Storm Forecast
Valid: Sat 20 Aug 2022 06:00 to Sun 21 Aug 2022 06:00 UTC
Issued: Fri 19 Aug 2022 22:58
Forecaster: PISTOTNIK

A level 1 and level 2 are issued from Poland to Greece mainly for excessive convective precipitation, towards the east also for severe convective wind gusts and large hail.

Level 1 areas are issued for central Sweden, S Germany, Slovenia, Croatia, Bosnia-Herzegovina, the E Ukraine and far-SW Russia mainly for excessive convective precipitation.

A level 1 is issued for the NE coastlines of the Black Sea for non-supercellular tornadoes (waterspouts).

SYNOPSIS

A deep, steering cyclone is located near the Faroe Islands. A mid-level jet and cool maritime air in its periphery affect the British Isles, BeNeLux, Denmark and Norway.
Further south and east, warm to hot conditions prevail across the continent under rather weak pressure and temperature gradients. Two elongated zones of slightly lower 500 hPa geopotential stretch from Germany to Greece and from St. Petersburg to the Crimean peninsula, respectively. A blocking anticyclone keeps its position over Russia.

DISCUSSION

... belt from Poland to Greece ...

A long, wavy cold front separates hot and fairly dry, deeply mixed air to the east from cool and moist, maritime air to the west. Modulated by various outflow boundaries and density currents, the front continues its slow eastward motion. In the transition zone, fairly rich low-level moisture with 2m dewpoints in the upper tens undercuts the hot air and allows CAPE on the order of 500 to 1500 J/kg, depending on the amount of solar heating. Thanks to a southerly mid-level jet aloft, a considerable part of the CAPE reservoir is overspread by moderate deep-layer shear between 10 and 20 m/s.
Scattered thunderstorms will likely be active from the morning and become widespread between early afternoon and midnight, aided by persistent synoptic lift (prefrontal warm air advection and various vorticity maxima) and mesoscale ascent along the frontal boundary. Dominant storm mode will be a mixture of single- and multicells with a tendency to grow upscale into large clusters later on. Moist air and front-parallel motion indicate a significant possibility of backbuilding and/or training storms. Especially in the level 2 area, several larfge MCSs are expected towards evening and pose a significant hazard of excessive rain with flash floods (and mudslides in hilly or mountainous terrain).
Storms that manage to detach from the front and propagate northeast- to eastward into the hot air can turn multi- or temporary supercellular and pose additional hazards of severe downbursts and large hail. It is also possible that an organized outflow front with more widespread strong to severe wind gusts outruns the parent convection and pushes deeply into the hot and dry air, bringing a cool change without precipitation or storms.
Convection will turn elevated overnight with gradually decreasing severe weather hazards, but the background of strong lift can keep some of the activity alive until the end of this forecast period (and beyond).

... from N Sweden to N Finland and Karelia ...

The northern part of the frontal zone is stationary or pushes slowy northward as a warm front. Lapse rates are weaker than further south, but rich low-level moisture with 2m dewpoints up to almost 20C still allows CAPE up to 1000 J/kg. Vertical wind shear is impressive across the lowest kilometer with values often between 10 and 15 m/s, but hardly increases across deeper layers anymore.
With a background of slight warm air advection but a strongly anticyclonic curvature of the jet stream, net synoptic vertical motion appears to be neutral to negative, which makes it difficult to overcome the capping inversion and achieve convective initiation. Forecast models therefore agree in rather little precipitation signals, warranting not more than a low probability lightning area for now. However, if surface-based storms form and persist, the strong low-level shear could result in good organization into (rather low-topped) supercells or bowing lines. In that case, severe wind gusts and one or two tornadoes are not ruled out.

... other thunderstorm areas ...

Various other regions will likely see scattered, mostly daytime-driven thunderstorm activity under low to moderate CAPE and weak vertical wind shear: (1) central Sweden in the range of a frontal wave, (2) central Europe and to a lesser degree Italy in the post-frontal maritime air, and (3) the E Ukraine and far-southwest Russia in the range of the shallow mid-level low. A few excessive rain events constitute the main hazard. In addition, one or two brief waterspouts are not ruled out over the large lakes in S Germany and Switzerland, which still feature water temperatures close to 25C. A few waterspouts are also possible over the Black Sea in the morning and again overnight (not necessarily with precipitating ot electrified clouds), when land breezes propagate offshore.