Phenomenology of extreme floods in Emilia-Romagna: the role of soil preconditioning and atmospheric low-frequency variability

Extreme weather and climate events constitute one of the most impactful hazards to the environment and human society, for which correct and comprehensive understanding and representation still pose several major challenges. The devastating floods resulting from severe rainfall events are an emblematic example of these, whose impacts might be exacerbated by global climate warming. For these reasons, this study aims to examine the occurrence of extreme floods in a specific sub-region of Italy (Emilia Romagna), an area particularly prone to flood risk, within the broader context of compound extreme events, which are characterized by multiple interacting factors that can amplify the resulting impacts. Floods are categorised as preconditioned, when extreme precipitation is preceded by high soil moisture levels, or non-preconditioned, when this condition is absent. Precipitation and soil moisture extremes are identified using a peak-over-threshold method applied to ERA5-Land data (1979–2024) and analysed through event coincidence analysis to assess their statistical relationships. Hydrological impacts are evaluated using river discharge data from EFAS (1992–2024), while geopotential height anomalies at 500 hPa from ERA5 (1979–2024) are examined to determine their role as potential modulators. Results indicate that preconditioning by high soil moisture occurs more frequently in winter and substantially exacerbates flood severity, leading to extreme river discharge and often coinciding with persistent negative geopotential height anomalies. These insights improve understanding of flood precursors and contribute to enhancing early warning and prevention strategies in Emilia-Romagna and other regions with similar climatic conditions.