Storm Forecast
Valid: Sat 16 May 2020 06:00 to Sun 17 May 2020 06:00 UTC
Issued: Sat 16 May 2020 07:41
Forecaster: PISTOTNIK

A level 1 is issued for SW Russia for excessive convective precipitation (mainly towards the North) and for large hail and severe convective wind gusts (mainly towards the south).

A level 1 is issued for parts of Spain mainly for excessive convective precipitation.

SYNOPSIS

Low 500 hPa geopotential covers the Norwegian Sea and Scandinavia. Another cut-off low is parked over Morocco and Spain. An unusually strong subtropic jet with speed maxima in excess of 40 m/s at 500 hPa curves around it and runs then towards Italy, the Balkans and the Black Sea region, where it gradually merges with the fringes of the polar jet.
The majority of Europe is still under cold polar air which only slowly heats up to seasonable temperatures again. In contrast, a strong SW-erly flow advects very hot air into SE Europe, where many regions experience record-breaking temperatures for May.
The sharp frontal zone separating these two air masses runs from the W Mediterranean Sea across central Italy and the Balkans to the Black Sea and adjacent Russia.

DISCUSSION

... SW Russia ...

An MCS will likely survive this night and will continue to travel eastward along the frontal boundary on Saturday. Similar to Friday, a short-wave trough and frontal cross-circulations create strong lift across the frontal plane in an environment of limited, elevated CAPE and strong vertical wind shear. On Friday the MCS did not succeed to propagate towards the right to encounter better and surface-based instability. With a similar synoptic background on Saturday, confidence in such a scenario, which would strongly enhance the severe weather threat, is low. As long as the MCS stays elevated, excessive rain is the main risk, while isolated large hail is not ruled out at its southern flank in case of an embedded supercell.
Further south, very steep lapse rates are present, thanks to an elevated mixed layer advected all the way from the Sahara across the Balkans into this region and also reinforced over the Caucasus mountain range. Degree of low-level moisture, and hence CAPE, is unclear. If low-level moisture is maintained, robust CAPE can develop under very strong vertical wind shear and storm-relative helicity with values up to 25 m/s and 400 m^2/s^2 across the lowest 3 km, respectively. This environment would allow a quick storm organization into supercells with a main risk of large to very large hail and a secondary risk of severe downbursts. However, the further inland and especially the closer to the Caucasus mountains, the more likely it becomes that the shallow moisture is mixed out during daytime and both CAPE and shear are strongly reduced. For now, a low probability lightning area and a level 1 seem adequate.

... central Italy to Bulgaria and N Turkey ...

In contrast to Friday, low-level cold air will spread more southward and a short-wave ridge at mid-levels crosses the region. Forecast models show patchy CAPE near the base of some mountains where enough moisture accumulation and diurnal heating coincide, overlapped by still strong deep-layer shear.
However, in the presence of a strong cap, there are no indications of convective initiation. Stabilization and synoptic subsidence will cause a gradual cool change most likely without any precipitation.

... SW Europe ...

In the range of the cut-off low, diurnal heating creates some hundred J/kg CAPE under weak vertical wind shear over Iberia. Scattered to widespread, mostly daytime-driven storms form mainly over the mountains and along sea breeze fronts. They pose a primary risk of excessive rain with a few flash floods, especially in SE Spain and Catalonia where the highest storm coverage is expected. In addition, large amounts of small hail as well as one or two non-mesocyclonic tornadoes along convergence zones are possible.
Elevated and weak storms may also overspread the W Mediterranean Sea in the warm air advection regime ahead of the cut-off low. By the end of the forecast period, this activity could spread as far north as S France and N Italy.

... N and NE Europe ...

Low-topped daytime convection in well-mixed polar air does not pose a noteworthy severe weather risk. Highest chances for some deeper and weakly electrified cells exist over the Baltic States, which are crossed by a short-wave trough.