Storm Forecast
Valid: Thu 10 Aug 2017 06:00 to Fri 11 Aug 2017 06:00 UTC
Issued: Wed 09 Aug 2017 23:39
Forecaster: GATZEN

A level 2 was issued for northern Austria to Czech Republic and south-western Poland mainly for excessive precipitation, large or very large hail, severe wind gusts and to a lesser extent tornadoes.

A level 2 was issued for central Italy mainly for large or very large hail and severe wind gusts and to a lesser extent tornadoes and excessive precipitation.

A level 2 was issued for northern Italy, Austria, eastern Switzerland mainly for excessive precipitation, large or very large hail and to a lesser extent severe wind gusts and tornadoes.

A level 1 was issued for Poland, northern Belarus, Lithuania mainly for excessive precipitation, large hail, severe wind gusts and tornadoes.

A level 1 was issued for south-eastern Germany mainly for excessive precipitation, large hail, severe wind gusts and tornadoes.

A level 1 was issued for the northern Balkans mainly for large or very large hail and excessive precipitation.

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

A level 1 was issued for central Finland and north-western Russia mainly for severe wind gusts and tornadoes.

SYNOPSIS

Amplified trough centered across France starts to cut off and slowly migrates east-southeastwards. A vort-max will eject from its base and crosses the west Mediterranean and northern Italy until Friday morning, then entering Germany. Downstream ridging occurs over eastern Europe and expands into southern Scandinavia in the wake of a short wave trough moving over Finland into Russia.

At lower levels, a plume of Saharan desert air mass and associated elevated mixed layer is advected from the Balkans into Poland, western Ukraine and Belarus. Steep lapse rates can be also found below the trough centers over France and east Scandinavia. Low-level moisture is maximized along a frontal boundary from south-western Mediterranean to central Italy, northern Adriatic, Czech Republic, northern Poland to Belarus.

DISCUSSION

East Switzerland, northeast Italy, Slovenia, Austria, Czech Republic, Poland, southern Baltic States

Substantial overlap of rich low-level moisture and steep lapse rates is present along a frontal boundary from northern Italy and Austria towards northern Poland. Forcing is expected due to warm air advection and thunderstorms will be possible along the front during the night and on Thursday morning according to latest models. Outflow boundaries of these storms may travel into central and southern Poland, central Czech Republic, and the southern Baltic States. Despite weak QG forcing, these outflow boundaries may support initiation of further storms during the day. Main limiting factor is that remaining clouds and cold pools cause large CIN what inhibits initiation, especially across western Poland. If diurnal heating is strong, widespread severe weather can result in northern Poland.

Further south-west, increasing QG forcing is expected to affect northern Italy, spreading northward into the Alps and Czech Republic. A very moist air mass will allow for high CAPE again. Storms are likely to develop especially over the mountains, moving northeastwards.

Deep layer vertical wind shear increases to the west and north, where supercells are forecast, capable of large or very large hail. As 0-3 km vertical wind shear is also strong, it is possible that storms merge to MCSs and form bow echoes. Severe wind gusts and excessive precipitation are possible in particular with such systems, but also along the southern slopes of the Alps due to numerous storms and backbuilding systems. Tornadoes are possible especially from northern Austria to Czech Republic and northern Poland given strong low-level vertical wind shear in the warm air advection regime.

Further south-east, vertical wind shear is weaker with also better low-level mixing what will reduce the chances of initiation. However, storms that can ride outflow boundaries may become quite intense, producing large or very large hail, severe wind gusts, and excessive rain.

Northern Balkans

QG forcing is weak and capping inversion will be too strong for initiaton over most areas. However, a few storms are not ruled out. Large hail is possible given high CAPE. Backbuilding storms are also capable of excessive precipitation.

Central Finland and north-western Russia

Rapidly moving short-wave trough approaches from the west, and a strong mid-level jet stream spreads across the Baltic Sea into southern Finland, causing strong DCVA at its cyclonically sheared flank. At lower levels, abundant moisture is advected northward into Finland with an occluded frontal system. As the trough approaches, lapse rates increase and CAPE will build in response to diurnal heating. Clouds and precipitation will limit this diurnal heating. Additionally, drier air masses will advect from the south-west. However, storms are forecast to develop within the moist air mass. They will rapidly organize due to strong vertical wind shear. Multicells and supercells are expected. Main threat are severe wind gusts and tornadoes.

Northern Algeria and Tunisia

A cold front moves into northern Africa, advecting Mediterranean moisture onshore and upslope. QG forcing increases in the afternoon at the right entry of a mid-level jet streak and may help to overcome the capping inversion. A few storms are expected over the mountains that will become supercells due to 25 m/s deep layer vertical wind shear. Large or very large hail is forecast together with severe wind gusts. Storms will weaken overnight.