The Vaia storm in Otober 2018

Together with the infamous flood of 1966, the "Vaia" storm (27-29 October 2018) represents one of the most intense precipitation events of the last century over Italy. It was characterized by an explosive cyclogenesis in the Mediterranean, heavy rain and floods, storm surge, and extremely intense wind gusts that caused casualties and extensive damage, especially to the Alpine forests. Several scientific papers investigated the different aspects of the storm, which is also known as "Adrian": storm surge aspects in the Adriatic (Cavalleri et al., 2019) and in the Ligurian Sea (Cavalleri et al., 2022); heavy precipitation and floods (Giovannini et al., 2021); strong wind over Corsica (Lfarh et al., 2023) and wind storm impact on Alpine forests (Motta et al., 2018, among others). Also, popular science articles described the storm and its impacts (Cavalleri et al., 2019, 2022). The Vaia storm was also characterized by an extremely intense atmospheric river which transported a significant amount of water vapour from the Atlantic Ocean tropical areas, across the African continent, into the Mediterranean basin, to feed the extreme precipitation over Italy. This aspect is analyzed in detail in Davolio et al. (2020).

GFS analysis, 29 Oct 2018 00 UTC

72h accumulated precipitation on 27-29 Oct 2019 (source DPC-Dewetra)

The synoptic characteristics leading to the Vaia storm development were those typical of autumn intense precipitation events over the western Mediterranean basin, associated with an amplified synoptic wave positioned over western Europe. A deep trough deepened over the western Mediterranean basin during 27 October and in the morning of 28 October a cut-off process began between France and Spain, while the trough extended further south reaching northern Africa, favouring cyclogenesis at the surface in the lee of the Atlas Mountains. On 29 October, the intense upper level forcing and a strongly baroclinic environment promoted an explosive cyclogenesis close to the Algeria coast. The intense Mediterranean cyclone (977 hPa) rapidly moved northward towards Corsica, reaching northwestern Italy in the evening. The slowly evolving large scale circulation and the progressively stronger pressure gradient between the western Mediterranean area and eastern Europe favoured intense meridional exchange, with strong advection of moisture from the Mediterranean basin toward the Italian peninsula and the Alps for several days. Heavy precipitation affected all the Alpine chain, with the maximum amount of 817 mm in 72 hours over the eastern Alps, but also hit Liguria (more than 500 mm in 72 hours) and central Italy (about 350 mm in 48 hours).

The detection algorithm clearly identifies an Atmospheric River, whose maximum integrated vapour transport (IVT) reached an impressive intensity exceeding 1200 kg/(ms) over the Tyrrhenian Sea on 29 October, in correspondence with the most intense phase of the event. The atmospheric river was several thousand km long and the transport of moisture was confined in the lower troposphere, below 3000m all along its path. A detailed diagnostic based on the water budget computation in the atmosphere, together with numerical experiments, revealed the critical role of the atmospheric river in supplying moisture to the heavy precipitation systems over Italy (Davolio et al., 2020), although the first phase of rainfall was mainly supported by moisture coming from the Atlantic.

It is worth mentioning that numerical simulations and weather predictions were able to capture the main exceptional characteristics of the storm in terms of precipitation, winds and storm surge, being this event associated with a well-defined large-scale forcing. The good forecasts allowed a timely action of the civil protection, thus limiting the adverse impacts on the population. Also, the Vaia storm came after a dry period, and the precipitation over the 3 days was characterized by a hiatus of several hours. These aspects certainly affected the hydrological response and river discharges, preventing much more adverse impacts.

Not only intense rainfall, but also floods, landslides, exceptional wind storms, waves and storm surges were experienced during those days. Over the eastern Italian Alps, the fierce wind, associated with gust values exceeding 50 m/s at some locations, had a dramatic impact on the environment, with 41,000 ha of forest damaged and a loss of about 8.5 million m^3 of growing stock. Although not comparable with the impressive damages due to extratropical storms over Europe, Vaia produced the worst wood destruction in Italy of all times, as a result of the compound effect of heavy rainfall and strong wind.

