Storm Forecast
Valid: Thu 10 Aug 2023 06:00 to Fri 11 Aug 2023 06:00 UTC
Issued: Wed 09 Aug 2023 16:54
Forecaster: PISTOTNIK

A level 1 and level 2 are issued for far-North Turkey, the Black Sea, SW Russia and the E and central Ukraine for large hail, severe convective wind gusts and tornadoes mainly towards the SE, and for excessive convective precipitation mainly towards the NW).

A level 1 is issued for parts of Greece, North Macedonia, Serbia and Bulgaria for excessive convective precipitation and large hail.

A level 1 is issued for parts of Romania, the far-West Ukraine and W Belarus mainly for excessive convective precipitation.

SYNOPSIS

An elongated mid-level trough filled with cool air stretches from Sweden all the way to Greece. It is associated with an old, filling, largely barotropic surface cyclone centered over central Sweden and a second, more diffuse, but also more baroclinic one centered over the Black Sea. Its forward flank will be the main site for partly organized and severe thunderstorm activity.
Further upstream, an equally elongated mid-level ridge extends from the W Mediterranean Sea across the North Sea to Iceland. It brings warm air far north and extremely hot air into Spain. The British Isles are later crossed by the occluding frontal system of the next pronounced cyclone over the North Atlantic Ocean with rain and perhaps shallow, weakly electrified convection.

DISCUSSION

... far-North Turkey, Black Sea, Ukraine, SW Russia ...

A broad area with unstable air is exposed to synoptic lift, to the west (NW Turkey and W Black Sea into W Ukraine) mainly from positive vorticity advection ahead of the main mid-level trough, to the east (E Black Sea into SW Russia and E to central Ukraine) mainly due to strong warm air advection from the southeast ahead of the surface cyclone over the Black Sea. The warm air advection will also carry an elevated mixed layer from the Turkish Plateau and Iran across the region. As it overspreads a very moist boundary layer, CAPE on the order of 500-1500 J/kg will build across wide areas, probably peaking around or above 2000 J/kg over the NE Black Sea, the Azov Sea and surrounding coastlines. Deep-layer shear varies within the 10 to 20 m/s range with higher values but mostly unidirectional wind profiles towards the SW and somewhat lower values but strongly veering wind profiles in the warm air advection regime towards the NE.
Thunderstorm activity will be widely scattered, and possibly at least partly elevated, until mid-afternoon. Even with synoptic lift support, further surface-based initiation will at first struggle against the strong cap and will only become more widespread in the mid- to late afternoon, mostly in the SE half of the level 2 area. Early and discrete storms will likely become multi- to supercellular and may produce all kinds of severe weather: large (to isolated very large) hail, severe downbursts and perhaps also one or two tornadoes in pockets of enriched low-level moisture and/or enhanced low-level shear, e.g. along the sea breeze front or outflow boundaries. In the evening, upscale growth into probably several large MCSs is expected, which can persist and track NW-ward across large parts of the Ukraine all night long. By then, excessive rain with flooding clearly becomes the main hazard, while other severe weather events gradually become less likely.
Thunderstorm activity will also exhibit an upswing across the central Black Sea in the second half of the forecast period, with tail-end storms possibly affecting far-North Turkey. Also there, all kinds of severe weather are possible.

... between the Balkans, the Baltic States and the Eastern Alps ...

Beneath the mid-level trough, scattered afternoon storms are expected especially over orographic features. The low CAPE magnitude (mostly below or around 500 J/kg), low cloud tops in the cool airmass and predominantly weak vertical wind shear are factors that keep the severe weather hazard low. A level 1 for localized excessive rain is issued for those areas with the expected highest storm coverage. Towards the SW, the mid-level jet on the back side of the trough can partly overlap with the slightly unstable airmass and allows deep-layer shear up to 20 m/s from the Eastern Alps to the S Balkans and Greece. In these regions, storms can turn multicellular and can also produced marginally large hail.

... belt from Lapland into NW Russia ...

Ahead of the occlusion of the Swedish cyclone, a plume of CAPE is still present in the deep southerly flow with 0-6 km shear around 20 m/s. However, it becomes narrower and weaker, and its overlap with synoptic lift more and more questionable. As of the model runs until Wed 06 UTC, ECMWF is the only forecast model with widespread convective precipitation which would require a level 1 mainly for severe wind gusts, as the kinematic background would be adequate for an organization into multicells and bowing line segments. All other models are much more reluctant and only simulate weak and patchy convective precipitation, probably owing to mesoscale synoptic subsidence overspreading the discussed belt and damping convective activity. Following the majority of the model pool, only a low-probability lightning area and no level 1 are issued, but a monitoring of the situation is recommended.