Storm Forecast
Valid: Fri 03 Jul 2020 06:00 to Sat 04 Jul 2020 06:00 UTC
Issued: Thu 02 Jul 2020 22:52
Forecaster: PISTOTNIK

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

A level 1 is issued for the W Ukraine, SE Poland, E Slovakia, Romania, Serbia, Bosnia-Herzegovina and Montenegro mainly for excessive convective precipitation and large hail.

A level 1 and level 2 are issued for N and central Italy for excessive convective precipitation, large hail and to a lesser degree for tornadoes and severe wind gusts.

A level 1 is issued for parts of the Adriatic Sea for tornadoes.

SYNOPSIS

A zonal flow regime is in place. Curving around a steering cyclone over Scandinavia, the mid-level jet axis runs from the British Isles to the Baltic Sea and Finland. Further south, a cut-off low travels leisurely from S France (Fri 06 UTC) to central Italy (Sat 06 UTC) and is flanked by another, weaker zonal wind maximum over the W and central Mediterranean region.
Closer to the surface, a long, diffuse and slowly advancing cold front stretches from NW Russia to the N Balkans, N Italy (delayed by a Genova cyclone) and the Balearic Islands. It separates temperature maritime air of Atlantic origin to the northwest from hot air to the southeast. Low-level moisture accumulates ahead of this front, and various areas in this belt will experience partly organized thunderstorms.

DISCUSSION

... W Russia into SE Belarus, east-central Europe to the Balkans ...

Despite rather poor lapse rates, 500-1500 J/kg CAPE are expected to build due to rich moisture accumulation and daytime heating along and ahead of the cold front. Scattered to widespread storms will form in the course of the day.
The most interesting environment in terms of organized convection develops from SE Belarus into NW Russia, where the belt of unstable air is overspread by synoptic lift and moderate to strong vertical wind shear (15-20 m/s across the 0-6 km layer, around 10 m/s across the 0-1 km layer). In this segment, forecast models also largely agree on the position of the frontal boundary and its sharpening due to strong daytime heating on the warm side. Storms in this environment can turn multi- to supercellular and pose a risk of large hail, exxessive rain, severe wind gusts and maybe one or two tornadoes. The risk diminishes some hundred kilometers beyond the Russian border, where the bulk of convective activity will shift into the evening and night and will have a tendency to decouple from the surface.
Further southwest toward east-central Europe and the Balkans, outflow boundaries from overnight convection, residual clouds and insolation gradients blur and modulate the front line and make forecasting its position quite difficult. A lack of synoptic lift and weak vertical wind shear will keep storms poorly organized. The primary risks are marginally large hail (mainly in initiating stages) and excessive rain (especially later on, when large storm clusters may develop). Convective activity will peak in the afternoon and evening before it fades after sunset.

... N and central Italy ...

The second area of enhanced thunderstorm activity develops ahead of the cut-off low. Supported by daytime heating, rather steep lapse rates aloft (remnants of a "Spanish plume") and synoptic lift, 1000-2000 J/kg CAPE are in place. Despite only weak to moderate SW-erly flow aloft, rather strong opposed surface winds - a blend of sea breezes and synoptic winds at the flank of the surface cyclone over the Gulf of Genova - will create 0-6 km shear up to 20 m/s.
Forecast models agree on scattered to numerous rounds of storms in the entire forecast period that gradually shift SE-ward, following the area of strongest synoptic and orographic lift. Much of the activity will be quite messy and partly embedded, therefore mainly prone to produce high rainfall accumulations with an enhanced flash flood risk.
More discrete storms - especially towards the SE - can organize well and pose additional hazards of large hail and one or two tornadoes, the latter especially along the east coast of central Italy in the evening and overnight. The wind risk, on the other hand, is kept in check by the moist air and the slow storm motion.
Additional tornadoes, this time of non-mesocyclonic origin, may spin up near the center of the cyclone over the Gulf of Genova.

... E Spain ...

A fragile overlap of CAPE up to 1000 J/kg and 0-3 km shear up to 20 m/s develops in the sea breezes along the Spanish east coast. Forecast models show isolated and short-lived storms over the mountains, but current thinking is that they will only form within the hot and dry inland air. The sea breezes, further enforced by the tail of the cold front, are probably not warm enough to be ingested into convective updrafts. Otherwise, storms could organize very well and pose a risk of large hail and isolated severe downbursts.

... Scandinavia, NW Russia, Baltic states, Poland ...

Scattered, daytime-driven thundery showers are expected across wide regions near the occlusion from Finland into NW Russia (in embedded form, though) and in the well-mixed maritime air in its wake. While vertical wind shear is moderately enhanced across deeper layers thanks to the mid-level jet, the low amount of CAPE and the limited depth of convection keep the severe weather risk low.