Storm Forecast
Valid: Thu 30 Aug 2012 06:00 to Fri 31 Aug 2012 06:00 UTC
Issued: Wed 29 Aug 2012 23:04
Forecaster: GATZEN

A level 2 was issued for eastern Spain mainly for large hail.

A level 2 was issued for north-eastern Italy, Slovenia, and southern Austria mainly for excessive precipitation.

A level 1 was issued for the west Mediterranean, the Alpine region, northern Italy, northern Balkans, eastern Germany, Czech Republic, and western Poland and surroundings mainly for large hail, excessive precipitation, severe wind gusts, and tornadoes.

A level 1 was issued for southern Sweden mainly for tornadoes, severe wind gusts, and large hail.

A level 1 was issued for the south-western North Sea mainly for waterspouts.

SYNOPSIS

An intense trough amplifies across western Europe. A cold front will enter the west Mediterranean at the end of the period. A frontal wave may develop ahead of the trough across the Czech Republic. Ahead of the cold front, a moist air mass is present from the west Mediterranean to the Alps, eastern Germany, and southern Sweden. Latest soundings indicate steep low-level lapse rates in the northern portions of the warm air mass and steep mid-level lapse rates in the south from southern France to the Alps.

DISCUSSION

West Mediterranean

Ahead of the cold front that enters the west Mediterranean Sea during the period, an unstable air mass is present characterized by rich boundary-layer moisture around 16 g/kg 0-1km mixing ratio and steep mid-level lapse rates. QG forcing is forecast ahead of the approaching trough. Additionally, a thermal low that builds across eastern Iberia will cause easterly low-level winds and upslope flow along the eastern coasts of Spain. Over the central west Mediterranean Sea near the Balearic Islands, low-level convergence is expected.

Current thinking is that numerous thunderstorms will initiate across eastern Spain that will spread into the Mediterranean Sea. Very strong deep layer vertical wind shear (30 m/s 0-6 km bulk shear and 20 m/s 0-3 km bulk shear) as well as favourably veering profiles (0-3 km SRH 150 m²/s²) are forecast to support well-organized storms. Supercells capable of producing large or very large hail, severe wind gusts, and excessive precipitation are forecast. A tornado is not ruled out, although low-level vertical wind shear is rather weak.

During the day, the cold front enters the area from the north, leading to dry air advection at low levels. As a consequence, convective activity is expected to shift further south. Ahead o the cold front, a line of thunderstorms may evolve overnight moving southward into northern Algeria at 6 UTC. Given the strong deep layer vertical wind shear mostly parallel to the convective line, a parallel stratiform MCS is expected, capable of producing excessive precipitation and severe wind gusts.

Alps, northern Balkans

Ahead of the approaching trough and cold front, models indicate warm air advection especially to the south and east of the Alps. Low-level moisture is rich and lapse rates are forecast to be at least slightly unstable. Upslope flow and convergence are forecast and thunderstorms are expected during the day ahead of the cold front. Moderate to strong vertical wind shear and favourably veering profiles are forecast to support supercells and multicells, capable of producing large hail, severe wind gusts, and excessive precipitation. Convective clusters, probably linear organized, may spread into northern Italy, Slovenia, Austria and the Czech Republic in the afternoon and evening hours. Severe wind gusts and excessive precipitation may be the main threat with these storms. During the night, focus of convection is expected to spread into the northern Balkans and northern/central Italy, where excessive precipitation is forecast to be the main threat. Severe wind gusts and locally large hail or a tornado are not ruled out.

Eastern Germany, Czech Republic, western Poland, and surroundings

As the trough spreads southward, the approaching cold front becomes parallel orientated to the mid-level flow and will slow down. Additionally, frontal waves are indicated by latest models that spread northward along the frontal boundary. Moist low-levels and remaining steep lapse rates will lead to weak to moderate CAPE. Thunderstorms are forecast along the cold front spreading into eastern Germany in the morning/noon hours. Strong deep layer vertical wind shear is forecast to support bowing segments and multicells. Isolated convection can evolve into supercells as well, but main convective mod will be linear clusters given the strong linear forcing. Large hail and tornadoes are not ruled out as well as excessive precipitation and severe wind gusts. Later in the day, frontal waves spreading northward are expected to be associated with increasing precipitation, and excessive precipitation is forecast to be the main threat during the night. At the eastern flank of the cold front, a tornado is not ruled out in the range of a weak low-level jet across south-western Poland.

Southern Scandinavia

A tongue of moist air spreads into Scandinavia ahead of the cold front. Daytime heating and QG lift will result in weak CAPE. Strong low-level convergence is expected to lead to initiation. Thunderstorms may organize given moderate to strong vertical wind shear. Multicells and mesocyclones are forecast, capable of producing large hail and severe wind gusts. Tornadoes may also develop as low-level vertical wind shear is rather strong. Storms are forecast to merge along the main convergence zone / cold front and spread across the Baltic Sea.

North Sea, The Channel

Near the base of the trough, numerous showers and thunderstorms are forecast across the North Sea. Rather steep lapse rates will be present. The best potential of waterspouts is expected from the region south of the British Isles to the coasts of The Channel and the Benelux countries, where best moisture and weakest vertical wind shear are forecast.