Storm Forecast
Valid: Sun 31 May 2020 06:00 to Mon 01 Jun 2020 06:00 UTC
Issued: Sat 30 May 2020 23:12
Forecaster: PISTOTNIK

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

A level 1 is issued for parts of N Algeria for large hail and severe convective wind gusts.

A level 1 is issued from S Ialy to Bulgaria and the Aegean Sea mainly for large hail and excessive convective precipitation in inland areas, and mainly for non-supercellular tornadoes (waterspouts) in coastal and offshore areas.

A level 1 is issued from the E Ukraine into W Russia mainly for large hail and excessive convective precipitation.

SYNOPSIS

A "high over low" blocking pattern is in place with an anticyclone over Scandinavia and a large, bean-shaped mid-level low from France to Italy, the Balkans and the Ukraine into NW Russia, where it connects with a new cut-off process.
Apart from Scandinavia and Iberia, cooler-than-normal conditions cover most of Europe. Towards the southeast, diurnal heating of this modified polar air creates vivid, albeit mostly disorganized, thunderstorm activity. Better organized storms are expected in the warmer airmass in parts of Portugal and Spain.

DISCUSSION

... Iberia, Morocco, Algeria ...

Daytime heating creates CAPE between 500 and 1500 J/kg in the NW half of the Iberian Peninsula, around the Portuguese/Spanish border perhaps even up to 2000 J/kg, as onshore and upslope flow feeds beneath steep lapse rates created inland. Despite a lack of synoptic lift, the capping inversion will be overcome around noon, and first storms are expected over the mountains. Later in the afternoon and evening, activity will become quite widespread and also propagate to lower areas along outflow boundaries.
With slightly enhanced SW flow at mid-levels and a complex low-level wind field, dominated by sea breezes and upvalley/upslope circulations, 0-3 km shear is expected to be mostly between 10 and 15 m/s. Storm will therefore organize into multicells and probably some temporary supercells with a main risk of large to locally very large hail and severe downbursts. Later on, upscale growth into several MCSs is likely which move east- to northeastward. By then, excessive precipitation becomes another hazard.
Convection will gradually become elevated and weaken after sunset.

Somewhat less CAPE and a stronger capping are present in the SE half of Spain and in Morocco and Algeria in an otherwise similar environment supportive of organized and severe storms. Vertical wind shear is even stronger in the well-established sea breeze regime, but this air needs a lot of diurnal heating to overcome the strong cap. Without synoptic lift, surface-based convective initiation is rather unlikely and will, if at all, be confined to orographic features. Other than that, weakly electrified and non-severe elevated convection is not ruled out near the nose of the Saharan warm air plume.

... Italy to S Balkans, NW Turkey, E Ukraine, W Russia ...

Another round of scattered to widespread thunderstorms is predicted in an environment of low to moderate CAPE and mostly weak vertical wind shear. The majority of storms will form in the afternoon over land, in particular over mountainous areas. The strongest pulse storms can bring localized excessive rain, marginally large hail or copious amounts of small hail. Level 1 areas are issued for those regions where the CAPE magnitude can reach 500-1000 J/kg and the highest storm coverage is expected.
Similar to the past few days, mid-levels are cold enough to support also some storms in coastal and offshore areas in the night and morning, when land breeze propagate offshore. The latter is particularly likely over the S Adriatic and N Ionian Sea early and over the Aegean Sea late in the forecast period. This activity may spawn a handful of waterspouts.