Storm Forecast
Valid: Thu 28 Jul 2011 06:00 to Fri 29 Jul 2011 06:00 UTC
Issued: Wed 27 Jul 2011 21:57
Forecaster: GATZEN

A level 1 was issued for the central Mediterranean, Italy, and the central Balkans mainly for large hail and severe wind gusts.

A level 1 was issued for western Russia, Belarus, and the southern Baltic States mainly for excessive precipitation and large hail.

SYNOPSIS

A trough quasi-stationary over central Europe yields an unseasonably deep jet over the Mediterranean. At the surface, low pressure centres are located over southern and eastern Europe. The warm subtropical air has been pushed eastward to Turkey and Russia.

DISCUSSION

Central Mediterranean to central Balkans

A short-wave trough curves around the southern flank of the European trough and spreads across the southern Mediterranean on Thursday. This is expected to yield warm air advection from Italy to the Balkans and QG forcing will spread eastward during the period. The affected air mass is characterized by a pool of rich boundary-layer moisture from the Tyrrhenean Sea to the Adriatic Sea and the central Balkans, while mid-level lapse rates are forecast to be quite steep near the cold centre of the trough. Latest GFS model guidance expects CAPE of more than 1500 J/kg that may materialize over the land masses with strong diurnal heating.

Current thinking is that numerous storms will go on over the Mediterranean during the period, starting over the sea near convergence lines of the land-see-breeze and spreading to the larger islands and Italy during the daytime. In the afternoon hours, increasing convective activity is also expected over the Balkans. The storms may easily organize given strong vertical wind shear of 10-15 m/s 0-3km and 15-20 m/s 0-6km bulk shear. Large hail and severe wind gusts are forecast with the stronger storms that may merge to bowing lines. Excessive rain may be possible with mesoscale convective systems, while tornadoes are not ruled out but will pose a minor threat. The un-focussed convective development prevents a level 2 at this time that may be required in an update on Thursday.

Western Russia to the Baltic States

To the east of the European trough, a very warm and well-mixed subtropical air mass is advected north-westward into the Baltic States. Near the frontal boundary to colder air masses over central Europe, a broad zone of rich low-level moisture has developed from the eastern Ukraine to western Russia, eastern Belarus, and the Baltic States. An overlap of this moisture and steep mid-level lapse rates is expected by latest models that indicate CAPE in the order of 2000 J/kg.

On Thursday, QG forcing is forecast as a short-wave trough currently over the Balkans will spread north-eastward into western Russia. Thunderstorms are forecast especially along the frontal boundary. While the mid-level winds are quite weak, the storms will move slowly and will pose a threat of excessive precipitation. Additionally, large hail is forecast with the stronger storms that may also produce isolated severe wind gusts. The storms will likely go on over the northern portions of the risk area where warm air advection and associated QG forcing will go on during the night hours.

Farther east, isolated thunderstorms are expected especially in the evening hours when the mid-level trough spreads across western Russia, leading to increasing forcing. These storms will be capable of producing isolated large hail, severe wind gusts, and excessive precipitation.

Norway, western Germany, Alps

Near the main axis of the trough, cold mid-level air masses and diurnal heating will create steep lapse rates that will allow for thunderstorms given moderate boundary-layer moisture. Weak vertical wind shear will be present what limits the severe potential. An isolated severe weather event is not ruled out completely, as small-scale variations of the thermodynamic profile may assist some stronger storms.