Storm Forecast
Valid: Fri 24 Jun 2022 09:00 to Sat 25 Jun 2022 06:00 UTC
Issued: Fri 24 Jun 2022 08:58
Forecaster: PISTOTNIK

A level 1 and level 2 are issued for the Alpine region mainly for excessive convective rainfall and, especially towards the SE, for large hail and to a lesser degree for severe convective wind gusts.

A level 1 is issued for E France and Germany mainly for excessive convective rainfall.

A level 1 and level 2 are issued for S Turkey, Georgia, Armenia and Azerbaijan mainly for excessive convective rainfall and large hail and to a lesser degree for severe convective wind gusts.

A level 1 issued for parts of SE Russia, the E Ukraine and N Turkey mainly for excessive convective rainfall.

SYNOPSIS

A blocking pattern is in place. A large, warm anticyclone is centered over the Baltic States and is flanked by stationary cut-off lows over the Celtic Sea and the Black Sea. The polar branch of the jet stream circles the blocking anticyclone to the north. Its subtropic branch splits from it over Iberia and runs across the central Mediterranean region, the W Balkans to Cyprus. Two prominent short-wave troughs embedded in this flow translate from France (Fri 06 UTC) to the E Alpine region (Sat 00 UTC) and from Bulgaria (Fri 06 UTC) to Cyprus (Sat 00 UTC), respectively.
Temperature-wise, a stream of very warm air from the Mediterranean is directed across central Europe to Scandinavia at the W flank of the anticyclone. A cold front moves east from France to Germany, the Czech Republic and the Alpine region. Another cold front, more diffuse and blurred by diurnal temperature oscillations, slowly moves southeast over Turkey.

DISCUSSION

... E France, BeNeLux, most of Germany ...

Moisture accumulates along the cold front, and diurnal heating plus synoptic-scale lift assist in building some hundred J/kg CAPE. At least scattered storms are expected to form, mostly in the afternoon to evening. Vertical wind shear is weak to moderate at best (<15 m/s across the 0-6 km layer), hence single cells, some multicells and loosely organized clusters are the expected storm mode. The main risk is excessive rain. Hail and wind events, if any, should stay isolated and marginally severe.
The primary source of initiation is the cold front, which crosses central and E Germany and Bohemia during peak heating hours. A lower-topped and probably weaker second round of storms is expected further west in E France, BeNeLux and W Germany, as the maritime postfrontal air is exposed to daytime heating.
Convection should slowly subside overnight.

... Alpine region into the Czech Republic ...

A more robust severe weather threat evolves further SE, where the same cold front crosses the Alpine region from west to east. The SW-erly mid-level flow advects somewhat steeper lapse rates over these regions, and more robust CAPE on the order of 500 to 1500 J/kg is expected to build. Vertical wind shear is also stronger with 15-20 m/s across the 0-6 km layer, locally even more thanks to the fringes of the subtropic jet aloft.
The Alpine region will see quite widespread thunderstorm activity, whose bulk shifts from the western Alps around noon to the eastern Alps and the Czech Republic until evening. In general, excessive rain with resulting flooding is also the main risk here. However, further southeast and/or earlier in the afternoon, some discrete storms may also organize into strong multicells and temporary supercells. In that case, large to isolated very large hail and a few severe wind events become additional risks. Some recent high-resolution model runs even show upscale growth with a more concentrated swath of severe winds somewhere in the bordering region between SE Germany and N Austria, but these solutions seem dependent on favorable mesoscale developments and do not appear particularly likely, considering the quite moist air that hampers strong downdrafts and the strong clustering tendency under a multitude of large- and mesoscale lift mechanisms.
Into the night, a small cut-off low may condense from the short-wave trough over Austria or the Czech Republic. The remnants of widespread convection could therefore grow into a mesoscale convective vortex (MCV) that may carry the heavy rain risk into the night for a prolonged period, possibly also affecting the (southern) Czech Republic then.

... S Turkey into Georgia, Armenia and Azerbaijan ...

Scattered to widespread, mostly diurnally-driven storms are expected in an environment of a few hundred to up to 1000 J/kg CAPE, moderate 0-6 km shear around 15 m/s, and various lifting mechanisms from large scales (mid-level vorticity lobe) to smaller scales (diffuse cold front, orography). Multicells are the most likely convective mode. The primary risks are large hail, excessive rain and to a lesser degree severe downbursts. Even more CAPE (up to 2000 J/kg) and stronger 0-6 km shear (around 20 m/s) are present in the coastal belt of S Turkey, but a strong cap may be an issue there. Nonetheless, storms that manage to detach from the mountains and propagate south into this potent setup can easily turn supercellular and may also produce very large hail.

... belt from SW Russia across E Ukraine to N Turkey ...

An occlusion wraps around the cut-off low and exposes moist, marginally unstable air to strong and persistent lift. Training and partly embedded convection can produce high rainfall amounts within a couple of hours. Despite regionally enhanced vertical wind shear, the flow-parallel orientation of this rainfall band should make storm organization difficult and keep the other severe weather risks low.
Around the center of the cut-off low over the warm Black Sea waters or at nocturnal sea breeze fronts, a few non-supercellular tornadoes may spin up.

... W Balkans ...

Strong daytime heating creates patches of CAPE under strong deep-layer shear up to 25 m/s. ICON-EU and GFS show hardly any precipitation signals, whereas ECWMF is more "generous" in simulating scattered afternoon storms. This forecast rather leans towards the former two models, since initiating convection will likely struggle against a background of strong shear and large-scale subsidence. Nonetheless, if the ECMWF scenario comes true and more persistent updrafts form, they could organize well and easily produce large hail.