Storm Forecast
Valid: Thu 05 May 2011 06:00 to Fri 06 May 2011 06:00 UTC
Issued: Wed 04 May 2011 21:54
Forecaster: GATZEN

A level 1 was issued for western Russia and the eastern Ukraine mainly for severe wind gusts.

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

SYNOPSIS

Unseasonably cold air has spread into central Europe near the center of a mid-level trough. A frontal boundary is expected to the east over Russia. A ridge amplifies over western Europe, whereas a rather strong south-westerly jet affects north-western Europe.

DISCUSSION

East central Europe

The axis of the polar trough will move eastward over Poland. The cold air mass will spread into western Russia, the western Black Sea region, and the northern and central Balkans. This air mass is characterized by very dry low-levels that will be well-mixed due to daytime heating. Showers and thunderstorms are expected especially underneath the trough axis, where mid-levels will be coldest and low-level convergence is also likely. The poor moisture and instability are forecast to lead to a very low threat of severe weather.

Western Russia, eastern Ukraine

Ahead of the cold front, a well-mixed boundary-layer is present over western Russia and the eastern Urkaine. Given some low-level moisture, CAPE of a few hundreds J/kg is expected on Thursday along the cold front where a strong wind-shift yields low-level convergence. These storms will likely be quite poorly organized as the vertical wind shear will be weak. However, latest Suhinici sounding indicates strong DCAPE on Wednesday and these profiles will likely remain on Thursday. Showers and thunderstorms are therefore expected to pose a threat of strong microbursts. Severe wind gusts are not ruled out with the showers, warranting a marginal level 1. In the evening, decreasing instability is expected to lead to decaying convection.

Turkey, Greece

Ahead of the cold air advection regime over eastern Europe, a zone of low pressure stretches across Turkey, Greece, and the Adriatic Sea. Soundings indicate a broad region where sufficient boundary-layer moisture overlaps steep lapse rates, and CAPE is forecast to build in response to diurnal heating. Besides the synoptic-scale convergent flow, sea breeze and orographic lift will likely assist initiation. Numerous storms are forecast in the weakly sheared environment. Stronger pulse storms may be capable of producing severe hail. The overall situation does not warrant a categorical risk area.

Southern and eastern Spain

The amplifying ridge will affect south-eastern Iberia on Thursday. Below the relatively warm mid-levels, inverted-v profiles have developed over south-eastern Spain. Easterly surface winds will lead to some moisture advection from the Mediterranean Sea into Spain during the day, and daytime heating is expected to result in a few hundreds J/kg mean layer CAPE. Upslope flow and convergence over the mountains are expected to lead to initiation in the noon/afternoon hours. Weak vertical wind shear will limit the severe potential, but locally severe wind gusts are not ruled out.

British Isles

A quite warm and moist air mass advects into the British Isles with southerly low-level winds to the east of an Atlantic low pressure system. Latest Valentia sounding indicates nearly neutral lapse rates that will easily allow for weak CAPE due to daytime heating. Latest model output indicates weak instability over the southern and western British Isles on Thursday afternoon. Sea breeze convergences are forecast to assist initiation that will become more likely near a mid-level jet streak pointing towards Ireland. The severe threat is forecast to increase in the western parts where rather strong south-westerly winds are expected. Especially in the evening hours models suggest increasing low-level vertical wind shear (SRH 100 m²/s² in the lowest kilometre) that will overlap with some weak CAPE. This may support tornadoes, and some events are not ruled out. Limiting factor will be the rather weak low-level convergence, and a level 1 threat is not forecast. Storms are expected to weaken after sunset.

Atlas mountains

Orographic lift of moist Mediterranean air masses is expected to lead to initiation in the afternoon hours. Storms may organize given 10-15 m/s deep layer bulk shear. Favourably veering profiles are forecast by latest models in the evening hours, and a few supercells are not ruled out capable of producing large hail and severe wind gusts. Storms will decay in the evening/night hours.