Storm Forecast
Valid: Thu 19 May 2011 06:00 to Fri 20 May 2011 06:00 UTC
Issued: Wed 18 May 2011 20:47
Forecaster: GATZEN

A level 1 was issued for northern Algeria mainly for large hail.

SYNOPSIS

A cut-off low moves from the Aegean Sea to southern Turkey. Another weak trough is situated over Iberia and the north Mediterranean. Weak mid-level dynamics are therefore expected over most of Europe. From the British Isles to Scandinavia, short-wave troughs travel eastward.

DISCUSSION

Central Europe

South-westerly winds advect rather warm air masses currently over the west Mediterranean and Spain towards the Alps and Poland. Mid-level lapse rates about 6K/km are expected in the 2-4km layer. In the boundary-layer, models suggest that moisture will slightly increase from France to Poland to 8 g/kg in the lowest km. From the morning on, a low-level cold front is expected to travel south-eastwards and is forecast to reach a line from central France to eastern Germany at noon and will move on in the afternoon hours.

Current thinking is that instability will develop ahead of this cold front. As a weak trough moves eastward during the day and daytime heating will increase the low-level temperature, lapse rates will likely improve in the afternoon. Latest GFS model suggests 30 hPa MLCAPE from 500 to 1000 J/kg. Thunderstorms are expected to develop ahead and along the cold front in a relatively large convergence zone from France to northern Poland. Latest models do not indicate favourably veering profiles ahead of the cold front, and organized storms are not forecast. However, some stronger pulse storms may be capable of producing large hail locally, especially near the Alps, where lapse rates are expected to improve in the evening hours.

Northern Algeria

At the southern flank of the European low geopotential, a strong mid-level jet streak spreads into Algeria. At lower levels, a frontal boundary will be placed over northern and western Algeria. To the north of this front, a moist and rather cool low-level air mass is expected to become unstable in response of diurnal heating. Mid-levels will be likely dominated by steep lapse rates given southerly winds atop of the boundary-layer. Some thunderstorms are expected to develop, especially where moist low-level air will be lifted due to upslope flow over the Atlas mountains. Given 15 m/s 0-3 km vertical wind shear storms will likely organize. Supercells may develop capable of producing large hail and severe wind gusts. A tornado is also not ruled out. Storms are expected to cluster and may become elevated in the night hours as the boundary-layer cools.

Iberia, Italy, Balkans to Turkey

Thunderstorms are forecast to develop in weak vertical wind shear. Stronger pulse storms may produce isolated large hail or excessive precipitation.