Storm Forecast
Valid: Sat 30 May 2020 06:00 to Sun 31 May 2020 06:00 UTC
Issued: Fri 29 May 2020 23:47
Forecaster: PISTOTNIK

A level 1 is issued for W Russia to E Belarus for excessive convective precipitation (mainly towards the W), large hail, tornadoes and severe convective wind gusts (mainly towards the E).

A level 1 is issued for parts of Italy, Montenegro, Bosnia-Herzegovina, Serbia, Romania and Bulgaria mainly for excessive convective precipitation and large hail.

A level 1 is issued for the S Adriatic Sea and coastal areas of central and S Italy for non-mesoscyclonic tornadoes (waterspouts).

A level 1 is issued for N Portugal, parts of N Spain, NE Algeria and NW Tunisia for large hail and severe convective wind gusts.

SYNOPSIS

A "high over low" blocking pattern has established with an anticyclone over Scandinavia and a large, gradually filling mid-level low between BeNeLux, the Ukraine, Turkey and Italy. Closer to the surface, a strong and almost stationary cyclone is placed over the central Ukraine, whereas pressure gradients are weak everywhere else.
Much of Europe and the Mediterranean region is still under rather cool air. Warm air advection is directed from Morocco to Iberia, Ireland and Scotland and from Siberia to W Russia and Belarus, respectively.

DISCUSSION

... W Russia, Belarus ...

Hot air from the Siberian prairie with steep lapse rates is advected westward in the warm sector of the Ukrainian cyclone. Friday's surface observations showed a V-shaped zone of enhanced moisture with 2m dewpoints between 15 and 18C both near the almost stationary warm front and near the cold front that quickly wraps northward around the cyclone. This distribution will be similar on Saturday and CAPE around 500 J/kg is likely in vicinity of the frontal system. Deeper in the warm sector, the convective boundary layer is deeper and drier and CAPE more limited, though probably non-zero. This unstable air is overspread by enhanced vertical wind shear (~15 m/s across the 0-3 km layer) with strongly veering winds near the warm front and more unidirectional profiles near the cold front.
In the range of the warm front, elevated and embedded storms will likely be active through much of the forecast period under strong isentropic lift. They pose a main risk of heavy rain and may spread quite some distance westward beyond the occluison point into Belarus, despite cool low-level air. Further east, scattered surface-based initiation is possible in the course of the day. These storms can easily turn supercellular with a risk of large hail, severe wind gusts and, given 0-1 km shear around 10 m/s and a strongly helical flow under fairly low cloud bases, also a few tornadoes.
More widespread initiation is expected at the cold front in the late morning to afternoon, probably including multicells and some supercells in early stages and at least one or two large clusters later on. As the storms mature and grow upscale, their main risk shifts from large hail to severe convective wind gusts (especially when they run into the hot and dry warm sector) and excessive convective precipitation (especially closer to the cyclone center). This convection may persist into the night while gradually weakening.
Current thinking is that some regions may see a severe weather coverage sufficient for a level 2, but two sources of uncertainty tip the scales for a broad level 1 area instead: Near the warm front, precipitation signals in the model pool are pretty sparse, possibly indicative of a pronounced inversion and a lack of synoptic lift (aside the warm front itself). The cold front phases better with synoptic lift from a short-wave trough, but ECMWF, GFS and ICON diverge by several hundred kilometers with respect to its position.

... Italy, Balkans to the Ukraine, W and N Turkey ...

Daytime heating creates some hundred J/kg CAPE under weak vertical wind shear. Another round of scattered to widespread, disorganized daytime-driven convection forms. The strongest pulse storms can bring localized heavy rain, marginally large hail, or copious amounts of small hail.
In the morning and then again in the following night, convection may also affect some coastal and offshore areas as a result of nocturnal land breeze fronts propagating offshore, especially around central and S Italy, the Adriatic Sea and the Ionian Sea. A few off-season waterspouts are possible with this activity.

... Iberian peninsula, Morocco, Algeria, Tunisia as well as E Turkey, Georgia, Armenia, Azerbaijan ...

Placed beneath the fringes of the (weak) subtropic jet, these regions will regionally see moderate vertical wind shear. Whereas low-level moisture is soon mixed out by daytime heating in deeply inland areas, pockets of enhanced moisture can be maintained and can support CAPE up to 1000 J/kg in confined areas of onshore, upvalley and upslope flow regimes. Isolated to scattered afternoon storms are possible mostly over mountains which have access to this moisture. In the absence of synoptic lift, they will struggle against a capping inversion and dry entraining environmental air. However, longer-lived storms can organize into multicells or temporary supercells. The main risk is large hail. In addition, a few evaporation-driven downbursts are possible.