A cul-de-sac effect makes Emilia-Romagna more prone to floods in a changing climate.

The disastrous flood of May 2023 in Emilia-Romagna, Italy, displaced thousands of residents and had severe impacts on the economy, with extensive damage to infrastructure-roads, buildings, bridges-and losses in agriculture and livestock. The flood was caused by two consecutive precipitation events, during which no hourly rainfall extremes were recorded, but for which accumulated rainfall over several days produced nonetheless extreme flooding, with a return period of over 500 years. The persistent, long-lasting precipitation was fueled by an uninterrupted vertically integrated water flux from the Adriatic Sea over the Po Valley, driven by a cyclonic circulation over Italy that remained stationary for several days. A "cul-de-sac" effect, due to mountains that blocked moisture fluxes from the Adriatic Sea, amplified rainfall and was a root cause of the disaster. In this study, we analyze the dynamics of this case study in the context of the large-scale atmospheric circulation, focusing on the role of the stationary cyclonic structure over Italy, a feature that also characterized a similar event over the same area in 2024. Furthermore, by examining the frequency of stationary cyclones in the Mediterranean region over recent decades, we are able to suggest that the persistent, dangerous configuration observed during the 2023 and 2024 events should be of concern to other Mediterranean areas that share similar conditions. A preliminary analysis also suggests that this class of events may become more frequent in a changing climate with important implications for the early warning systems.