Storm Forecast
Valid: Thu 22 Sep 2011 06:00 to Fri 23 Sep 2011 06:00 UTC
Issued: Wed 21 Sep 2011 15:44
Forecaster: GATZEN

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

A level 1 was issued for northern Algeria mainly for large hail and severe wind gusts.

SYNOPSIS

The cut-off low over the east Mediterranean moves slowly east into Turkey. While the trough starts to weaken, the strong mid-level flow at is southern flank decreases. In the wake of the trough, a broad ridge has spread into the west and central Mediterranean. Embedded weak thermal troughs are expected to the south of France and to the north-west of the Iberian Peninsula. At the northern flank of the Mediterranean ridge, an intense trough moves eastward over Scandinavia.

DISCUSSION

Turkey and surroundings

CAPE is expected to weaken rapidly from the west over Turkey behind a cold front moving into east Turkey. This is associated with decreasing low-level moisture and weak lapse rates. However, latest model output indicates easterly surface winds along the southern coast of Turkey, so that the low-level moisture cannot be replaced at once. Together with upslope flow and QG lift due to approaching trough, conditions are expected to be favourable for some thunderstorms in the morning hours as the cool mid-level air mass will be likely unstable above the warm sea surface.

Thunderstorms that form may organize when easterly surface wind will materialize. Mesocyclones are not ruled out capable of producing tornadoes. However, limiting factors are the rather weak low-level moisture expected behind the cold front and probably weak low-level lapse rates in the cold air advection regime.

Farther north, a moist north-easterly flow is expected at the northern flank of the cut-off low. This moist and rather warm air mass originating from the Black Sea will likely become unstable during the day given the cold mid-level air mass. Lift can be anticipated as the trough centre moves across the region and upslope flow will be present at low-levels. Thunderstorms are forecast that can become organized given moderate vertical wind shear of about 10 m/s in the lowest 3 km. Local large hail is forecast to be the main threat.

North Algeria

An intense frontal boundary is forecast over the Atlas mountains, separating a well-mixed air mass over the Sahara from cooler Mediterranean air masses. During the day, diurnal heating will contribute to upslope northerly flow along the coasts that will advect rich low-level moisture into the Atlas. An overlap of the moist low-level air with the well-mixed air is likely along the frontal boundary, where favourably veering profiles are likely as well.

Current thinking is that some thunderstorms will develop over the mountains and will likely evolve into supercells. Very large hail and severe wind gusts are possible. Given the dry air mass to the south of the Atlas, the storms will likely decay rather quickly.

East Scandinavia

An intense short-wave trough migrates eastward across Scandinavia and the Baltic Sea. Strong QG lift is forecast along the trough axis. While the warm air mass ahead of the cold front is forecast to be likely stable, the lift of the polar air mass will likely lead to neutral lapse rates that can become slightly unstable in response to diurnal heating over Sweden and further diabatic heating over the warm sea surface. Current thinking is that numerous showers and thunderstorms will develop along the trough axis that spread eastward. Especially near the base of the trough, strong vertical wind shear may support storm organization, and some bowing segments are well possible, capable of producing severe wind gusts. Given only 20-25 m/s wind speed at 850 hPa, the threat seems to be too low for a risk level at this time. A tornado and marginally severe hail are not ruled out. Convective activity is expected to decay during the night hours over western Russia.