A comparison between the two "century floods", the 1966 event and the Vaia storm (Sioni et al., 2023), showed that the events shared a similar large-scale configuration, which is typical of heavy precipitation episodes over northern Italy during autumn: a vigorous amplification of a baroclinic wave extending over the western Mediterranean and slowly evolving eastward, which favors the meridional transport of moisture.

In both events, a significant amount of water vapour was transported by an intense atmospheric river, from tropical regions into the Mediterranean basin, and the characteristics of the atmospheric river in terms of location and evolution explained most of the observed precipitation patterns. In both cases, the water vapour transport was confined within the lowest 3000 m of troposphere, but during Vaia the atmospheric river was broader and oscillated back and forth over the Tyrrhenian Sea, thus affecting a wider area. On the other hand, during the 1966 flood, the water vapour transport was more persistent on the same location, from Tuscany to north-western Alps, and an important contribution of moisture came through low-level advection from the Eastern Mediterranean.

The contribution of the Sirocco was critical in both events: it provided further moisture from the Adriatic Sea and it caused exceptional wind storms and storm surges, although with different characteristics and impacts. The "Acqua Alta" in Venice was higher in 1966 (194 cm vs 156 cm), but the winds were much more fierce over the Alps in 2018.
Finally, also the geo-hydrological response and damages were different, depending on the preceding soil moisture conditions (dryer in 2018), the snowfall elevation, the snow cover at the ground possibly melted by rainfall, and the (time) distribution of precipitation.

References - more on this event:

Sioni F., S. Davolio, F. Grazzini, L. Giovannini, 2023: Revisiting the atmospheric dynamics of the two century floods over north-eastern Italy. Atmospheric Research, 286, 106662, https://doi.org/10.1016/j.atmosres.2023.106662
Davolio S., S. Della Fera, S. Laviola, M.M. Miglietta, V. Levizzani, 2020: An atmospheric river in the Mediterranean basin: the 29 October 2018 “Vaia” storm over Italy. Mon. Weather Rev. 148, 3571–3588. https://doi.org/10.1175/MWR-D- 20-0021.1.
Cavaleri L., M. Bajo, F. Barbariol, M. Bastianin, A. Benetazzo, L. Bertotti, J. Chiggiato, S. Davolio, C. Ferrarin, L. Magnusson, A. Papa, P. Pezzutto, A. Pomaro, G. Umgeisser, 2019: The October 29, 2018 storm in Northern Italy - an exceptional event and its modeling. Prog. Oceanogr. 178, 102178 https://doi.org/ 10.1016/j.pocean.2019.102178.
Cavaleri L., F. Barbariol, L. Bertotti, G. Besio, F. Ferrari, 2022: The 29 October 2018 storm in Northern Italy: Its multiple actions in the Ligurian Sea. Progress in Oceanography, 201, 102715, https://doi.org/10.1016/j.pocean.2021.102715.
Giovannini L., S. Davolio, M. Zaramella, D. Zardi, M. Borga, 2021: Multi-model convection-resolving simulations of the October 2018 Vaia storm over Northeastern Italy. Atmos. Res., 253, 1–20. https://doi.org/10.1016/j.atmosres.2021.105455.
Lfarh W., F. Pantillon, J. Chaboureau, 2023: The downward transport of strong wind by convective rolls in a Mediterranean windstorm. Mon. Wea. Rev., 151, 2801–2817, https://doi.org/10.1175/MWR-D-23-0099.1.
Motta R., D. Ascoli, P. Corona, M. Marchetti, G. Vacchiano, 2018: Selvicoltura e schianti da vento. Il caso della ‘‘tempesta Vaia.’’ Forest, 15, 94–98, https://doi.org/10.3832/efor2990-015.
Magnusson L., Cavaleri L., 2019: Predicting multiple weather hazards over Italy. ECMWF Newsletter, 158, 2-3.