Storm Forecast
Valid: Sat 22 May 2021 06:00 to Sun 23 May 2021 06:00 UTC
Issued: Sat 22 May 2021 01:09
Forecaster: PISTOTNIK

A level 1 and level 2 area issued for parts of Turkey, Armenia, Azerbaijan, Georgia and SW Russia mainly for large hail and to a lesser degree for severe convective wind gusts (mainly towards the SE) and excessive convective precipitation (mainly towards the NW). In the level 2 areas, all risks overlap and are maximized.

A level 1 is issued for parts of NW Russia, Latvia, Lithuania and Poland mainly for severe convective wind gusts and to a lesser degree for large hail and tornadoes.

SYNOPSIS

A very large, persistent area of low 500 hPa geopotential and low surface pressure covers the entire N Atlantic and NW Europe. It is flanked by an unseasonably strong SW-erly jet stream with an axis from Iberia to the Alpine region, Belarus and NW Russia. Two embedded short-wave troughs cross NW Russia and move from Switzerland to Belarus, respectively, during daytime on Saturday.
Cool maritime air is present in the NW half of Europe on the cyclonic side of the jet stream. On its anticyclonic side, very warm to hot airmasses are in place in the W and central Mediterranean region as well as over the far SE of our forecast domain. In-between, a cut-off low that has leisurely drifted from Romania into central Turkey has brought somewhat cooler air into the Balkans, temporarily softening the frontal zone.

DISCUSSION

... E Turkey, Armenia, Azerbaijan, Georgia, SW Russia ...

The most interesting setup for organized, severe convection develops ahead of the Turkish cut-off low.
Hot air with very steep lapse rates from the Near East is advected northward and overspreads decent low-level moisture in the lowlands of Armenia, Georgia and the Russian Caucasus forelands. The degree and depth of this moist boundary layer are difficult to assess, as it may be highly variable and is poorly sampled by the coarse station network, but Friday's observations suggest that 2m dewpoints around 15C may be maintained and allow the buildup of CAPE on the order of 1000 J/kg, perhaps locally up to 2000 J/kg with strong daytime heating. Ahead of the the approaching cut-off low, 0-6 km shear increases to 15-20 m/s.
Warm air advection and a few subtle vorticity lobes indicate that some synoptic lift is available, though it looks rather weak compared to the strength of the capping inversion. Current thinking is therefore that convective initiation will be isolated to scattered. However, storms that form will quickly organize into multi- and supercells and are likely to produce large hail, which could well exceed 5 cm in diameter in case of supercells. In addition, dry environmental air and steep lapse rates promote the risk of evaporation-driven severe downbursts.
The areas at highest risk are quite confined in space: Over elevated terrain and in general towards the SE, too dry entraining air could be deadly for any towering cumulus clouds before they become robust enough. Close to the Black Sea and in general towards the NW, on the other hand, a lack of moisture is not a big issue anymore, but the low-level air may be too strongly capped or even not buoyant at all. However, ths transition belt offers reasonable chances for convective initiation. The "sweet spots" will likely be tied to drylines, i.e. airmass boundaries separating hot and dry, deeply mixed air over the mountains from cooler and moister, capped air over the forelands. Such drylines are expected to develop on the downwind side of mountain chains with respect to the mid-level flow, most notably in far-southwest Russia (in the wake of the Caucasus mountains) and in Georgia and Armenia (in the wake of the Anatolian plateau), but also more locally in the south Turkish provinces of Adana and Osmaniye as soon as the mid-level flow turns to the W to NW with the passage of the cut-off low. (Interesting detail for the latter area: forecast soundings indicate strongly backing hodographs, therefore left-moving supercells with anticyclonic rotation could even be favored over their right-moving counterparts.) These areas are upgraded to a level 2.
Depending on the amount of synoptic lift that finally arrives, storms might grow upscale into an MCS in the evening especially in the W parts of Georgia or far-southwest Russia, which could (in elevated form) survive some distance onto the Black Sea while it starts circling around the cut-off low. Otherwise, convection weakens after sunset and the severe weather risk quickly diminishes.

... central Turkey ...

Near the center of the cut-off low, forecast models still predict a few hundred J/kg CAPE under variable, but in general relaxing vertical wind shear. Scattered, partly embedded thunderstorms are expected. The main risk is heavy to excessive rain. Large hail is not ruled out, either, especially in areas of more insolation or pockets of still enhanced vertical wind shear. A low-end level 1 covers these risks.

... far-northwest Russia, Baltic States, Belarus, Poland ...

Near the main frontal zone and on its cool side, some hundred J/kg CAPE develop in response to daytime heating and overlap with moderate vertical wind shear. Deep-layer shear turns even strong (~20 m/s) towards the south, where convective cloud top heights could be too low to fully benefit from these strong kinematics, though.
Scattered, mostly daytime-driven showers and thunderstorms are expected and can turn into low-topped multi- and supercells or short bowing lines. The main risk are some severe wind gusts. In addition, marginally large hail is possible and one or two tornadoes are not ruled out.
The highest storm coverage is expected over Poland, where the diurnal timing of the mentioned short-wave trough is best. Some high-resolutions models (e.g., ICON-DE and UM) show quite aggressive solutions for W and central Poland with scattered swaths of severe wind gusts, but neither CAPE nor vertical wind shear nor other factors favoring severe wind gusts (e.g., dry layers fof environmental air at mid-levels) appear impressive enough to upgrade to a level 2.
Further NE into the Baltic States and Russia, uncertainties are higher, owing to less confidence in both enough CAPE buildup and a timely arrival of sufficient synoptic lift. Overall, the discussed area is encompassed by a broad level 1.

... belt from Hungary to N Italy ...

On the warm side of the strengthening frontal zone, a few hundred J/kg CAPE are simulated under impressive deep-layer shear (20-30 m/s). Despite some patches of synoptic lift and stronger mesoscale lift due to the frontal cross-circulation, little to no signals for convective initiation can be seen, possibly due to the imbalance between too strong shear and weak buoyancy in the layers relevant for cumulus cloud growth.
Nonetheless, any signs of convection in this belt should be monitored. Storms near the front line would likely be embedded and elevated, hence their severe weather risk is low apart from a possibility of heavy rain. Storms forming ahead of the front or moving away from it could organize well and bear an enhanced hail and wind risk.

... other thunderstorm areas from Finland and Sweden to France ...

Scattered, daytime driven thundery showers in well-mixed, maritime air should mostly stay non-severe, though isolated, marginally severe rain, hail and wind events are not completely ruled out.
Despite quite weak winds aloft, a confined belt with curved hodographs and enhanced 0-1 km shear is in place in the Netherlands and NW Germany ahead of a sharp surface trough that passes this area in the morning to early afternoon. Together with low cloud bases and converging surface winds, this points to a possibility of one or two short-lived tornadoes. Limiting factor which precludes a level 1 is the expected cloud cover, which keeps daytime heating - and therefore low-level buoyancy - on the low side.