Storm Forecast
Valid: Sun 28 Oct 2018 07:00 to Mon 29 Oct 2018 06:00 UTC
Issued: Sun 28 Oct 2018 07:40
Forecaster: PISTOTNIK

A level 1 and level 2 are issued for parts of the Italian main land, parts of Sicily, S Austria, coastal Slovenia, coastal Croatia, Bosnia-Herzegovina and Montenegro mainly for flash floods and to a lesser degree for severe convective wind gusts, large hail and tornadoes.

A level 1 is issued for inland Slovenia, inland Croatia, SE Austria and W Hungary for severe convective wind gusts and to a lesser degree large hail and tornadoes.

A level 1 is issued for the W-central Mediterranean Sea (including Sardegna) mainly for severe convective wind gusts and to a lesser degree for large hail and tornadoes.

A level 1 is issued for parts of the Bay of Biscay and the Spanish N coast mainly for severe convective wind gusts.

SYNOPSIS

A positively tilted, stationary and highly amplified mid-level trough stretches from the Russian Arctic Sea SW-ward to Spain, where its tip cuts off into a separate upper-level low. Maritime polar air flows far southward and brings unsettled, showery and very cool conditions to the NW half of Europe. In contrast, the strong SW-erly flow ahead of the trough advects extremely warm air into its SE half.
The resulting frontal zone further tightens and sharpens on Sunday, especially along the Alps: temperatures barely above freezing and snowfalls down to low levels in Switzerland and SW Germany will be opposed by temperatures up to 25C from N Italy to Hungary. A new surface cyclogenesis between the Balearic Islands and Corsica starts to push the frontal zone northward as a pronounced warm front across France, Germany, Poland and Belarus. An active day of organized storms is forecast in the warm sector that is overspread by the strong mid-level jet.

Far SE Europe is dominated by high pressure. A "cold" anticyclone near the surface also forms over Scandinavia, which has temporarily turned wintry beneath the base of the mid-level trough.

DISCUSSION

... S Alpine region, mainland Italy, W Balkans into Hungary ...

Strong Scirocco winds advect very warm and moist air northward across the central Mediterranean region. Low-level moisture increases and grows deeper in response to synoptic lift ahead of the trough, mesoscale lift by converging surface winds, and orographic lift as this flow impinges the Apennines, Alps and Dinaric mountains. Depending on the length of the fetch, a few hundred to 1000 J/kg of CAPE will build, maximized in coastal areas of Croatia. A capping inversion is created by warm air advection and eroded by the lift processes, with the latter gradually getting the upper hand in this interplay and making convective initiation more likely.
Heavy "stau" precipitation with scattered embedded thunderstorms is expected on the upwind side of the mentioned mountain chains. Of major concern are the S Alps, while a secondary maximum is foreseen on the upwind side of the Dinaric mountains somwhere between Sibenik (Croatia) and Kotor (Montenegro). Forecast models agree on series of intense backbuilding storms in these regions, and 50 to 150 mm rain may often accumulate on Sunday, locally even more in case of persistent training bands of convection. Flash floods are a distinct threat, and the risk of river flooding will start to increase as well, especially as the snow level will remain around or above 2500m. Vertical wind shear varies, but is in general around or above 15 m/s across the lowest 3 km, hence tail-end storms in coastal or offshore areas may turn supercellular and pose additional risks of severe downbursts, large hail and tornadoes.

The activity will gradually shift eastward with a first, subtle vorticity maximum ahead of the main trough that overspreads the areas from central Italy and the W Balkans to Austria and Hungary. In the afternoon and evening, its lift and daytime heating will also create 100 or 200 J/kg of CAPE over land as the warm and moist low-level jet advects Adriatic moisture far inland. Isolated to scattered storms are expected over E Austria, Slovenia, inland Croatia, N Serbia and W Hungary. With vertical wind shear between 15 and 20 m/s that is mostly concentrated across the lowest few kilometers, any storm can turn supercellular and bring severe wind gusts, a tornado or marginally large hail. As the strong low-level wind field will limit nocturnal cooling, convection may stay surface-based well into the night, and a level 1 covers the associated risks.

... W-central Mediterranean between the Balearic Islands and Italy ...

The surface cyclone is predicted to move from the Balearic Islands to the Cote d'Azur and to deepen to ca. 990 hPa by Monday morning. The warm air advection regime ahead of it advects an elevated mixed layer (EML) from N Africa towards Malta, Sicily, Sardegna and the Tyrrhenian Sea, creating more CAPE, possibly in the 1000-2000 J/kg range until the end of the forecast period. 0-3 km shear gradually increases to 15-25 m/s as the long-wave trough approaches and a particularly strong mid-level jet streak swings around its base overnight. If synoptic lift in its left exit region suffices to break the cap and initiate storms, supercells with all kinds of severe weather are likely, including very large hail and tornadoes. However, latest forecast runs reduced the probability for this scenario, hence only a level 1 and a low-probability lightning area are issued for the Tyrrhenian Sea.
Convective initiation is more likely W of Sardegna and Corsica beneath the fringes of the EML, but CAPE is limited and the wind field stays somewhat weaker until 06 UTC. An eastward acceleration of the cold front with an increasing risk of severe wind gusts is foreseen towards the end of the forecast period, but latest forecast runs postponed this scenario until beyond Mon 06 UTC, i.e. into the next forecast period.
Parts of Sicily were also added to a level 1 for isolated nocturnal thunderstorm activity in a decent shear/CAPE environment. Ongoing uncertainties in initiation and coverage of storms preclude higher probabilities for now. Any storm, which develops can produce all kind of severe.

... coastal areas of France, Spain and Portugal ...

Scattered to widespread showers will be active in the stream of polar air. Some of them will become deep enough to produce a little lightning.
Parts of the Bay of the Biscay and the Spanish N coast are upgraded to a level 1, as the strong background winds (~20 m/s at 850 hPa) and evaporative cooling may drive convective wind gusts just beyond the 25 m/s threshold. In addition, isolated instances of excessive precipitation are not ruled out in the upslope flow in N Spain, like localized flooding or, at higher elevations, roads blocked by snow.
In the rest of the area, the severe weather risk is low, as deep mixing will strongly limit both CAPE and vertical wind shear.