Storm Forecast
Valid: Fri 30 Jul 2021 06:00 to Sat 31 Jul 2021 06:00 UTC
Issued: Fri 30 Jul 2021 00:01
Forecaster: PISTOTNIK

A level 1 and level 2 are issued for the greater Alpine region for large hail, excessive convective precipitation and severe convective wind gusts.

Level 1 areas are issued for E Slovakia and for central Romania mainly for large hail and to a lesser degree for excessive convective precipitation and severe convective wind gusts.

A level 1 and level 2 is issued for the E Ukraine and W Russia mainly for excessive convective precipitation and to a lesser degree for large hail and severe convective wind gusts.

A level 1 and level 2 are issued for parts of the Baltic Sea, Estonia and Latvia mainly for severe convective wind gusts and to lesser degree for tornadoes.

SYNOPSIS

Two pronounced cyclones (at 500 hPa and at the surface) cover Scandinavia and the British Isles. An unseasonably strong W-erly to SW-erly flow remains in place along their southern flank from Spain across France, the greater Alpine region and Poland to Belarus. It is associated with a diffuse frontal zone that separates temperate, maritime air to the north from hot subtropic air to the south, with temperature levels close to all-time heat records in SE Europe and parts of Spain. The hot and dry air of Saharan origin is mixed down to the surface in inland areas (Spain, Balkans, Turkey) during daytime, whereas better low-level moisture can be maintained over the Mediterranean Sea (where the maritime boundary layer is strongly capped) and closer to the frontal zone. The latter environment is also once more the main site for organized thunderstorm activity.

DISCUSSION

... belt from central France to Slovakia and Romania ...

A cocktail of a very warm airmass with fairly steep low- to mid-level lapse rates, strong daytime heating, moisture accumulation along the frontal zone and further moisture enhancement by local "recycling" of evaporation once more creates CAPE on the order of 500 to 2000 J/kg. CAPE values increase in general from north to south and from the mountains to the lowlands. This moderately to highly unstable airmass is overspread by moderate deep-layer shear between 15 and 20 m/s underneath the mid-level jet.
Scattered thunderstorms are expected to form from noon onwards over orographic features, in particular the Alps. The forecast model pool somewhat agrees on two rounds of convection in the Alpine region, supported by two subtle short-wave troughs at mid-levels. The first round of storms shall affect the eastern Alps in Austria, Alto Adige / South Tyrol and Slovenia, the second one shall initiate over the Massif Central and the Swiss Alps and shift eastward in the evening. Further mountains looking ready for storm initiation are the High Tatras and the southern Carpathians in central Romania, but without synoptic lift storm coverage will be lower and convection will struggle more to detach from their initiation sites.
The primary convective mode will be multicells and probably several large clusters as the day proceeds. In the belts of suppressed vertical mixing in the upvalley flow regimes, local CAPE and shear enhancement may support some supercells or a few bowing line segments, if storms manage to survive a little distance into the forelands. (Side nide: wind profiles favor right-moving storms on the southern side and left-moving storms on the northern side of the Alps.)
A high coverage of large hail, severe wind and excessive precipitation events is expected that should qualify for a level 2 across a rather wide area in and around the Alps. Isolated extreme events are possible as well (hail with supercells and wind with bowing line segments). The other mountain ranges with less prolific "storm production" mentioned above are covered by level 1 areas.
Storm clusters continue to move eastward well into the night, while they gradually turn elevated and their severe weather risk decreases. In the range of the second trough, residual storms may affect larger parts of SE Germany, the Czech Republic, Austria and Slovenia overnight. Otherwise, capping over flat terrain is very strong and mostly suppresses thunderstorm activity without orographic help, even at peak heating times.

... E Ukraine into W Russia ...

Despite weaker lapse rates, particularly rich low-level moisture ahead of a cold-active segment of the frontal zone creates another CAPE maximum with expected amounts up to 1500 J/kg in the northern and often between 1000 and 2500 J/kg in the southern half of the highlighted area. Scattered to widespread, mostly daytime-driven thunderstorm form along the slowly advancing convergence zone and with the support of another short-wave trough aloft. Under weak vertical wind shear and moist profiles, excessive precipitation is the primary risk. Large hail and severe downbursts are possible with stronger pulse storms and multicells, especially where CAPE is maximized, but also towards the north, where vertical wind shear is slightly enhanced.
Convection will slowly weaken but probably not die overnight, while it continues to move E- to NE-ward and turns elevated. A couple of storms may also form on the Sea of Azov overnight, where a few waterspouts are possible at the nocturnal land breeze front.

... Baltic States ...

At the southern flank of the Scandinavian cyclone, a few hundred J/kg CAPE develop over the unusually warm Baltic Sea. Vertical wind shear is strong throughout the lower half of the troposphere (e.g., around 20 m/s across the 0-6 km layer and in inland areas also exceeding 10 m/s across the 0-1 km layer). Scattered, low-topped thundery showers form and move onshore mainly in the first half of the forecast period, when strong synoptic lift is in place. They can mix down 850 hPa winds around 25 m/s and produce at least scattered severe wind gusts at the surface. In case some of the storms manage to organize better despite the limited amount of CAPE, the severe wind risk further increases, and one or two tornadoes are not ruled out with (supercellular) storms deviating from the mean wind.

... SE England ...

Almost saturated, neutrally to marginally unstable vertical profiles are predicted near the center of the British cyclone. With a little daytime heating, a few weakly electrified thunderstorms could be embedded in the otherwise stratiform rain field. While heavy rain and in the strong pressure gradient towards the south also strong to isolated severe wind gusts are possible, a convective contribution is too questionable for a level 1.