Revisiting the Atmospheric Dynamics of the Two Century Floods over North-Eastern Italy

The two most intense precipitation events that occurred over northern and central Italy in the last century are analyzed in this study: the November 1966 flood and the Vaia storm in October 2018. These two events share a similar large-scale evolution, characterized by a vigorous baroclinic wave, deepening on the western Mediterranean basin and slowly evolving eastward. Although this is a common synoptic setting for severe weather over Italy in autumn, in these cases the interaction between incoming Rossby wave packets produced a particularly strong downstream development over the Mediterranean. In both events, the large-scale dynamics was able to focus towards the Mediterranean basin an exceptional high amount of moisture transported from the Tropics in the form of Atmospheric Rivers (ARs), although with local and remote differences related with the sea and atmospheric state conditions characterizing these two episodes occurring 52 years apart. As a result, the precipitation patterns, in terms of both duration, intensity and distribution, were quite different: while in 1966 heavy rainfall affected for 48 h mostly Tuscany region (infamous Florence flood) and north-eastern Italy, in 2018 almost the entire Alpine chain, as well as Liguria region and central Italy, were hit by severe events during almost three days. Only over the north-eastern Italian Alps the rainfall fields look similar in the two events. The mesoscale dynamics and the moisture supplies are investigated in detail, highlighting peculiarities and common aspects. It is found that the different characteristics of the ARs (intensity and steadiness) partially explain the rainfall patterns, but the complete picture has to take into account also local (e.g. Mediterranean) sources of moisture and smaller scale circulation features that turned out to be relevant. Through a relative comparison, this study points out some important aspects of the genesis of extreme precipitation events over Italy, identifying important precursors and moisture sources. In an operational context, this may help to recognize the environmental conditions potentially leading to the most severe precipitation episodes.