Storm Forecast
Valid: Mon 22 Jun 2020 06:00 to Tue 23 Jun 2020 06:00 UTC
Issued: Sun 21 Jun 2020 13:45
Forecaster: PISTOTNIK

A level 1 and level 2 are issued for parts of Poland, Belarus, W and SW Russia, the Ukraine, Romania and Moldova for excessive convective precipitation and to a lesser degree for large hail, severe convective wind gusts and tornadoes.

A level 1 is issued for Greece and S Bulgaria for excessive convective precipitation and to a lesser degree for large hail and severe convective wind gusts. At the Aegean coasts, non-mesocyclonic tornadoes (waterspouts) are possible.

A level 1 is issued for N Turkey mainly for large hail and to a lesser degree for excessive convective precipitation and severe convective wind gusts.

SYNOPSIS

A messy spring-like pattern remains in place.
At 500 hPa, an elongated low stretches from the Ionian Sea to the Crimean Peninsula. Two other small cut-off lows slowly move SE-ward in NW Russia (close to the Estonian, Latvian and Belorussian borders) and in S Sweden, respectively. In contrast, a ridge extends from NW Africa to Germany. Closer to the surface, pressure and temperature gradients are weak. The atmospheric state is largely barotropic with cooler air beneath the cut-off lows and warmer (in inland parts of Iberia and Turkey even hot) air in the regions under more anticyclonic influence.
The only region under stronger dynamics are the British Isles, where the SW-erly flow increases ahead of a large cyclone over the North Atlantic.

DISCUSSION

... E Europe and Balkans ...

Diurnal heating creates CAPE in the order of 300-1000 J/kg over the Balkans, placed beneath the mid-level low, and 500-2000 J/kg from the Ukraine into parts of Belarus and Poland, where subtle warm air advection delays vertical mixing and supports 2m dewpoints around 20C. Isolated leftover convection may already be active in the morning. By early afternoon, widespread convective initiation is expected especially along insolation gradients, outflow boundaries or other convergence zones, as well as along sea breeze fronts and upslope flow convergences where available. Convection will likely grow into numerous large clusters afterwards which can persist into the night.
The wind field is weak throughout the troposphere, so there are no indications for a better storm organization or backbuilding, but the mere presence of moderate to high CAPE and 30-40 mm precipitable water implies a significantly enhanced risk of excessive rain. The daily "convective overworking" of the air mass limits the lapse rates and thus the hail and wind risk, but isolated to scattered cases are foreseen as well. Short-lived non-mesocyclonic tornadoes are possible in case of a favorable mesoscale interactions, e.g. collisions of outflow boundaries or other convergence zones.
The highest storm coverage is expected on the warm (east) side of a synoptic-scale convergence zone, interpretable as an almost stationary warm front, which runs from N Poland to Moldova. Another hot spot may evolve from the Ukraine eastward into adjacent Russia in the evening and overnight, where continued warm air advection and a subtle mid-level trough could carry an MCS through much of the night.

... Turkey ...

Some hundred J/kg CAPE build when sea breezes and upvalley/-slope flows are exposed to daytime heating. Slightly enhanced SW- to W-erly mid-level flow and opposed sea breeze / upvalley circulations at low levels can augment 0-3 km shear shyly above 10 m/s along the Turkish north coast. Scattered afternoon storms can organize into multicells with a main risk of large hail and secondary risks of heavy rain and severe downbursts. Limiting factor is a capping inversion on top of the sea breezes, which could keep the storm coverage rather low.
In the inland areas, strong daytime heating quickly mixes out low-level moisture and reduces vertical wind shear. Storms in this dry, deeply mixed environment will be more isolated, high-based and poorly organized. The main risk consists of a few evaporation-driven downbursts.

... NW Black Sea, Aegean Sea, Ionian Sea ...

Rather cool mid-levels could allow surface-based convection in coastal and offshore areas in the morning and then again overnight, when land breezes propagate offshore. The forecast models strongly disagree on this issue: ECMWF and GFS show little signals, whereas ICON simulates widespread convective precipitation over the mentioned sea areas. Though it is not the high season for this phenomenon, this convection could spawn a handful of waterspouts. Chances for this scenario appear highest along the coasts of the Aegean Sea, where the level 1 from Greece is expanded a bit offshore for that reason.