Storm Forecast
Valid: Thu 22 Jun 2017 06:00 to Fri 23 Jun 2017 06:00 UTC
Issued: Thu 22 Jun 2017 00:00
Forecaster: GATZEN

A level 2 was issued for eastern France into central and eastern Germany and western Poland and Czech Republic mainly for severe wind gusts and large hail and to a lesser extent excessive precipitation and tornadoes.

A level 1 was issued for the Benelux countries, eastern France, Poland, Slowakia mainly for severe wind gusts and large hail and to a lesser extent excessive precipitation and tornadoes.

A level 1 was issued for northern Spain, southern France, Switerland, Austria, northern Italy, Slowenia, northern Croatia, Hungary mainly for large hail and excessive precipitation and to a lesser extent severe wind gusts.

A level 1 was issued for eastern Romania into Bulgaria mainly for large hail and severe wind gusts and to a lesser extent tornadoes and excessive precipitation.

A level 1 was issued for western Russia mainly for severe wind gusts and large hail.

SYNOPSIS

Unseasonably intense subtropical ridge is present across south-western Europe. It is flanked by a polar trough over north-eastern Europe. A strong mid-level flow extends from the British Isles to Poland to the northern Black Sea. Embedded in this flow, two short-wave troughs travel rapidly east-southeast, one from Poland to the northern Black Sea and another, quite intense one follows from Ireland reaching western Poland on Thursday morning. Strong quasi-geostrophic forcing can be associated with this dynamic synoptic situation.

A highly unstable air mass over western Europe will be advected east ahead of the approaching trough and will be the focus of severe potential. Furthermore, a belt of moist low-level air extends across the Balkans where severe thunderstorms can also form during the forecast period.

DISCUSSION

Benelux and eastern France, Germany into western Poland, Czech Republic, and Slovakia

In between a short-wave trough that moves east over Poland and a new trough approaching from the British Isles, upper-level coupled jet configuration evolves late in the day over north-western Germany and moves south-east in the evening. This can support ventilation in the anvil layer and increases the potential of upscale growth of convective storms. At mid-levels, a sharp short-wave trough will move into northern Germany in the evening hours and is expected over western Poland on Friday morning. Intense mid-level height falls will affect northern Germany in the evening hours. Strong mid-level flow affects northern Germany through-out the period. In the evening and night hours, a strong mid-level jet streak will move from the Benelux States towards eastern Germany and provides further differential cyclonic vorticity advection.

At low levels, plume of highly unstable air mass spread into Germany. Frontogenesis can be expected in the north-westerly flow over north-eastern Germany in the morning hours, whereas some subsidence near the mid-level ridge axis expands east in the daytime. In the evening hours, the tongue of warm air spreads over northern Germany with a strong low-level jet that exceeds 15 m/s at 850 hPa. Later on, a cold front pushes south-east from the North Sea and crosses northern and central Germany until Friday morning. It will replace the humid and unstable air mass over most of Germany and northern France / Benelux.

At 21 UTC, thunderstorms are spreading east over the British Isles and the North Sea in the warm air advection regime. It is expected that these elevated storms will cross the North Sea and that its remnants will move over Denmark and northern and north-eastern Germany on Thursday morning. Until the early afternoon, embedded new thunderstorms are expected close to the North and Baltic Sea, whereas some subsidence associated with the mid-level ridge could cause decaying storms further south. Strong diurnal heating further south-west will increase the temperature difference over north-eastern Germany with a convergence zone in between that could be enhanced by outflow boundaries of the morning storms.

Initiation is expected in the late noon in two areas: One from north-eastern France into western and central Germany, where strong diurnal heating will reduce the cap and local topography is supportive, the other over the North Sea into the Netherlands ahead of the approaching trough and cold front and associated frontogenetical lift. The southern storms are forecast to slowly spread eastwards during the afternoon and will be the first focus of severe weather. With CAPE in the order of 2000 J/kg and deep-layer vertical wind shear around 10 m/s, intense multicells are forecast, capable of producing large or very large hail, excessive precipitation, and local severe wind gusts. In the evening, storms spread east where drier air and stronger capping can cause some weakening of the activity.

The northern storms are expected to be well-organized given 20 m/s deep-layer vertical wind shear. Straight hodographs indicate the potential of splitting storms capable of producing large or very large hail and severe wind gusts. A tornado is not ruled out but limited low-level vertical wind shear and buoyancy reduce the risk. In the afternoon and evening, storms will likely cluster along their cold pools and intensify due to increasing deep layer vertical wind shear. Bow echoes may evolve that move east or south-eastward, capable of producing swaths of severe wind gusts and large hail. Tornadoes may be possible along the convergence zone that might be still present over central or north-eastern Germany. This convergence zone could be also the focus for a long-lived high wind event, but current uncertainty is too large to issue a level 3.

Main limiting factor is uncertainty of surface-based storms across north-eastern Germany given a rather cool boundary layer and possible clouds and embedded storms through-out the day in the warm air advection regime. Although severe storms might be still possible, coverage of severe weather could be limited.

With the trough axis, convective activity will spread into Poland, the Czech Republic, and Slovakia in the night hours. Severe wind gusts will be the main threat especially across western Poland, and long track high wind events are not ruled out. Large hail and tornadoes as well as excessive precipitation are possible. Later in the night, severe threat gradually decreases over eastern and southern Poland, the Czech Republic, and Slovakia.

Pyrenees, southern France, Alps, northern Balkans

Mid-level ridge will cover this area and large-scale lift will be weak. However, rich low-level moisture is present that is overlapping with steep lapse rates especially in the western portions, but diurnal heating is also expected to cause instability in the east. CAPE will range around 1500 J/kg in the afternoon. Initiation is forecast over the mountains in the noon and afternoon. Pulse storms and multicells are forecast to produce large hail and excessive precipitation. Local severe wind gusts are not ruled out. Later in the day, clusters of storms may also spread into the lowlands, capable of producing large hail and severe wind gusts and also excessive precipitation. Storm activity will be rather isolated and widespread severe weather is not forecast.

Eastern Romania into Bulgaria

A moist and unstable air mass is situated across Romania. Models indicate CAPE due to diurnal heating and first storms are expected to initiate over the Carpathians. In the afternoon, a short-wave trough will approach from the north-west, causing increased quasi-geostrophic forcing and vertical wind shear. It is expected that storms start to move off the mountains into the low lands, where easterly sea breeze results in 15 m/s 0-3 km vertical wind shear, supportive for the development of bow echoes. Severe wind gusts and excessive precipitation are forecast as well as large hail. Tornadoes are also not ruled out especially along the sea breeze convergence where increased low-level vertical wind shear and moisture are likely. In the evening, storms spread east to the Black Sea where they decay during the night.

Western Russia

At the south-western flank of the polar trough, strong DCVA is forecast on Thursday at the cyclonically sheared flank of a passing mid-level jet streak. Some CAPE is forecast and storms are expected in the noon and afternoon. With moderate vertical wind shear and strong low-level buoyancy, severe wind gusts are possible. Large hail is also forecast due to 15 to 20 m/s deep layer vertical wind shear, supportive for multicells and also some splitting storms.