Storm Forecast
Valid: Fri 14 Sep 2018 06:00 to Sat 15 Sep 2018 06:00 UTC
Issued: Thu 13 Sep 2018 22:24
Forecaster: PISTOTNIK

A level 1 and level 2 are issued for the E Black Sea and coastal areas of the Crimean Peninsula, SW Russia and W Georgia for flash floods, large hail, tornadoes and severe convective wind gusts.

A level 1 and level 2 are issued for the Turkish N coast mainly for flash floods and to a lesser degree for waterspouts.

A level 2 is issued for NE Morocco and NW Algeria for flash floods, large hail and to a lesser degree severe convective wind gusts.

Level 1 areas are issued for S Spain, S Italy, NE Italy, S Austria, NW Slovenia, and E Slvakia mainly for flash floods.

SYNOPSIS

A strong zonal mid-level jet (20-30 m/s at 500 hPa) stretches across the British Isles and Scandinavia into NW Russia. Cool maritime air floods these areas. The associated cold front stretches from S Belarus and S Poland along the Alps into S France, but makes only little southward progress.
Very warm to hot air is still present everywhere further south. Stationary cut-off lows are placed near the Strait of Gibraltar and over the central Black Sea. Plentiful CAPE over the warm sea waters is overspread by mid-level jet streaks at the forward flanks of these cut-off lows, creating favorable severe weather setups. Anticyclonic influence dominates over the central and E Mediterranean region and especially over the Bay of Biscay.

DISCUSSION

... E Black Sea and surrounding coastlines ...

A mid-level jet curves around the S and E flank of the cut-off low and pushes an elevated mixed layer (EML) from the Turkish highlands onto the E Black Sea. An impressive overlap of up to 3000 J/kg CAPE with 20-30 m/s deep-layer shear is predicted to build in this process on Friday. The Thursday 12z Samsun (Turkey) sounding showed this forming CAPE, but also the extreme cap beneath the EML.
Synoptic lift will be plentiful ahead of the cut-off low, but may still not suffice to break this capping inversion for a longer time. Convective initiation may therefore happen rather late and/or at the W fringe of the EML. The overlap of CAPE and shear starts to crumble there but still suffices for organized storms, including supercells, with a risk of large to very large hail, severe wind gusts and a few tornadoes. Flash floods become another risk in the coastal belt. The center of storms moving or forming onshore will slowly shift from the Crimean Peninsula eastward into SW Russia and W Georgia towards evening and night.

At the back side of the cut-off low, the flow turns westerly to northwesterly and steers maritime air with still plentiful low-level moisture and some hundred J/kg of CAPE against the N Turkish coastlines. Vertical wind shear relaxes. Flash floods become the main concern. Over the water, a few tornadoes, this time non-supercellular, remain possible throughout the night.

... W and central Mediterranean region ...

CAPE on the order of some hundred to 1000 J/kg is in place over much of the W and central Mediterranean Sea. The unstable maritime boundary layer will once more mostly be capped over open sea waters, but may be converted into scattered afternoon storms where it is ingested into thermal upslope circulations.
More CAPE and also better deep-layer shear up to 20 m/s are present over NE Morocco and NW Algeria ahead of the mentioned Gibraltar cut-off low. After synoptic lift erodes the initially strong cap, scattered to widespread storms become likely by evening and the first half of the night, possibly including one or two MCSs. Discrete storms will likely turn multi- or even supercellular with a risk of large hail and severe wind gusts. Excessive precipitation becomes the dominant risk with clustering and/or training storms. Activity may persist during much of the night, as NE-erly surface winds continue to pump unstable air with plentiful moisture onshore. Life-threatening flash floods in sloping catchments are not ruled out!
A similar but less threatening scenario is expected over S Spain, where synoptic lift will likely keep scattered storms going into the night. In the synoptically quiescent environment over Corsica, Italy and the W Balkans, convection should mostly decay around sunset.

... S Belarus into S Alpine region ...

The trailing cold front and a vorticity maximum aloft "activate" slightly unstable warm air with a few hundred J/kg of CAPE. Scattered storms are expected from the late morning, mostly over the eastern Alps and the High Tatras, from where they may propagate east- to southeastward into adjacent forelands towards evening. These areas are covered with a level 1 mostly for a possibility of isolated flash floods. In addition, marginally large hail might occur in initiating stages, though this risk is limited by weak lapse rates and deep-layer shear barely above 10 m/s. Convection will be more isolated over flat terrain, and will in general mostly decay in the evening.

... NW and N Europe ...

The stream of cool maritime air over fairly warm sea waters creates a little instability over wide offshore and coastal areas. According to Thursday's soundings, the depth of convection was limited to ~2 km from the British Isles to the Baltic Sea and only increased to thunder-allowing heights (>5 km) towards the Shetland Islands and Norway.
On Friday, two short-wave troughs will move eastward in the zonal flow, the first one from S Sweden into NW Russia and a second, stronger and larger one from the British Isles to the Baltic Sea. Convection will grow deeper and may produce a little lightning ahead of their paths.
Despite enhanced vertical wind shear, meager CAPE and still rather low cloud top heights limit the severe weather threat. However, one or two tornadoes and isolated severe wind gusts are not ruled out. This risk is relatively highest around Denmark overnight, but still too diffuse for a level 1.