Storm Forecast
Valid: Thu 21 Jul 2011 06:00 to Fri 22 Jul 2011 06:00 UTC
Issued: Wed 20 Jul 2011 22:15
Forecaster: GATZEN

A level 1 was issued for most of eastern Europe mainly for large hail and excessive precipitation.

A level 1 was issued for eastern Poland mainly for tornadoes.

A level 1 was issued for the southern Alpine region mainly for large hail and excessive precipitation.

SYNOPSIS

A trough extends from the North Sea region across most of central Europe into Romania, where a mid-level low is centred. A strong mid-level westerly jet will be located over the Mediterranean Sea. Ahead of the low geopotential, a warm and moist south-easterly flow affects portions of eastern Europe. Cool air masses have spread across central Europe and also spread into the Mediterranean.

DISCUSSION

Eastern Romania to the Baltic Sea

On Thursday, a cold front is forecast from the southern Baltic Sea to eastern Romania. Ahead of this cold front, a humid low-level air mass is advected north-westward. This air mass is unstable as rather steep mid-level lapse rates and rich low-level moisture are forecast to overlap in the warm air mass. Storms are forecast to develop in response to diurnal heating of the unstable air mass and will be most likely along the frontal boundary, where low-level convergence will support initiation. West of the frontal boundary, some of the storms may be also elevated.

Thunderstorms that form will likely organize in the strongly-sheared environment. Winds are expected to reach 20 m/s at 500 hPa, leading to a strong deep layer bulk shear. Multicells and mesocyclones are forecast that may evolve into bowing lines. Large hail and severe wind gusts as well as excessive rain are forecast. Isolated supercells seem to be not very likely as storms are expected to cluster along the front and SRH is not too strong over most places. However, some tornadoes are not ruled out, though. The highest potential seems to exist over Poland, where the low-level vertical wind shear is strongest and a rather large overlap of instability and strong vertical shear is forecast by latest GFS.

Baltic Sea region

A well-developed warm air advection regime remains over the Baltic Sea region. The main frontal boundary will slowly move westwards towards Sweden, and moist and warm air masses will spread into the Baltic States, Finland, and the eastern Baltic Sea. Diurnal heating will result in instability given the rich low-level moisture and rather steep mid-level lapse rates. Thunderstorms are forecast to develop especially near numerous convergence zones that can build due to the sea breeze and outflow-boundaries. Although the vertical wind shear is not strong, local enhancements near convergence zones may be favourable for mesocyclones. These will pose a thread of large hail and isolated tornadoes.

Western Russia

At the western flank of high geopotential over Russia, a mid-level trough migrates north-westward. It will be associated with QG forcing and will likely enhance the development of deep convection in a moist and unstable air mass. Storms that form may be severe capable of producing large hail and excessive rain. Given rather weak vertical wind shear, storms are forecast to cluster and will likely decay during the night hours.

Southern Alpine region

A short-wave trough will travel across the Mediterranean on Thursday. It will provide QG forcing that will also affect the southern Alps. At low levels, models indicate that southerly winds will evolve that will advect moist air northwards. Together will cool mid-level air masses present over the Alps, instability will likely develop. Storms are expected to form in the afternoon hours. While this activity will spread eastwards, storms may gradually become organized as the vertical wind shear will increase. Supercells are not ruled out capable of producing large hail and excessive precipitation.