seem to be two regimes with convection ahead of the surface cold front. To the east, models indicate a very strong low-level jet evolving in the warm air advection regime ahead of the surface trough. Winds at the 850 hPa level are forecast to increase to 25 to 30 m/s, indicating that convective downdrafts will have a great potential of producing severe wind gusts. Although instability is limited in the warm air advection regime, strong lift and low-level convergence may be sufficient for initiation and embedded showers are forecast. These will have the potential of producing severe wind gusts and also some tornadoes as low-level wind shear will likely be associated with locally strong veering profiles in the range of the coasts. Lightning activity is not ruled out with stronger storms, but warm equilibrium level limits lightning activity. Highest potential for severe thunderstorms exists in the range of the northern Aegean late in the period. Intense precipitation and local flash flooding is also possible.

To the west of the warm air advection regime, latest GFS indicates that warm and moist low-level air mass initially remains in the range of the surface trough, while cold and drier air masses follow further west. Given weak CIN and rather strong low-level convergence underneath the trough axis, widespread deep moist convection is forecast. Relatively weak vertical wind shear, low-level convergence, and rather high low-level buoyancy may create a favourable environment for waterspouts. Intense precipitation is another threat associated with this convection. Later in the period, dry low-level air mass floods eastward destroying convection.

South-western North Sea

Underneath the trough axis, convection has formed over the North Sea that will go on during the period. Given a rather cold equilibrium level, isolated thunderstorms are not ruled out. Weak vertical wind shear and low-level convergence may be favourable for some waterspouts, but threat will be low.