Storm Forecast
Valid: Thu 05 Mar 2009 06:00 to Fri 06 Mar 2009 06:00 UTC
Issued: Wed 04 Mar 2009 22:57
Forecaster: GATZEN

SYNOPSIS

A deep long-wave trough is placed across western and southern Europe. At its western flank, a strong mid-level jet streak moves into Portugal region and further into northern Algeria. The trough is filled with well-mixed polar air mass characterized by lapse rates of more than 8K between from 2000 to 4000 metres over British Isles, Bay of Biscay, eastern Iberia and west Mediterranean Sea. Polar air mass is relatively dry at low levels. To the south and east of this cold air mass a couple of cold front occlusions are present moving across west Mediterranean Sea, southern Mediterranean, and the Balkans.

DISCUSSION

Bay of Biscay, eastern Iberian Peninsula, and west Mediterranean

Focus of expected convective activity will be mid-level troughs filled with deeply mixed polar air mass. The cold front occlusions ahead of these troughs may also lead to convection, but instability will be limited due to rather poor lapse rates. The easternmost trough axis is expected to cross the Balkans and Adriatic from the south in the morning hours. Given rather moist low-level air mass due to southerly winds, some instability will likely develop in the range of the Mediterranean Sea. Although widespread clouds are expected given quite strong QG forcing and moist air mass, embedded convection is forecast. Chance for some lightning is highest along the southern coasts where upslope flow will likely lead to some additional low-level forcing. Severe threat is limited given weak low-level instability, but strong mid-level winds may pose a risk of some organized storms. Sever wind gusts and tornadoes are not completely ruled out in the morning hours. Low thread will even decrease during the day, and a threat level seems to be not necessary.

Central Mediterranean

Another trough axis follows to the west and crosses the central Mediterranean. Polar air mass will likely be well-mixed, but boundary-layer moisture will be quite dry as indicated by latest observations across the Iberian Peninsula. Instability will likely be limited. Given low-level cold air advection, strong forcing is also not expected. Some showers may develop, and the chance for waterspouts is not forecast to be significant although weak vertical wind shear and good low-level lapse rates will be present.

Bay of Biscay to south Mediterranean

A third upper trough axis will move from the Bay of Biscay across the Iberian Peninsula and west Mediterranean into northern Algeria. It is also accompanied by a strong mid level jet streak moving south-eastward. Strong DCVA is forecast underneath the cyclonic flank of the mid-level jet. At lower levels, cold air mass spreads southward across the Iberian Peninsula behind a cold front occlusion that crosses northern Africa during the period. This air mass will be characterized by steep low- to mid-level lapse rates in excess of 8 K/km. Although low-level moisture will be rather limited, diurnal heating will likely contribute to significant buoyancy and CAPE in the order of a few 100s J/kg. Showers and thunderstorms are forecast to spread southward during the period and will likely produce soft hail. Given strong vertical wind shear underneath the jet streak, organized convection is possible and linear segments are forecast capable of producing severe wind gusts. Although tornadoes are not ruled out completely, chance seems to be low given rather poor low-level moisture. Best potential seems to exist near the casts, where low-level moisture is more favourable.

Over the Mediterranean, additional thunderstorms will likely develop lat in the period and spread southward reaching the Algerian coast. Better moisture and good low-level buoyancy will overlap with strong low-level vertical wind shear and forcing, and thunderstorms may be severe capable of producing severe wind gusts. Tornadoes and marginally severe hail are not ruled out, but potential is limited given weak low-level veering profiles.

Greece, Aegean, western Turkey

Cold front that has crossed most of the Mediterranean will become parallel to the flow over Greece during the period. While southerly low-level winds will advect rather moist air masses, a mid-level trough moves northward across the region, leading to some QG forcing. Given mid-level cooling and steepening lapse rates, instability will likely develop along nd ahead of the cold front. Embedded convection is forecast specially along the western coasts, where low-level convergence is expected. Strong low-level vertical wind shear near the coasts is likely, and a few rotating storms are not ruled out as well as bowing segments embedded in the cold front. Chance of tornadoes is slightly enhanced until the evening hours, while the threat will likely decrease from the west during the night. Severe wind gusts and large hail seem to be rather unlikely.