Storm Forecast
Valid: Thu 27 Jun 2019 09:00 to Fri 28 Jun 2019 06:00 UTC
Issued: Thu 27 Jun 2019 09:40
Forecaster: GATZEN/PISTOTNIK

A level 3 is issued for E-most Hungary and W Romania mainly for large hail and severe convective wind gusts.

Level 1 and 2 areas are issued for parts of Belarus, SW-most Russia, the Ukraine, Moldova, Romania, Slovakia, Hungary, Serbia, Bosnia-Herzegovina, Croatia, Slovenia and Austria mainly for large hail and to a lesser degree severe convective wind gusts and excessive convective precipitation.

A level 1 is issued for the Celtic Sea south of Ireland mainly for large hail.

A level 1 is issued for NE Turkey, Georgia and the Russian Caucasus region mainly for large hail and to a lesser degree for severe convective wind gusts.

A level 1 is issued for the E Black Sea mainly for non-supercellular tornadoes (waterspouts).

SYNOPSIS

A high across northern Africa extends over France and ridges into the British Isles. At its western flank, a highly amplified trough is situated west of Europe. A plume of very warm, well-mixed air had spread into east Iberia, most of France, the Bay of Biscay, and southern British Isles. Another trough across Scandinavia amplifies over eastern Europe, associated with intense cold air advection over large portions of Europe. A frontal boundary extends from northern France to southern Germany, Hungary, and across the Ukraine and east Belarus, where a surface low moves east. Rich low-level moisture along this frontal boundary overlaps with steep lapse rates especially from France to Romania, where high CAPE develops. Finally, a weak trough is situated from Turkey to the south Mediterranean.

DISCUSSION

...from Romania to Austria and the north Balkans...

The cold front of the low across east Belarus moves southward today. Ahead of the front, abundant low-level moisture is present. Latest observations (Thu 08 UTC) confirm 2m dewpoints often between 20 and 23C over Hungary and surroundings. Aided by fairly steep lapse rates aloft and strong daytime heating, high CAPE between 2000 and 4000 J/kg is about to build.
A small but pronounced vorticity maximum overspreads the cold front during peak heating hours. Around 12 UTC, scattered convective initiation becomes likely along the cold front, especially where orography helps (e.g. over the north Carpathians, Tatras and eastern Alps). Under 0-6 km shear between 15 and 20 m/s and mostly unidirectional wind profiles, splitting supercells are the expected primary storm mode and have a high potential to produce large hail. A few cases of very large hail (>5 cm) are well possible. In addition, fairly deep subcloud layers promote a risk of severe downbursts which might turn extreme (>32 m/s) as well, especially in case upscale growth takes places. The latter scenario appears most likely from easternmost Hungary into western Romania, and these areas are therefore upgraded to a level 3, whereas stronger capping and somewhat weaker lift make it less likely over the eastern Alps.
Storms will gradually cluster towards evening and will move southeastward into larger parts of Romania, Serbia, Croatia and Slovenia. Storms will gradually become elevated after sunset and the severe weather risk starts to decrease, but large hail, severe wind gusts and excessive precipitation are still possible into the first half of the night.

... Eastern Belarus into western Russia and central Ukraine ...

The cold front is currently moving across northern and western Ukraine where a line of storms has formed. As this line moves farther east, weak low-level moisture and only modest lapse rates will limit its severe potential. A few severe downbursts and locally large hail are forecast. In the wake of the frontal boundary, rich low-level moisture is advected into the area that overlaps with steep mid-level lapse rates. CAPE in excess of 1500 J/kg is forecast. Furthermore, large-scale lift is expected ahead of the intense trough that amplifies into eastern Europe. As a consequence, scattered storms are forecast today. Vertical wind shear is strong, with long straight line hodographs and deep layer shear around 20 m/s. Supercells are forecast and large hail and damaging wind gusts are forecast. Isolated supercells may also produce tornadoes as low-level vertical wind shear is rather strong (5-10 m/s 0-1 km bulk shear). Storms are expected to weaken after sunset due to rather rapid stabilization of the boundary layer.

... rest of Belarus, Lithuania, NE Poland, S-most Sweden ...

The cooler air behind the cold front features neutral to marginally unstable profiles under strong vertical wind shear (0-6 km: ~20 m/s, 0-1 km: ~10 m/s). Scattered showers and low-topped thunderstorms may organize into multicells or even a few supercells. Severe wind gusts or an isolated tornado are not ruled out. Limiting factor is the low amount of CAPE / low equilibrium height. Confidence in some electrified convection is highest over Belarus, which is therefore placed under a level 1.

... Bay of Biscay to the southern British Isles ...

North of a surface low west of the Iberian Peninsula, an elevated mixed layer advects north-westward and a plume of most unstable CAPE up to 2000 J/kg is expected to the south of the British Isles. In the warm air advection regime south of the British Isles, elevated storms are likely to go on and intensify during the night. Main threat will be large hail with these storms that form at the eastern flank of a strong mid-level jet. However, severe wind guts are not ruled out although low-level will be stable. Latest models do not indicate that this activity will affect the coasts of Ireland.

... NE Turkey, Georgia and Russian Caucasus region ...

Rather steep lapse rates are present due to cool mid-level air close to a trough axis. During the day, the sea-breeze will advect rich moisture from the Black Sea inland where CAPE around 1500 J/kg will develop. Convection initiation is forecast along the sea breeze front and over the mountains. With 10-15 m/s deep layer vertical wind shear, storms can become organized and some large hail and severe wind gusts are forecast. Additionally, local flash floods are not ruled out.
Convection may move offshore onto the Black Sea overnight, where a few waterspouts may develop under plentiful CAPE and converging surface winds (land breezes).

... S Italy ...

Scattered oragrphic storms are possible in the afternoon in an environment of moderate CAPE and weak vertical wind shear. An isolated hail event or flash flood is not ruled out.