Storm Forecast
Valid: Thu 01 Oct 2009 06:00 to Fri 02 Oct 2009 06:00 UTC
Issued: Wed 30 Sep 2009 13:24
Forecaster: GATZEN

A level 1 was issued for the Tyrrhenian Sea mainly for excessive rain.

SYNOPSIS

A mid-level trough over Scandinavia evades westward and southward during the period. An intense short-wave trough propagates eastward across the Baltic Sea and will amplify over eastern Europe, leading to cyclogenesis to the east of the Baltic Sea. Most of western and central Europe will be influenced by the anticyclonic flank of the strong mid-level jet surrounding the long-wave trough. Over the Mediterranean, the mid-level low over Iberia travels eastward affecting the Tyrrhenian Sea. At lower levels, moist air masses are advected into the Baltic States and eastern Europe ahead of a cold front, while cold air advection affects most portions of western and central Europe in the wake of the short-wave trough. Weak cold air advection is also likely in the wake of the Mediterranean trough, while warm air advection takes place over the east Mediterranean Sea.

DISCUSSION

Central Mediterranean

Ahead of the approaching mid-level trough, latest soundings indicate rich low-level moisture over the central Mediterranean. The warm sea surface and lift ahead of the trough will likely produce steep lapse rates spreading eastward on Thursday. At the surface, models indicate convergence over the Tyrrhenian Sea along a weak cold front that is expected to be the focus of deep moist convection.

Thunderstorms will likely go on and spread eastward ahead of the trough. The weak vertical wind shear as well as limited low-level forcing will be a limiting factor for storm organization, and well organized convection is not expected. Some storms may produce excessive rain given the slow propagation and high moisture content of the boundary-layer. The potential seems to increase along the western coasts of Italy, where upslope flow may favour convection in the afternoon and evening hours.

To the west, weak northerly winds are forecast. Weak low-level convergence will likely reduce the chance of intense convection significantly. Some storms may develop over the warm sea surface in the wake of the mid-level trough, though.

Baltic Sea region

Ahead of the approaching intense surface low pressure system, a tongue of moist low-level air spreads north-eastward. Models seem to agree that stable lapse rates are present ahead of the cold front that travels into western Russia and the Ukraine in the afternoon hours, and deep convection is not expected along the cold front over most places. The best potential seems to exist over the central Baltic States along the nose of the low-level moisture tongue. While the mid-level jet streak spreads eastward across Poland, strong QG forcing is likely ahead and along of the surface cold front. As a consequence, neutral to slightly unstable lapse rates may develop due to a thermal indirect circulation. A narrow convective line is forecast to spread eastward along the cold front that may be weakly electrified. Isolated severe wind gusts and tornadoes are not completely ruled out given the rather strong low-level jet ahead of the cold front. Limiting factor is the uncertainty of convective instability.

In the wake of the front, models indicate another convergence line underneath the trough axis of the mid-level trough that propagates south-eastward across the southern Baltic Sea in the afternoon hours. To the south-east of this convergence line, low-level instability will likely develop due to steepening lapse rates underneath the cyclonic flank of a strong mid-level jet streak spreading south-eastward. The low-level moisture will be rather high over the warm sea surface.

Numerous thunderstorms are expected to develop over the Baltic Sea underneath the trough axis and will likely spread south-eastward into Poland and the southern Baltic States during the period. Given quite strong low-level vertical wind shear, a few embedded mesocyclones are not ruled out, capable of producing a tornado or severe wind gusts given 20 m/s westerly winds at the 850 hPa level. The overall potential of severe storms seems to be quite weak, though.

Southern Alpine region

Rather moist low-level air and steepening lapse rates ahead of the approaching mid-level trough as well as some diurnal heating may result in weak CAPE in the afternoon hours. Upslope flow and warm air advection may be sufficient for initiation. Severe thunderstorms are not forecast given weak westerly mid-level winds and associated weak vertical wind shear.