danni tempesta vaia altopiano asiago

Tempesta Vaia, the cyclone that devastated Italy in 2018: the meteorological analysis of an extreme event

The damage of the Vaia storm in the Asiago Plateau, in Veneto. Credit: Nordavind, CC BY–SA 4.0, via Wikimedia Commons

There Vaia storm which hit central-northern Italy between Saturday 27 October and Tuesday 30 October 2018 was undoubtedly among the most disastrous meteorological events of the last decades in our country, with very violent rains especially in the North-East, gusts of wind above the 200 km/hstorm surges and tidal waves on the upper Adriatic, floods and inundations. THEThe combined action of rain and wind devastated 42,500 hectares of forest in 494 municipalities in Northern Italy between Lombardy, Veneto, Trentino-Alto Adige and Friuli-Venezia Giulia, with a dramatic balance of 8.6 million cubic meters abated, 7 times the quantity of wood processed in a year by all Italian sawmills.

The meteorological history of storm Vaia

Vaia was born as a depression on Saturday 27 October 2018 within a vast depression channel stretching from the Baltic Sea to the western Mediterranean, which remained on the seas between the Gulf of Lion, the Balearics and Sardinia until the morning of Monday 29th.

In one first phasebetween 27 and 28 October, an intense flow of currents from the south and south-west, impacting along the southern slopes of the northern Apennines and the Pre-Alps, generated significant rainfall along the exposed slopes and on the adjacent foothills between northern Piedmont and Carniauntil reaching anomalous intensities The Comelico and Alto Adige on the border with Austria. Heavily hit by intense storms thehinterland of Genoa on the border with the Po basin, which generated a first wave of flooding in the Apennine watercourses.

In one second phasesince the morning of Monday 29 October, a impetuous reinforcement of the sirocco and the simultaneous development of violent storm cells between Sardinia, the Tyrrhenian Sea and the Ligurian Sea, rapidly moving towards the mainland and the Adriatic Sea. The triggering cause was a massive irruption of cold air over the warm Mediterranean Sea, where the Vaia depression was already present, centered west of Corsica, favoring its intensification. It is what is called in technical jargon explosive cyclogenesiswhen the atmospheric pressure decreases by at least 24 hPa in 24 hours.

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Cyclone Vaia over the Mediterranean Sea seen by satellite on 29 October 2018. Credit: Sat24.com/Eumetsat/Met Office

In the photo above you can see the spiraling cloud cover around the center of the cyclone, made up in part of the long crown of white cells associated with the stormy part of the front itself. At midnight on Monday 29 October the minimum pressure inside the cyclone dropped to 995 hPa, between the Balearics and Sardinia: it then moved towards the North-East, reaching theexceptional depth of 978 hPa on the Turin plain in the evening, as it migrated towards the north-western Alps, with a pressure drop of 17 hPa in about 18 hours: for this area this is one of the lowest values ​​ever recorded in over two centuries of observations.

Between the night and the morning of Tuesday 30 October the vortex then moved to Switzerland, and finally to Germany in the afternoon, allowing an attenuation of the winds and rains over Italy, as well as the entry of cooler currents from the west.

Record rainfall: over 600 mm in 3 days

According to the data provided by the stations of the various regional functional centers of Northern and Central Italy (ARPA Veneto, ARPA Liguria OSMER del Friuli Venezia Giulia, SIR Toscana) the precipitation epicentres of the entire event were located on the northern Apennines and on the Alpine reliefs between the Belluno area and western Carnia, where they were sometimes exceeded 600 mm of precipitation, almost entirely concentrated in three days from 27 to 29 October.

The cumulative totals of in particular stand out 623 mm in Torriglia (Genoa, Scrivia Po basin), 595 mm in Sappada (Udine), 716 mm in Soffranco (Belluno, Piave basin) e 817 mm at Malga Chiampiuz (in the Municipality of Forni di Sotto, Udine). Also noteworthy are the 534 mm of Lake Larecchio (Isorno Valley, Ossola, Toce basin).

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Cumulative precipitation over 72 hours (from 00 UTC on day 27 to 00 UTC on day 30). Source: National Department of Civil Protection through the Dewetra platform.

The violence of the phenomena was amplified by marked horizontal differences in temperature (as well as pressure) between west and east right next to Italy, with cold air of Arctic origin descending from Northern Europe towards France and warm air rising from Northern Africa towards the Italian peninsula.

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Map of temperatures and geopotential at an altitude of approximately 1400 metres, at 00 UTC on Monday 29 October 2018. Between the cold air over France and the warm air over Italy there is a difference of a dozen degrees less than 700 km, the origin of violent thunderstorms and stormy winds. Source: Wetterzentrale.de

Among the most striking effects on the ground we are talking about dozens of road interruptions, bridge collapses, bank erosion, debris flows on inhabited areas and landslidesconcentrated mostly between Trentino, the Dolomites, the Belluno area and Carnia.

One of the most intense skirmishes ever in Italy

Among the phenomena of unprecedented violence generated by its passage, the Vaia storm is remembered above all for its strength of the sirocco which blew during Monday 29th, then replaced by Libeccio in the evening starting from the seas west of the Peninsula.

The powerful gusts of southern wind were in some cases combined with further reinforcements and local wind turbulences, associated with storm systems, as occurred in the central hours of the same day in eastern Liguria and along the Tyrrhenian side, with hundreds of trees felled in the Lazio, especially in Terracina.

Here are some maximum speeds of the wind reached (always according to the weather data of the regional centres):

  • 128 km/h at Passo Valles (Dolomites);
  • 140 km/h Urbino-Oss. Serpieri;
  • 148 km/ha Capo Carbonara (south-eastern Sardinia);
  • 155 km/h at Colle di Cadibona (Savona);
  • 171 km/h in La Spezia and Follonica;
  • 180 km/h at Marina di Loano;
  • 192 km/h at Monte Cesen (Valdobbiadene);
  • 200 km/h on Monte Rest (Carnic Prealps);
  • 204 km/h on Monte Gomito (Tuscan-Emilian Apennines).
trees felled Trentino Vaia storm
Trees felled by storm Vaia in Trentino. Credit: Civil Protection Department from Italy, CC BY 2.0, via Wikimedia Commons

Destructive storm surges in Liguria and high water in Venice

Among the consequences of the impetuous sirocco winds on large marine surfaces, the following cannot be overlooked devastating storms especially in Liguriawith serious damage to coastal roads and railways and dozens of boats destroyed in ports, particularly between Rapallo and Portofino, but also further west, around Genoa.

The ARPA Liguria buoy in front of Capo Mele recorded, on the evening of Monday 29 October, a maximum wave height of well 10.3 meters. It was one of the most powerful storm surges ever observed in the area.

The episode of high water in Venicewith a maximum of 156cm at 2.10pm on 29 October, a value surpassed only by three other events in the historical series since 1872. A high tide of meteorological origin which, fortunately, did not combine with the maximum excursion of the sizygial tide which usually occurs in correspondence with the phases of the full moon: in this case the water levels would have been even higher and more harmful in the lagoon.