Storm Forecast
Valid: Thu 05 Jan 2012 06:00 to Fri 06 Jan 2012 06:00 UTC
Issued: Wed 04 Jan 2012 19:28
Forecaster: GATZEN

A level 2 was issued for northern and central Germany and parts of Austria mainly for severe wind gusts.

A level 1 was issued for the Benelux countries, Germany, western Poland, eastern France, the Czech Republic, and the Alpine region mainly for severe wind gusts.

A level 1 was issued for the Adriatic Sea and surroundings mainly for severe wind gusts.

+++Adjustment of the level 2 header, deleting Switzerland+++

SYNOPSIS

At the western flank of low geopotential over Scandinavia, another strong vort-maximum will race across Central Europe into the Mediterranean. The associated 70 m/s mid-level jet streak will nose into eastern Germany at Thursday morning. As the short-wave trough amplifies, the jet axis will rapidly cross the Alps in the afternoon and evening hours.

At lower levels, models predict a deepening low that moves to the southern Baltic Sea region. South of the Alps, a second intense low will develop centered over the southern Aegean at the end of the forecast period.

DISCUSSION
The Benelux countries, Germany, Alps, and surroundings
Latest models indicate an intense synoptic-scale situation across central Europe associated with 850 hPa winds of 40-50 m/s ahead of a cold front and 35-40 m/s near an upstream back-bent occlusion.

Ahead of the cold front, low-level moisture seems to be rather limited given 5 to 7 g/kg mixing ratio over northern Germany. However, strong lift can be expected especially at the cyclonically sheared nose of the mid-level jet and associated dry intrusion over northern and eastern Germany.

Together with strong low-level forcing at the leading gust front, a strongly-forced convective line is expected. Given 800 m²/s² 0-1km SRH ahead of the convective line, conditions for rotating updrafts seem to be rather favorable, especially near bowing segments. Despite the weak low-level buoyancy due to the cool and rather dry boundary-layer, conditions seem to be supportive for tornadoes that may be strong.

The main threat will be severe wind gusts that will likely occur along the gust front. The best potential is expected at the northern portion of the cold front thats projection perpendicular to the mean wind vector is greatest. A swath of damaging surface wind gusts is forecast that may exceed 33 m/s. After 6 UTC, convection will likely weaken given decreasing low-level moisture and forcing as the mid-level jet streak spreads southward.

Further west, the cold front becomes more parallel to the mid-level flow but will continue southward as the mid-level trough expands towards the Alps. The tongue of higher low-level moisture and the mid-level forcing at the cyclonically-sheared flank of the jet streak are expected to create weak CAPE sufficient for a convective line that reaches the Alps in the afternoon hours. Severe wind gusts and isolated tornadoes are forecast given the strong vertical wind shear.

In the wake of the cold front, models indicate instability near the occlusion of the low that enters the Benelux countries and western Germany in the noon/afternoon. The main uncertainty will be the low-level moisture that is expected to be around 4 g/kg in the lowest km. The lapse rates will also likely decrease as the rather warm occlusion approaches.

Current thinking is that showers and thunderstorms may develop near the coasts of the North Sea where low-level heating and lapse rates will be best that will weaken over land. Organized storms are not expected given the rather weak low-level forcing and buoyancy. However, showers will well contribute to the vertical momentum transport through-out the period.

Adriatic Sea and surroundings

As the trough expands into the Mediterranean the jet axis moves across the Adriatic Sea. Strong DCVA will lead to lift and steepening lapse rates, while the low-level moisture of 6-7 g/kg will only slowly decrease in the wake of the Alps. CAPE will likely develop across the Adriatic Sea and thunderstorms are forecast to develop around midnight. These storms will spread southward ahead of a developing cold front that is expected over the southern Adriatic region at the end of the period.

Strong vertical wind shear will overlap the instability to the south of the Alps. 20 m/s 0-6km and 15 m/s 0-3km vertical wind shear are forecast and storm organization is rather likely. Multicells and mesocyclones will mainly pose a threat of severe wind gusts given 20-25 m/s winds at 850 hPa. However, locally large hail and a tornado are not ruled out.