Intense Mediterranean cyclone's alpine crossing

By Ivan Smiljanic (SISYS), Michal Žák (CHMI) and Yasmin Markl (ZAMG) and Sancha Lancaster (Pactum)

An exceptionally strong cyclogenesis took place in the western Mediterranean during the last five days of October 2018. The cyclone (named as Adrian by Météo-France and Vaia by the Freie Universität Berlin (FUB)) brought extreme weather, especially to Italy where intense thunderstorms, high winds and very heavy rainfalls lead to flash floods and landslides. More than 30 fatalities were reported.

Impacts in Italy

The violent storms battered Italy for days, causing 29 deaths. The storms, combined with very high tides, caused the worst flooding in Venice for a decade.

• In Venice water levels rose by 156cm (more than 5ft), flooding many parts, including St Mark's Square and the Basilica.
• In Sicily 12 people died after swollen rivers burst their banks.
• In Rapallo, north-west Italy, the breakwater walls, yachts and the port area were destroyed by a storm surge.
• The resort town of Portofino was cut off by a landslide.
• The fishing village of Vernazza was flooded with seawater.
• High winds toppled trees that killed passersby in three accidents in Naples and Lazio.
• Around 300,000 trees were flattened, after winds swept through the Val d'Assa in the Asiago plateau.
• Two people were killed when a tree hit a car they were travelling in near Frosinone, south of Rome.

Synoptic development

The catalyst for this event was a pronounced dip in the polar jet stream that extended into northwest Africa. A very cold air mass was advected over southern parts of the UK on 26 October.

This cold and dry air can be seen as the red/magenta areas on the Airmass RGB satellite imagery (Figure 1).

Following this cold air intrusion, an intense cyclogenesis occurred over Spain and the western Mediterranean during 28 October.

On the eastern side of the cyclone (which was also very well developed in the upper levels) a strong flow of unseasonable warm air extended over the Balkans up to southern Scandinavia, while the cold air flowed on the western side to north-western Africa.

A very large temperature gradient (about 15°C) and the very warm waters of the Mediterranean Sea contributed to the cyclone's intense development.

The cyclone's maximum intensity was reached on 29 October, when severe storms formed on the cold front. These crossed over almost all of Italy, then moved on to Croatia and Slovenia during the evening.

The cumulonimbus clouds reached very high levels, as can be seen from the bright white colour of their tops in the Meteosat-11 Airmass RGB (Figure 3) — indicating dangerous weather phenomena were occurring.

A high resolution view of the storm, perhaps when it was at peak intensity, is provided by the Suomi-NPP VIIRS True Color RGB (Figure 4).

During the following night, the cyclone moved across the Alps, taking an unusual track directly from south to north. The satellite imagery shows how the cloudiness became disorganised in the vicinity of the low centre during this mountain crossing.

Later, on 30 October, the cyclone continued its track north, over Germany, where it was already slowly filling up.

The flow of dry air, originating from stratosphere (red colour), led to almost clear skies in the belt stretching from the Adriatic Sea, over Czechia, to eastern Germany.

Finally, on 31 October, the cyclone moved over Scandinavia, reaching its final stage of development, filling up and merging with an older cyclonic system.

In the meantime, another cyclogenesis was starting over the western Mediterranean region. Although its development was not so impressive, the convection connected with the low brought further heavy and deadly thunderstorms to Italy.

High winds, waves and sea levels

Intense and persistent wind forcing associated with the cyclone was responsible for the build up of the waves and sea levels. This led to the destruction of coastal areas, and heavy flooding, mostly in the downwind areas.

The northwest coast of Italy suffered the most from the high waves meeting coastal infrastructures and communities. From the wave model it is obvious that significant wave heights exceeded 6m. (Figure 5)

High winds also caused a lot of damage, along the path of the cyclone:

• On the Mediterranean island of Corsica powerful winds left 21,000 homes without power, and ports and airports were closed.
• In Croatia gales caused flooding and loss of some power.
• Storms also swept into the Ticino region of Switzerland causing flooding, fallen trees and damage to properties.
• In Austria rivers burst their banks and high winds caused damage to a section of the Medieval ramparts of Salzburg.

South Foehn wind on October 29/30 in a widespread Alpine region

Due to a strong pressure gradients, well-pronounced wind fields directed towards the southern side of the Alps produced the perfect conditions for foehn winds north of the main alpine ridge.

The ECMWF chart for wind gusts and surface pressure forecast wind speeds up to almost 20 km/h. (Figure 6).

The special feature of this foehn case was that the cold front reached the southern side of the Alps, before it reached the northern side. This had an important influence on the temperature gradient and the static pressure field, and triggered strong wind gusts at the exact moment when the cold front passed through.

This could be observed in weather station data, by the typical increase of temperature, and a rapid decrease of humidity (dew point), with a synchronous increase of wind speed and a change of wind direction to a mostly southerly direction.

Also with different satellite images it was possible to observe interesting clouds structures, such as the lee clouds northeast of the Alps, and an almost cloud-free area north of the Alps. (Figure 10).

Heavy rainfall October 29/30 in a widespread Alpine region

Due to a passage of the cyclone directly over the Alps, many clouds built up on the southern slopes of the Alps, producing heavy and long-lasting rain events in the region.

These clouds are seen well on the 24h Microphysics RGB sequence as a red shaded features (high and thick clouds). (Figure 11).

The structure of these clouds could be better seen in the the higher resolution MODIS Natural Color RGB (Figure 12).

Snowfall in southern France

Looking at the Natural Color RGB (Figure 14) over France a huge area of snow-covered ground was visible in south-eastern France. The snow areas appear as cyan (same colour for ice clouds, although there are not many in this imagery).

• More than 1,000 drivers were trapped in their cars for the night in the mountains of the Massif Central region, as snowstorms engulfed the roads.
• Another 400 had to spend the night in train carriages at the main station in the eastern city of Lyon after heavy snow blocked the tracks.
• More than 100,000 homes were left without power across mainland France, most of them in eastern and central regions.

Additional content

9 people from 2 families killed in Sicily flooding as Italy death toll rises to 29 (CNN)
Three-quarters of Venice flooded by exceptional high tide (The Guardian)
Three killed, one missing in storms (ANSA)
Heavy snow causes travel chaos and power cuts in central France (The Local)
Weather Alert: U.S. Embassy Rome, Italy, October 29, 2018
Heavy rains and high winds from Storm Vaia cause flooding, infrastructure damage, and road closures as of October 30; severe weather expected to continue through at least October 31 (Garda World)