Recombining past event precipitation and antecedent catchment states generates unprecedented floods

Abstract River floods that exceed historical records often occur unexpectedly, causing widespread damage and disruption. To improve disaster preparedness, it is crucial to explore exceptional flood scenarios that surpass past observations. Here we introduce a perfect storm approach that recombines historically observed extremes in precipitation and soil moisture to generate plausible flood scenarios that are easy to understand by lay people and flood risk managers. Our analysis covers the ten most damaging floods recorded in Germany, two additional extreme precipitation events and two cases of exceptionally wet soil moisture conditions. The results show that the severity and damage of these recombined floods can systematically exceed those of the observed events, demonstrating the potential of the perfect storm approach to generate plausible scenarios of unprecedented floods. While shifting extreme rainfall to wetter soil consistently amplifies flood severity, moving events by just 1 month can also intensify flooding, which underscores the critical role of temporal alignment between catchment state and precipitation. Our study enables planners and policymakers to anticipate extreme flood scenarios beyond historical precedents, and offers a simple, yet powerful, strategy for obtaining flood scenarios to enhance disaster preparedness.