Western Mediterranean flash floods through the Lens of Alcanar (NE Iberian Peninsula): Meteorological drivers and trends

Abstract

Flash floods in the Western Mediterranean pose a growing hazard due to the effects of climate change, rapid urbanization, and land-use changes. This study focuses on flash floods in the Montsia region of southern Catalonia (NE Iberian Peninsula), with particular emphasis on the municipality of Alcanar, because it illustrates the recent intensification of flash flood dynamics in the Western Mediterranean. The research is motivated by three recent severe flood events in Alcanar (2018, 2021, and 2023), each characterized by extraordinary rainfall totals and significant economic losses, unprecedented in the municipality’s 30-year observational record. Methodologically, we integrate multiple data sources—including meteorological station observations, weather radar products, lightning detection networks, high-resolution mesoscale model outputs, and a flood database spanning 1980–2023, complemented by economic compensation records from 1996 to 2020. Through this approach we (i) assess the regional frequency of heavy rainfall and flood episodes, (ii) quantify the economic impacts in Alcanar, (iii) characterize the meteorological and thermodynamic conditions on the three most intense recent events (including moisture source tracking via a Lagrangian methodology), and (iv) analyze spatio-temporal trends in extreme rainfall indicators (percentiles, threshold exceedances, kurtosis, and skewness). Our findings suggest that, from a meteorological perspective, current flash flood behavior in the Western Mediterranean likely emerges from the interplay of localized orographic triggers, elevated sea surface temperatures, strong instability associated with low-level moisture, particular positioning of jet streaks, synoptic-scale cut-off lows, and remote moisture sources. The results also point to an increase in rainfall intensity, explained by the presence of high precipitable water content and shallow convection, which enhances precipitation efficiency. These insights highlight the critical need for robust flood early warning systems, strategic watershed management, and improved risk communication to mitigate escalating flash flood risks in the Montsia county and similar areas throughout the Western Mediterranean region.