Storm Forecast
Valid: Thu 08 Aug 2019 06:00 to Fri 09 Aug 2019 06:00 UTC
Issued: Thu 08 Aug 2019 05:24
Forecaster: GATZEN

A level 2 was issued for western Russia mainly for severe wind gusts and large hail and to a lesser extent tornadoes and excessive rain.

A level 2 was issued for the western and central Ukraine mainly for large hail and severe wind gusts.

A level 2 was issued for the south-eastern Alps mainly for large or very large hail and to a lesser extent severe wind gusts.

A level 1 was issued for Belarus into western Russia mainly for large hail, excessive rain, severe wind gusts, and tornadoes.

A level 1 was issued for northern and central Italy, southern Austria, and surroundings including the north-western Balkans mainly for large or very large hail and severe wind gusts.

A level 1 was issued for southern Scandinavia mainly for excessive rain.

SYNOPSIS

A Scandinavian trough yields a strong westerly mid-level flow across the central parts of Europe. A rapidly moving short-wave trough crosses Belarus and the Ukraine into western Russia today. In its wake, a weak upper trough migrates over the central Mediterranean. Late in the period, an Atlantic trough enters Europe.

DISCUSSION

Western Russia

A strongly-forced synoptic situation is expected across western Russia today due to the rapidly progressing, intense short-wave trough that is associated with rapid height falls in the afternoon. A warm front has formed that progresses northward, and a rather moist low-level air mass has spread into parts of western Russia. During the next hours, models expect some insolation and with increasing low-level temperature, CAPE around 1000 J/kg will develop. Given rather weak CIN, frontogenetic forcing along the warm front is expected to assist in convection initiation early around noon. Storm coverage is forecast to increase in the afternoon and spreads eastward along the warm front.

Storm organization will gradually improve during the day with increasing vertical wind shear from the west, with deep layer shear about 15 to 20 m/s across the warm front. Main limiting factor is a stable boundary layer along and north of the warm front, where rather dry air is actually observed. Higher CAPE is forecast near the Ukraine border. Multicells and eastward moving supercells that ride the warm front are forecast to produce severe weather, including tornadoes due to the strong vertical wind shear, favourably curved low-level hodographs and locally more than 100 J/kg 0-1 km SRH. Strong tornadoes are possible. Large hail and severe wind gusts are also forecast. The main limiting factor is the uncertainty of the low-level buoyancy, and current clouds and limited dew points support a weaker risk of severe weather.

From the west, a cold front enters Russia in the afternoon. We expect that storms will have merged to mesoscale convective systems that rapidly move eastwards, producing swaths of high winds given strong vertical wind shear and forcing along the cold front. Every isolated storm ahead of the cold front will likely be a supercell in the strongly sheared environment. Late in the evening, CAPE will decrease and low-level stabilization will cause a decreasing severe potential. Especially in the northern portions of the risk area, some storms are forecast go on until the morning.

Ukraine, Belarus, and surroundings

Severe thunderstorms are expected across the Ukraine today. A cold front enters the Ukraine from the west. Ahead of the front, a moist and warm air mass becomes unstable in particular in the central and south-western parts where insolation is expected. MLCAPE around 1000 J/kg is forecast. Additionally, deep boundary layer mixing is likely. Storms along the cold front are forecast to cluster and organize along their outflow boundaries given rather strong vertical wind shear and straight-line hodographs. High winds are the main threat together with large hail. It is not ruled out that long swaths of high winds will occur. Main limiting factor is the rather widespread cloud coverage currently across western and central Ukraine that can limit severe potential due to reduced CAPE later on.

Across Belarus, weak CAPE is forecast in the cloudy, moist air mass ahead of the approaching trough and cold front. Rain with embedded thunderstorms is forecast. Most storms will be rather weakly organized, however, some storms that root to the boundary layer may organize and pose a threat of large hail, excessive rain, severe wind gusts, or a tornado. Main potential is expected across the southern parts where CAPE is higher.

Northern and central Italy, southern Austria, and surroundings

With the progressing trough, cool mid levels will develop across the northern Mediterranean. Latest models indicate CAPE in the range of 1500 J/kg and also quite large CAPE in the hail growth zone. Despite limited vertical wind shear that is between 10 and 15 m/s and straight line hodographs, some storms may organize. The potential for supercells is highest near the southern Alps where locally higher deep layer shear is expected. Main threat is large or even very large hail. We issued a level 2 in the area of highest potential for large hail. Additionally, slow moving storms can produce excessive rain. Severe wind gusts are also not ruled out. Storms will slowly decay in the evening.

Southern Scandinavia

Slow moving storms pose a threat of local flash floods today. Pulse storms may also cause some large hail and severe gusts. Better organized storms can develop across Denmark into northern Germany with stronger deep layer shear. This will result in a higher severe wind gust threat.

Southern British Isles, Bay of Biscay

Along an approaching cold front, some thunderstorms are forecast during the night. Although the air mass is moist and some CAPE will likely develop, latest models do not indicate substantial low-level buoyancy. With limited CAPE and elevated storms expected, severe potential is low. However, strong vertical wind shear is present and if surface-based storms form, they may produce severe wind gusts and tornadoes.