Storm Forecast
Valid: Thu 12 Aug 2010 06:00 to Fri 13 Aug 2010 06:00 UTC
Issued: Wed 11 Aug 2010 17:30
Forecaster: GATZEN

A level 1was issued for north-eastern Spain mainly for large hail.

A level 1was issued for northern Italy, the Alps, central and eastern Germany, Czech Republic, western Poland, and southern Sweden mainly for excessive rain and large hail.

A level 1was issued for the western Ukraine, Belarus, and portions of western Russia mainly for large hail.

SYNOPSIS

A long-wave trough is centred over the North Sea. A mid-level jet curves around this trough from the Bay of Biscay to the west Mediterranean and further into the Baltic Sea and north-western Russia. Mid level jet streaks are expected over the British Isles, northern Iberia, and eastern Germany during the period. Warm air with steep lapse rates is present over most of eastern Europe and over Iberia. Ample low-level moisture can be found from northern Russia and Scandinavia across central Europe to the Mediterranean Sea.

DISCUSSION

Eastern Spain

Ahead of the approaching mid-level jet streak, models indicate an easterly low-level flow that advects rich low-level moisture present over the Mediterranean Sea into eastern Spain. Diurnal heating over the high terrain will result in steep lapse rates spreading north-eastward during the day, while some QG forcing will result in cooling mid-levels. Models suggest CAPE in the order of 1000 J/kg near the coasts. Given low-level forcing due to convergence and upslope flow, thunderstorms are forecast to develop. These are expected to organize given about 15 m/s 0-3km vertical wind shear and favourable veering profiles. Supercells will be capable of producing large hail and isolated severe wind gusts. Storms will spread into the Mediterranean Sea during the evening and night as the mid-level jet streak will spread across this region. The severe thread will likely persist given the instability over the warm sea surface. To the south of the jet streak, large capping inversion will limit the chance of convection significantly.

Northern Italy, Alpine region, Germany, southern Sweden

Along a frontal boundary rich low-level moisture and low-level convergence will be present on Thursday. While lapse rates will be best over Italy and Poland, moist neutral lapse rates are expected to the west of the front, where diurnal heating will likely lead to weak CAPE during the day. Storms are likely through-out the period and are expected to cluster. Main severe thread will be excessive rain. Large hail is most likely over the southern portions of the risk area, where lapse rates will be rather steep. Severe wind gusts and large hail is forecast for western Poland as well given steep lapse rates and rather strong vertical wind shear in the range of the southerly jet streak. Storms are expected to spread into Sweden late in the period, where excessive rain may occur.

Eastern Europe

In the range of a weakening frontal boundary from the western Ukraine into Russia, some storms will evolve during the day. Focus of this development will be outflow-boundaries as the synoptic lift will be weak. Given steep lapse rates, stronger pulse storms may produce large hail locally. In the evening hours, storms are forecast to decay given the weak forcing.

Central Scandinavia

In the range of warm air advection over central Scandinavia, a moist easterly upslope flow evolves and will likely assist storms over central Scandinavia. Although vertical wind shear is rather weak, a few isolated cells may evolve strong updrafts with a chance of large hail.