Storm Forecast
Valid: Thu 22 Apr 2010 06:00 to Fri 23 Apr 2010 06:00 UTC
Issued: Wed 21 Apr 2010 21:59
Forecaster: GATZEN

A level 2 was issued for northern Algeria and Tunisia mainly for excessive rain and very large hail.

A level 1 was issued for Bulgaria mainly for large hail.

SYNOPSIS

The axis of an intense long-wave trough turns north-eastward over northern Europe. In the wake of this trough, a weak ridge moves eastward over the Mediterranean. To the west, a trough present over Iberia slowly moves eastward. Cold and dry polar air has spread across most of northern and central Europe that is stable in the south-western regions due to mid-level sinking. Diurnal instability is most likely underneath the trough axis over western Russia, Belarus, and the northern Ukraine. Rather cool low-level air is also present over the Mediterranean, and rather warm air spreading northward ahead of the Iberian trough will likely lead to further stabilization over most places.

DISCUSSION

Algeria and Tunisia

The trough over south-western Europe affects northern Africa during the period, given a strong mid-level jet streak that curves around this trough and spreads across central Algeria into Tunisia during the day. Models do indicate precipitation over the northern portions of Algeria and Tunisia in the warm air advection regime that will likely lead to increasing boundary-layer moisture. During the day, the warm air advection will likely increase as dry and well-mixed desert air spreads into central Algeria. To the north of this air mass, quite large CAPE can build underneath the cap given the rich low-level moisture.

In the afternoon hours, a cold front with several embedded waves is forecast to enter the area. Lift is expected due to DCVA and warm air advection with local enhancement due to upslope flow and outflow boundaries of early showers. Initiation seems to be quite likely. Storms that develop will have a large potential to organize given about 25 to 30 m/s deep layer vertical wind shear, and 100 to 250 m/s 0-3km SRH. Supercells are forecast capable of producing very large hail (5 cm) especially over Algeria, where initiation is most likely. Severe wind gusts are also possible. The chance of tornadoes gradually increases further east in the evening hours, when the low-level vertical wind shear increases to 10 m/s. Limiting factor is that the low-level buoyancy may be limited due to rather dry low-level air over Tunisia, but even strong tornadoes may develop given the expected good dynamics. Later in the period, storms are forecast to cluster along the wavy cold front and may move north-eastward along the frontal boundary, what enhances the potential of excessive rain and local flash floods.

Bulgaria

In the range of the east-European trough axis, a tongue of rather moist low-level air remains to the south of the cold front across Bulgaria. Diurnal heating and mid-level cool air mass will likely lead to some instability release, and given weak low-level convergence, storms are expected to develop. These may organize underneath the mid-level jet given about 20 m/s deep layer vertical wind shear, and multicells seem to be likely, capable of producing isolated large hail. The weak low-level vertical wind shear is forecast to limit the potential of tornadoes significantly. The storms will likely decay in the evening hours as the mid-level trough moves eastward and sinking will likely reduce the instability.

Western Russia

Ahead of the trough axis of the east-European trough, a tongue of rather moist low-level air is present from the northern Ukraine to eastern Belarus and western Russia. This air mass may become instable due to diurnal heating. Along the approaching cold front, thunderstorms are forecast. While the vertical wind shear will be weak, severe thunderstorms are not forecast.

Iberia

Underneath the western trough, the cool but rather moist low-level air will become unstable due to diurnal heating, and widespread showers and thunderstorms are forecast. Although most of the storms will be single cells, an isolated large hail event or brief tornado (land spout-type) are not ruled out. Storms will rapidly weaken after sunset.