Storm Forecast
Valid: Fri 20 Jul 2018 06:00 to Sat 21 Jul 2018 06:00 UTC
Issued: Fri 20 Jul 2018 00:41
Forecaster: PISTOTNIK

A level 1 and level 2 are issued for N Italy, S Switzerland, S Austria and Slovenia for large hail, flash floods and to a lesser degree severe convective wind gusts, in NE Italy also for tornadoes.

A level 1 and level 2 are issued for N and E Spain and S France for large hail, severe convective wind gusts and flash floods.

A level 1 and level 2 are issued for central and E France, SW Germany, Switzerland and W Austria mainly for large hail and flash floods.

Level 1 areas are issued for N Turkey and the Russian Caucasus forelands mainly for large hail.

Level 1 areas are issued for parts of Lithuania, Belarus, the Ukraine and Russia mainly for flash floods.

SYNOPSIS

A long-wave trough stretches from the Norwegian Sea to Spain and gradually connects with a large, diffuse upper-level low centered over the Ukraine. This configuration brings weak pressure and temperature gradients across most of Europe, which is covered by a broad body of warm air.
Stronger mid-level flow stretches from Spain across the W Mediterranean Sea into Italy and the Balkans, where it fans out. An elevated mixed layer from the Sahara is advected NE-ward with this flow. In contrast, cooler Atlantic air infiltrates into France, Spain and Portugal.
Extremely hot conditions are also in place over Scandinavia, where continental air is advected far northward between the Atlantic long-wave trough and a blocking high over NW Russia. Many stations even up to the shore of the Arctic Sea beyond 70N recorded tropical nights and maximum temperatures up to 34C in the past few days, breaking even long-time temperature records by several degrees.

DISCUSSION

... N Italy, Swiss Ticino, S Austria, Slovenia ...

The most robust severe weather threat evolves in the warm air advection regime on the southern side of the Alps.
A first vorticity maximum will facilitate numerous storms over the mountains of NE Italy and S Austria from the early afternoon onwards. As soon as they detach from the mountains and tap into the thermal upvalley flow regime, they will encounter plentiful CAPE (around 1000 J/kg in S Austria and Slovenia, up to 2500 J/kg in N Italy) and increasing 0-6 km shear (around 15 m/s with slightly veering profiles). An organization into multi- or supercells is expected. The main risks are large hail (possibly approaching 5 cm in case of supercells) and flash floods, as the wind profiles and the orographic forcing will favor side- and backbuilding. The risk of a few severe downbursts increases as well in case of upscale growth. It is likely that an MCS with an ongoing severe weather risk will move from S Austria into Slovenia before it gradually decays in the evening.
Secondary storms may initiate along outflow boundaries in the NE Italian forelands, where a CAPE maximum and enhanced low-level shear are expected in the sea breeze regime. Some convection-resolving models showed a dangerous scenario with strongly right-moving supercells zipping along the sea breeze front, which would enhance the tornado chances considerably next to the mentioned other risks. A level 3 was considered for this area, but was discarded due to the lack of steeper lapse rates, which would favor more extreme hail and wind events, and due to the possibility that the capping inversion will hold.
Convective initiation will be more sluggish in NW Italy, but a second round of storms will probably detach from the western Alps by evening and travel eastward across Piedmonte and Lombardia after nightfall. The arrival of the main trough axis will likely keep at least scattered convection going through the night, possibly including one or two large clusters. Flash floods become a distinct possibility, especially in the Swiss Ticino and neighboring parts of Italy which may be exposed to intense storms for several hours. Discrete or tail-end storms may still be highly organized and pose additional risks of large hail and severe wind gusts under further increasing 0-6 km shear up to 20 m/s, if they manage to stay surface-based. The severe weather situation extends into Saturday.

... E Spain, S and E France into SW Germany, Switzerland and W Austria ...

CAPE between 500 and 1200 J/kg is expected in large areas ahead of the slowly approaching cold front, overspread by moderate vertical wind shear (0-3 km: up to 10 m/s, 0-6 km: up to 15 m/s). A few elevated and non-severe storms may already be active in the morning in parts of France. At least scattered storms become more and more likely as the day proceeds, aided by diurnal heating and travelling vorticity maxima. Multicells with a risk of large hail and flash floods will be the dominant mode. Maxima of afternoon activity will likely be over the Swiss and Swabian Jura and the Alps.
Both CAPE and vertical wind shear increase towards the south, but capping will be strong beneath the fringes of an elevated mixed layer from the Sahara. The most impressive overlap of CAPE and shear along the Spanish east coast will likely remain untouched, but forecast models largely agree that synoptic lift ahead of the trough and mesoscale lift of onshore and upslope flow will successfully work together to enable scattered convective initiation in NE Spain and S France until evening. These storms may organize very well and pose a risk of large to very large hail, severe downbursts and flash floods, the latter especially in case a large MCS forms. Storms will gradually weaken overnight and become embedded and elevated while travelling over E France, Switzerland and possibly SW Germany and W Austria.

... Scandinavia ...

Some hundred J/kg of CAPE will form in a hot but rather dry airmass. Scattered afternoon storms are expected. A mid-level jet which grazed the unstable air on Thursday will depart to the NE, leaving weak vertical wind shear and an accordingly low severe weather risk in the current forecast period. However, considering the hot and dry history, additional forest fires may be ignited by lightning strikes.

... E Europe into W Russia ...

A wide area will see scattered, daytime-driven thunderstorms in an environment of moderate CAPE and weak vertical wind shear. CAPE is composed of abundant low-level moisture, while lapse rates are not particularly steep, hence flash floods are the main hazard. Due to a lack of obvious convergence zones, two broad level 1 areas but no level 2 are issued.

... N Turkey, Georgia and Russian Caucasus forelands ...

Under strong daytime heating, dry environments and a lack of synoptic features, predicted pockets of moderate CAPE crucially depend on the bottling up of some low-level moisture, e.g. in sea breeze and upvcalley flow regimes. Isolated to scattered afternoon storms will form over orographic features and may organize into multicells under 15 m/s deep-layer shear. The main risk is large hail. In case of a higher storm coverage, isolated downbursts or flash floods are possible as well.