Tourists make their way along water-covered sidewalks near the famed Rialto Bridge in Venice, Italy, on Monday, 29 October 2018. High water levels forced schools and hospitals to close, and authorities advised citizens against leaving their homes. Photo: Stefano Mazzola / Awakening / Getty Images

By Bob Henson
30 October 2018
(Weather Underground) – An exceptionally strong upper-level storm and low-pressure center has played havoc this week with the weather across large parts of Europe, especially Italy, where high winds, intense thunderstorms, and a landslide caused at least 11 deaths. Most of the fatalities were from falling trees as a powerful squall line ripped across the length of the Italian peninsula on Monday afternoon. One man died in a wind surfing accident off the coast of the Emilia-Romagna district, and a woman near Trento in northern Italy died after a landslide engulfed her home, according to the Associated Press.Northwest of Rome, a wind gust to 202 km/hr (126 mph) was recorded on the island of Montecristo. This is just shy of the nation’s record-high gust of 210 km/hr, according to independent weather researcher Maximiliano Herrera.The costliest damage may have been on the Liguarian coast of western Italy, where close to 200 luxury yachts were reportedly destroyed in the port of Rapallo. Waves up to 10 meters (33 feet) high slammed into the marina, reported ANSA. The marina’s director said that a large section of a protective dam gave way despite having been bolstered after a storm in 2000.

Destroyed yachts and boats lie in the harbour of Rapallo, near Genoa, on 30 October 2018, after a storm hit the region and destroyed a part of the dam the night before. Photo: Marco Bertorello / AFP / Getty Images

The catalyst for the spate of extreme weather across Europe—including record October heat on one side of Switzerland followed by record October snow on the other—was a pronounced dip in the polar jet stream that extended across Spain well into northwest Africa. East of this cold upper-level trough, a strong flow of unseasonably warm air extended from northeast Africa all the way to Scandinavia. In between these contrasting air masses, an intense surface low moved from the Mediterranean across western Italy on Monday, accompanied by a cold front that spawned the bout of severe thunderstorms.

Venice’s worst “acqua alta” flooding in a decade

Some 70% of Venice was inundated on Monday by high water driven into the city’s lagoons by the circulation around the powerful surface low to the west. Venice is notorious for its “acqua alta” (high water) events, which periodically flood the renowned St. Mark’s Square and many of the city’s waterfront businesses and sidewalks. Acqua alta occurs when water is pushed lengthwise from south to north across the Adriatic Sea, parallel to Italy’s east coast. During such events, water naturally sloshes back and forth in the Adriatic over a 22-hour cycle, a phenomenon called a seiche. When this happens to be synchronized with the 24-hour astronomical tidal cycle, the seiche can accentuate the effects of an already-extreme weather-induced tide.

The frequency of tidal flooding in Venice of at least 110 cm (43.3")—enough to inundate 12 percent of Venice—has ramped up from about once per decade in the early 20th century to once every three to eight weeks in the 2010s. Graphic: City of Venice

Much like king tides in Miami, acqua alta events are most common in autumn and winter, when astronomical tides are at their highest. […]Tides in Venice are classified as “exceptional” when they exceed 140 cm (55.1″), which is enough to flood 59% of the city. According to City of Venice records, exceptional tides have occurred:

  • only once from 1872 to 1950;
  • about once every 5 to 10 years in the late 20th century;
  • and—ominously—10 times in just the last two decades, including this week’s event. [more]

Flooding in Venice and Deadly Storms across Italy