Storm Forecast
Valid: Sat 12 Jun 2021 06:00 to Sun 13 Jun 2021 06:00 UTC
Issued: Fri 11 Jun 2021 23:01
Forecaster: PISTOTNIK

A level 1 is issued for parts of Russia, the Ukraine, Romania, Bulgaria, Serbia and North Macedonia mainly for excessive convective precipitation.

A level 1 and level 2 are issued for parts of Austria, Slovenia, Italy, Greece, Turkey, Georgia, Armenia and the Russian Caucasus region mainly for large hail and excessive convective precipitation.

A level 1 and level 2 are issued for parts of Spain and Portugal for large hail, severe convective wind gusts and excessive convective precipitation.

SYNOPSIS

A broad and diffuse mid-level low stretches between Finland, the Black Sea and Italy. Pressure gradients near the surface are very weak, and moderately warm and moist air allows scattered to widespread convective activity.
Stronger dynamics come into play further upstream. A cyclone translates from Iceland to N Scandinavia and sends a cold front and a pronounced mid-level short-wave trough into east-central Europe. In contrast, a warm anticyclone builds from the Bay of Biscay to France and the W Mediterranean Sea, which is surrounded by a strengthening, anticyclonically curved jet stream.
Finally, a stray cut-off low is located over Portugal.

DISCUSSION

... from Russia to the Balkans ...

Scattered to widespread, mostly daytime driven afternoon storms form in a synoptically quiescent environment with CAPE up to 1000 J/kg and very weak vertical wind shear. The main risk is heavy to excessive rain, while the lack of steeper lapse rates and vertical shear keeps the hail and wind risk low. One or two non-supercellular tornadoes are not ruled out.

... belt from SE Austria, Slovenia and Italy to Greece, Turkey, Georgia, Armenia and the Russian Caucasus region ...

The fringes of the unstable airmass are characterized by steeper lapse rates and overspread by slightly stronger mid-level winds, yielding deep-layer shear around or slightly above 10 m/s. The steepest lapse rates are advected off the mountains and create robust CAPE where they overspread lowland or coastal moisture. In particular, Friday's soundings indicated that an "Alpine plume" of very steep lapse rates managed to create 1000-1500 J/kg in the lowlands and coastal areas of Italy, and similar or even higher CAPE values are expected on Saturday.
Scattered storms with a strong diurnal cycle are predicted which can organize into multicells with a risk of large hail and localized excessive rain. The initiation sites are strongly tied to orography. Those storms that manage to move over south- to east-facing slopes and forelands have the best chances of organization. In these situations, transient supercells with very large hail are not ruled out.
Limiting factor is the lack of synoptic lift, which results in a limited storm coverage. In particular, coastal areas are strongly capped.

... Spain and Portugal ...

CAPE up to 1000 J/kg develops in vicinity of the cut-off low, as very steep lapse rates from the Spanish Plateau are advected northwestward towards the coasts and overspread augmented low-level moisture. Scattered afternoon storms are expected and can organize into multicells and a few supercells under deep-layer shear around 15 m/s. Scattered severe hail and wind events are likely, and the inverted-V profiles further inland can also promote severe downbursts. A few extreme hail and wind events are not ruled out.
Some storms can continue or re-form into the night under some synoptic lift (warm air advection), but will turn elevated and pose a lesser severe weather risk.