Storm Forecast
Valid: Sat 06 Jun 2020 06:00 to Sun 07 Jun 2020 06:00 UTC
Issued: Sat 06 Jun 2020 00:55
Forecaster: PISTOTNIK

Level 1 areas are issued for parts of Finland and NW Russia for severe convective wind gusts and tornadoes.

A level 1 and level 2 are issued from E Belarus into W Russia for large hail, tornadoes, severe wind gusts and excessive convective precipitation.

A level 1 and level 2 are issued for parts of the Czech Republic, S Germany, parts of Austria, parts of Switzerland, N Italy, S France and N Spain for large hail, excessive convective precipitation and severe convective wind gusts.

A level 1 and level 2 are issued from N Romania into the central Ukraine mainly for large hail and excessive convective precipitation.

A level 1 is issued for the SW Balkans for excessive convective precipitation and to a lesser extent for large hail.

Level 1 areas are issued for the coastal belt in NE Italy and for the NW Aegean Sea for non-supercellular tornadoes (waterspouts).

SYNOPSIS

The current period of lively cyclonic activity in the European sector continues. Two cyclones, which are very pronounced throughout the depth of the troposphere, move southward over the North Sea and northward over the Baltic Sea, respectively, pivoting around a common barycenter. A strong mid-level jet with up to 50 m/s at 500 hPa surrounds their southern flank and tilts from a W-E to a SW-NE orientation on Saturday in response to the swerving cyclone cores. A long, diffuse and largely stationary frontal zone accompanies the mid-level jet at its anticyclonic side and runs from the Finnish/Russian border to Germany to Spain. Maritime polar air is present to its NW.
The Mediterranean region and the Balkans are facing warmer but not entirely stable conditions. Another cut-off low moves from the Aegean Sea to Bulgaria and a short-wave trough swings eastward across Spain.
Weak mid-level ridging with very warm and more stable weather is confined to Turkey and the Black Sea region.

DISCUSSION

... Finland into NW and W Russia, Belarus, the central Ukraine, N Romania ...

The cyclone over the Baltic Sea creates quite versatile environments for organized convection:
(1) Finland is overspread by a powerful vorticity maximum from the SW, whose lift in combination with daytime heating creates marginal CAPE. The resulting convection will initiate in the wake of the occlusion close to the Finnish south coast from 12 UTC onwards and will quickly spread NE-ward until it gradually outruns the strongest forcing and starts to weaken in the evening. Strong vertical wind shear, in particular low-level shear around or above 10 m/s, creates very good conditions for storm organization. Both a linear storm mode with a risk of isolated to scattered severe wind gusts and a discrete storm mode in form of low-topped multi- to supercells with a risk of a few tornadoes are imaginable. While synoptic lift and vertical wind shear are quite impressive, the marginal and rather shallow instability is the limiting factor. Chances for electrified storms are higher towards the NW, where synoptic lift is maximized.
(2) NW Russia will see a similar environment to Finland, but with slightly more CAPE and less forcing instead. It is also covered by a level 1, though the risk appears somewhat lower here.
(3) Further south, the occlusion opens up into a warm front running eastward into W Russia and a cold front stretching southwestward into SE Belarus and the central Ukraine. Both fronts are moving slowly and allow rich moisture accumulation on their warm sides. Forecast models are quite bullish and predict CAPE up to 1500 J/kg, which is not unrealistic but will require sufficient insolation. Scattered storms are expected to initiate by afternoon. 0-3 km bulk shear between 10 and 15 m/s provide favorable conditions for multi- to supercellular organization, especially near the warm front, where strongly veering wind profiles are available. These storms may produce all kinds of severe weather, most prominently large to very large hail. An enhanced tornado risk is in place near the warm front. The storm intensity and their severe weather risk will gradually decrease in the evening, when they cluster and decouple from the surface.
(4) Along the tail of the cold front, the model pool largely agrees on another small convection hot spot in the afternoon to evening over the Carpathians around the Romanian and Ukrainian border. CAPE around 1000 J/kg overlaps with 0-3 km shear between and 10 and 15 m/s, probably more in the upslope circulations NE of the mountains. Multi- to supercellular storms pose a main risk of large hail and flash floods, in particular as they may be anchored over topographic features for several hours. In the evening a propagation to the forelands is expected, but probably not for long distances before they decay.

... Czech Republic to N Spain ...

Strong daytime heating and solid moisture recovery are foreseen on the warm side of the trailing frontal zone. Some hundred J/kg CAPE will likely develop. Thanks to fairly strong WSW-erly mid-level flow and warm air advection, they overlap with enhanced vertical wind shear (0-3 km bulk shear around 15 m/s) and veering wind profiles (0-3 km storm-relative helicity between 100 and 200 m^2/s^2). Both CAPE and shear are maximized in the upvalley and upslope circulations on both sides of the Alps as well as in and around the Ebro Valley in NE Spain. The most impressive kinematics will likely evolve in the north Alpine forelands in SE Germany and N Austria, where the surface flow turns NE-erly in response to an Alpine heat low and may boost 0-3 km storm-relative helicity to up to 400 m^2/^2. The highest CAPE values around or even above 1000 J/kg are expected over N Italy.
Helped by warm air advection and subtle vorticity maxima aloft, scattered storms will initiate in the afternoon. A fairly quick organization mostly into multicells and into some supercells is expected. The primary risk is large hail, but heavy rainfall and a few severe wind gusts may occur as well, especially in case of upscale growth later on. Especially the level 2 areas may be exposed to one or two longer-lived supercells, and isolated very large hail is not ruled out in that case.
Storms in and around the Alps will decay in the evening, while the synoptic lift by the strongest mid-level trough may carry the Spanish storms well into the evening and night, when they will gradually spread into S France. The severe weather risk is still enhanced but gradually shifts to heavy precipitation.

... Bosnia-Herzegovina to Greece ...

Low to moderate CAPE and mostly weak vertical wind shear are in place in the range of the cut-off low. Scattered to widespread but poorly organized, daytime-driven storms pose a risk of localized flash floods and marginally large hail.

... coastal belt in NE Italy, Bay of Thessaloniki in Greece ...

Coastal and offshore convection (not necessarily electrified) at the land breeze / sea breeze front could spawn one or two non-mesocyclonic tornadoes. The coastal belt of the Veneto and Friuli regions in NE Italy is exposed to this localized risk on Saturday morning, the Bay of Thessaloniki in Greece towards Sunday morning.

... Morocco and Algeria ...

Robust CAPE and strong vertical wind shear overlap beneath the subtropic jet. However, forecast soundings show a fairly strong cap, and not much synoptic lift is available to erode it. Elevated, weakly electrified and non-severe convection is the most likely scenario. In case surface-based storms can yet initiate over the mountains in the afternoon, a conditional large hail and severe downbursts risk unfolds.

... Sweden to England ...

Low-topped afternoon convection in the well-mixed polar air should stay non-severe. The only possible exception is E England, where marginally severe wind gusts of partly convective origin are not ruled out at the flank of the strong cyclone core that moves from the English coast towards the Netherlands.