Storm Forecast
Valid: Thu 24 Jun 2021 06:00 to Fri 25 Jun 2021 06:00 UTC
Issued: Thu 24 Jun 2021 00:21
Forecaster: GATZEN

A level 3 was issued across northern Austria into southern Czech Republic for large hail and severe wind gusts.

A level 2 was issued across the western and northern Alpine region, SE Germany, Czechia, Slovakia, and southern Poland for large hail, severe wind gusts, heavy rainfall, and tornadoes.

A level 2 was issued for eastern Romania and Bulgaria mainly for large hail.

A level 2 was issued for the Caucasus region mainly for large hail and severe wind gusts.

A level 2 was issued for northern Russia mainly for severe wind gusts and tornadoes and to a lesser extend large hail.

A level 1 was issued across eastern Poland, SE Baltic States, W Belarus, W Russia, SW Ukraine for large hail, severe wind gusts and heavy rainfall.

A level 1 was issued for northern Algeria and Tunisia for severe wind gusts.

SYNOPSIS

A mid-level jet extends from the west Mediterranean towards Poland and farther into east Scandinavia ahead of a slowly lifting west-European trough, with several vorticity maxima that travel north-east. Downstream ridging remains from the central Mediterranean to western Russia upstream of a quasi-stationary cut-off trough centred across the Black Sea and Turkey.

A frontal boundary from northern Iberia and the west Mediterranean to the central Alps, Baltic States and northern Russia separates Atlantic air masses from steep lapse rates across most of southern and eastern Europe. Rich low-level moisture is present along this frontal boundary and overlaps with steep lapse rates. Additionally, ample moisture affects the Black Sea region, and robust MLCAPE is expected over eastern Europe. To the west, less CAPE but strong vertical wind shear create an environment favorable for organized convection.

DISCUSSION

Western and northern Alpine region into SE Germany, Czechia, Slovakia and S Poland

A strong south-westerly mid-level jet slowly progresses eastward on Thursday. An embedded short-wave trough spreads north-east and associated differential vorticity advection increases during the day. At lower levels, a frontal boundary extends from the western Alps towards SE Germany and into Czechia. Moist and unstable air are present to the south-east of this boundary. In response to pressure falls north of the Alps, moisture will return westward during the day and warm air advection sets in.

Convection initiation will become more likely during the day, with a peak of convective activity in the evening. Main focus will be storms forming along a cold front that progresses over southern Germany in the afternoon, but more isolated storms are also possible in the warm air advection regime farther east. Strong deep layer shear of 20 to 30 m/s will support supercells capable of giant hail and severe wind gusts. A level 3 was issued for the area with highest expected storm coverage.

Due to the high coverage of storms and strong 0-3 km wind shear, formation of foreward propagating MCSs and bow echoes is expected, with severe winds becoming the main threat. A tornado is not ruled out, with increasing potential across Slovakia due to strong low-level vertical wind shear that develops in the evening.

Clusters elevated storms are expected to persist for most of the night while progressing across Poland, with slowly decreasing risk of severe wind gusts and large hail.

Eastern Romania and Bulgaria

At the western flank of the cut-off trough over the Black Sea, a very humid and unstabkle air mass persists over Romania and Bulgaria. Due to weak CIN and strong diurnal heating, storms are expected to form again along outflow boundaries and over the mountains. 1-2 kJ MLCAPE and strong (15-20 m/s) deep layer shear provide a favorable environment for splitting storms capable of large hail and severe wind gusts.

Caucasus region

Ahead of the Black Sea trough, some QG forcing is expected to spread over the region ahead of an approaching vort-max. In an unstable air mass, diurnally driven storms are expected over the mountains. Due to moderate vertical wind shear around 15 m/s, some organized storms are forecast, with multicells and transient supercells capable of producing large hail.

Northern Russia

The northern boundary of an unstable and moist air mass is affected by QG forcing ahead of an progressing short-wave trough. Models indicate the possibility of an MCS moving east along the boundary, with gradually turning towards the unstable air mass to the south. Strong vertical wind shear at low-levels (19 m/s 0-3 km shear) support a high wind producing convective system. Additionally, convection at the southern flank of this system will be well-organized with supercells capable of producing strong tornadoes due to low LCL heights and strong 0-1 km shear (13 m/s), along with severe wind gusts and large hail.

Northern Algeria and Tunisia

Weak thunderstorms are not ruled out in a well-mixed air mass. Inverted-v profiles are forecast and severe wind gusts are expected.

Remaining level 1 areas

Across a large area of eastern Europe, high CAPE is expected. Thunderstorms are expected to form along the main frontal boundary, and additionally over mountains and outflow boundaries. Vertical wind shear is limited and storms are expected to be not well-organized. However, pulse-type storms are forecast to produce locally large hail and heavy rain, as well ad severe wind gusts.