Storm Forecast
Valid: Fri 29 May 2020 06:00 to Sat 30 May 2020 06:00 UTC
Issued: Thu 28 May 2020 23:54
Forecaster: PISTOTNIK

A level 1 is issued from the E Ukraine into W Russia for excessive convective precipitation, large hail, severe convective wind gusts and tornadoes.

A level 2 is issued for the Russian Causcasus forelands for large hail, severe convective wind gusts and excessive convective precipitation.

A level 1 is issued for NE Turkey, Armenia, Azerbaijan and Georgia mainly for large hail and to a lesser degree for severe convective wind gusts and excessive convective precipitation.

A level 1 is issued for parts of Romania, Bulgaria, Serbia, Albania, Greece, central Italy and the French Alps mainly for excessive convective precipitation and large hail.

A level 1 is issued for NE Algeria for large hail and severe convective wind gusts.

SYNOPSIS

An amplified mid-level ridge from Portugal to the North Sea and a surface anticyclone centered over the North Sea and S Norway introduce a beginning blocking pattern. It creates stable and warm (in Iberia even hot) conditions in W Europe.
Further downstream, a complex and extensive mid-level low filled with cool and moist air stretches between the Baltic States, Italy and Turkey. At its forward flank, a pronounced surface cyclone has formed over the central Ukraine which brings wrapped-around warm air advection into W Russia and Belarus.
While scattered thunderstorms are expected across wide regions beneath the mid-level low, organized convection is limited to the periphery of the Ukrainian surface cyclone.

DISCUSSION

... E Ukraine into W Russia ...

Moisture is accumulated near the warm front (though on Thursday this belt seemed more emphasized by model forecasts than it appeared in actual surface observations), and a few hundred J/kg CAPE likely become available under strong 0-3 km shear (around 15 m/s) and storm-relative helicity (150-250 m^2/s^2). Similar to Thursday, scattered convection may be limited to elevated and embedded activity for a prolonged time in the strong belt of isentropic lift in the warm air advection regime. In the afternoon and evening, surface-based initiation in the warmer air is possible, though far from certain (on Thursday, there were no clear indications that it indeed occurred). Storms that manage to root down to the surface can easily turn supercellular with a risk of large hail, severe wind gusts and, given 0-1 km shear around 10 m/s and a strongly helical flow under fairly low cloud bases, also a few tornadoes. As long as convection stays elevated, heavy precipitation is the main risk.
Due to the confined time frame and limited confidence in surface-based initiation, a level 1 seems sufficient for now.

... NE Turkey, Armenia, Azerbaijan, Georgia, Russian Caucasus forelands ...

The cold front of the Ukrainian cyclone pushes eastward, but is strongly modulated by the land and sea distribution, which awards it the character of an enhanced sea breeze front. Further inland, CAPE on the order of 500 to 1000 J/kg (perhaps locally up to 2000 J/kg) is likely to form during daytime, as an elevated mixed layer originating from the Turkish Plateau and from the Caucasus mountains, respectively, spreads northward over adjacent lowlands. Supported by synoptic lift from a short-wave trough swinging northeastward, scattered storms are expected to form by afternoon. 0-3 km shear around 15 m/s suggests that the main storm mode will be multi- to supercellular. The primary risk is large to locally very large hail, though other kinds of severe weather cannot be ruled out.
While most storms will weaken and decay in the evening, a prolonged or even enhanced severe weather risk can emerge over the Russian Caucasus forelands, where synoptic lift seems to coincide best with mesoscale circulations across a dryline north of the Caucasus mountains and the approaching cold front. It is well possible that the initially discrete (and struggling) storms can consolidate and grow into an MCS in the evening, which moves east- to northeastward well into the night with a risk of severe wind gusts, excessive rain and still isolated large hail.

... east-central and SE Europe, rest of Turkey, Italy, Corsica and French Alps ...

Daytime heating creates some hundred J/kg CAPE under weak vertical wind shear. Another round of scattered daytime-driven storms forms, the strongest of which may bring localized heavy rain, marginally large hail, or copious amounts of small hail.
Due to the unseasonably cold mid-level air and above-average sea surface temperatures, some convection may again propagate offshore along land breeze fronts overnight. A few off-season waterspouts were reported off the Greek coastlines in the past few mornings, and a few more are possible again on Friday morning.

... NE Algeria ...

An overlap of moderate CAPE and moderate vertical wind shear is likely along the Tell Atlas mountains. Scattered afternoon storms pose a risk of large hail and severe downbursts. Limiting factors are too cool air for surface-based storms in the sea breeze regime to the north and too little moisture to sustain convective updrafts in the desert air to the south.