Storm Forecast
Valid: Thu 16 May 2013 06:00 to Fri 17 May 2013 06:00 UTC
Issued: Wed 15 May 2013 22:31
Forecaster: GATZEN

A level 1 was issued for northern Tunisia and north-eastern Algeria mainly for large hail.

A level 1 was issued for north-eastern Poland, the southern Baltic States, and Belarus mainly for excessive rain and to a lower extend for large hail.

A level 1 was issued for the northern Ukraine mainly for large hail.

A level 1 was issued for south-western Russia mainly for large hail and severe wind gusts.

SYNOPSIS

The 500 hPa map show a deep trough that covers western Europe and a broad ridge over eastern Europe. A small cut-off low slowly moves into the western Black Sea. A unseasonable strong mid-level jet stream affects northern Africa and the central Mediterranean. Cold air advection continues across south-western Europe, whereas warm air masses across eastern Europe are advected into central and northern Europe with south-easterly winds.

DISCUSSION

Northern Tunisia and north-eastern Algeria

Dry and cool air masses are advected into northern Africa. However, a region with rich low-level moisture remains during the day across northern Tunisia and surroundings due to a diurnally driven thermal low further south. Ahead of the trough axis that enters the region in the evening hours, DCVA is expected to increase during the day. Additionally, daytime heating will result in weakening convective inhibition.

Initiation is expected across the mountains in the noon and afternoon hours. Strong vertical wind shear exceeding 15 m/s 0-3 km bulk shear and 25 m/s 0-6 km bulk shear together with favourable veering profiles will support supercells that move eastward. These storms will be capable of producing large hail and severe wind gusts. Additionally, tornadoes are not ruled out, but low-level vertical wind shear will be rather weak, so that threat will be lower compared to the last two days. In the evening hours, drier air will spread into this region and convective activity will decay from the west.

Eastern Europe

Latest soundings indicate an unstable air mass across western Russia, where the old convergence zone dissolves until Thursday. The very moist boundary-layer will start to spread westward again and will affect a region from the northern Ukraine to the southern Baltic States. Daytime heating will likely create CAPE in the order of 500 J/kg and weak WAA will lead to local QG lift.

Numerous thunderstorms are forecast. Weak vertical wind shear will limit storm organization, but pulse storms will be capable of producing large hail. Severe downbursts are not ruled out at the eastern edge of the thunderstorm region where latest soundings indicate deeply-mixed subcloud layers. Additionally the slow storm motion vector indicates a threat of excessive precipitation. In the evening hours, storms will likely cluster and weaken.

Northern Germany

Ahead of the west European trough, a convergence line has developed across central Germany. On Thursday, this line is expected to move westward as well. Remaining plume of boundary-layer moisture will affect central and especially northern Germany and southern Denmark.

Although mid-level warming will limit instability during the day, some thunderstorms may develop in the region with maximum WAA that will even increase in the evening hours along the nose of a developing low-level jet. Storms may cluster across northern Germany and southern Denmark and will move into the North Sea later on. Although weak mid-level shear will limit severe potential of these storms, the increasing low-level shear indicates a risk of tornadoes in the evening hours. Severe potential will rapidly decrease during the night.

West Mediterranean

On late Thursday, a trough axis crosses the west Mediterranean Sea. In the wake of this trough, an intense jet streak enters the west Mediterranean from the south-west, leading to QG forcing. The affected air mass is relatively dry, but latest model output suggests that well-mixed cool air masses from the Iberian Peninsula will overspread the maritime boundary layer. Latest GFS and ECMWF indicate that thunderstorms will develop. Given the strong deep layer vertical wind shear, multicells and supercells are forecast, capable of producing large hail.