Storm Forecast
Valid: Thu 06 Aug 2015 06:00 to Fri 07 Aug 2015 06:00 UTC
Issued: Wed 05 Aug 2015 21:48
Forecaster: GATZEN

A level 2 was issued for north-eastern Spain mainly for large or very large hail.

A level 1 was issued for eastern Spain, southern and central France mainly for large hail and to a lesser extend excessive rain and severe wind gusts.

A level 1 was issued for the Black Sea region, Aegean, Turkey, Balkans, southern Italy mainly for large or very large hail, excessive precipitation, and to a lesser extend waterspouts and severe wind gusts.

SYNOPSIS

Quasi-persistent polar long-wave trough across the north-east Atlantic and a strong subtropic high over the west Mediterranean that ridges into west Russia yield a strong west-southwesterly mid-level flow from the Atlantic into the Bay of Biscay and further into southern Scandinavia. A plume of well-mixed hot air over the Atlas mountains is advected into south-western and central Europe. To the east of this plume, colder air forms a positively tilted long-wave trough over south-eastern Europe. A weak cut-off trough will slowly move southward into Romania during the period. At low levels, moisture is best across the east and central Mediterranean, across the southern Black Sea, and to the south and east of the Iberian Peninsula. Rather good moisture is also present along a frontal boundary from the Bay of Biscay across central France into the Alps and Germany, Poland, and the Baltic States.

DISCUSSION

South-eastern Europe into the southern Mediterranean

Main focus of convective activity will be the mid-level trough that extends across south-eastern Europe. Latest soundings indicate steep lapse rates over most places, overlapping with adequate moisture to produce CAPE. In the Mediterranean, high CAPE is present due to rich low-level moisture.

Diurnal heating results locally in inverted-v profiles across eastern Europe, where CIN will be pretty weak. Outflow boundaries, sea-breeze convergences, and upslope flow will allow for early initiation of storms.

Overall vertical wind shear is low, and storms are forecast to cluster along the cold pools. Better low-level shear is expected from the Black Sea to the Aegean due to a easterly low-level jet that intensifies in the evening hours. 0-1 km vertical wind shear may exceed 10 m/s over the Black Sea and Aegean Sea. If storms can root the rather cool maritime boundary layer, they can organize into multicells and supercells, capable of producing large hail and excessive precipitation. Main potential is expected across northern and western Turkey.

Over the other regions, pulse storms are forecast to produce locally large hail, excessive precipitation, and severe wind gusts. Waterspouts are not ruled out given weak vertical wind shear. Highest potential is expected over the Mediterranean given high CAPE.

Eastern Iberia into southern France

The upslope advection of rich Mediterranean moisture together with an elevated mixed layer is expected to yield high CAPE over the eastern Iberian Peninsula during the day. Diurnal heating will also reduce the large capping inversion over Iberia.

Initiation is expected due to a passing short-wave trough in the afternoon. Best dynamics are expected in the evening hours over north-eastern Spain and the Pyrenees. Isolated storms may be also possible further south. Storms that form are expected to be well-organized given high CAPE and rather strong deep layer vertical wind shear. Supercells with very large hail are forecast to be the main threat. Severe wind gusts and excessive precipitation are not ruled out. Convective activity may go on across southern France during the night hours, whereas storms further south will likely decay after sunset.

France, Benelux, Germany

In the warm air advection regime near the frontal boundary, elevated storms are forecast late in the period. Given the strong vertical wind shear, these storms may organize into multicells or supercells, and large hail is not ruled out. Overall threat is rather weak, though.

Alps

Diurnally driven storms in a weakly-sheared warm air advection regime will pose a threat of locally large hail, excessive precipitation, and severe wind gusts across the western and central Alps. Storms will be poorly organized and isolated. A level 1 is not issued.

Baltic States into western Russia

A plume of rich low-level moisture will be advected onshore ahead of a weak frontal wave. Although lapse rates are rather poor, CAPE is expected. Storms are forecast to form along the convergence line and will likely become a linear cluster later on. Given strong low-level vertical wind shear of 10 m/s, storms may become well-organized, including supercells. Isolated supercells may be capable of producing tornadoes. Large hail and severe wind gusts are not ruled out. The potential is limited due to the weak lapse rates and CAPE.