The Value of Precipitation Forecasts to Anticipate Floods

Abstract Recent disasters have highlighted the severe local impacts of extreme precipitation, including flash floods, landslides, and urban inundation. Despite significant investments in early warning systems, these events often catch many people off guard, emphasizing the need for a better translation of warnings into early actions. In this study, we directly address this gap by translating precipitation forecasts from the ECMWF, specifically the extreme forecast index (EFI) and shift of tails (SOT), to concrete triggers for early action. Our analysis reveals that such actions, triggered with a 2–3-day lead time, have a high potential economic value (PEV) across large parts of Europe. The SOT forecasts generally have higher economic value than EFI, especially at longer lead times. However, the effectiveness of both disappears across most of Europe with a 5-day lead. These results are based on comparing forecasts to the E-OBS dataset for extreme rainfall (>5-yr return period) over 8 years. We apply the optimal warning thresholds found in this analysis to the rainfall event that triggered the July 2021 western Europe flood disaster. Our results indicate that the EFI and SOT forecasts provided accurate and timely warnings at least 2–3 days in advance, aligning with flood impacts recorded in the Emergency Events Database (EM-DAT) dataset. Notably, on a 1-day lead time, the SOT forecasts accurately pointed to the Ahr catchment in Germany with highly exceptional values, providing strong indications of the disaster that unfolded there. This study underscores the value of these rainfall indicators and calls for further testing on more high-impact events. Significance Statement Recent floods triggered by extreme rainfall, such as the 2021 western Europe floods, the 2023 Emilia–Romagna floods, and the 2022–23 California floods, have had an enormous impact. These events highlight the urgent need to scale up early actions. In this study, we use two rainfall indicators from ECMWF: the extreme forecast index (EFI) and the shift of tails (SOT). We find evidence that these indicators have high economic value when used to trigger early actions 2–3 days ahead of extreme rainfall events in Europe. Furthermore, these indicators provided accurate warnings at least 2 days before the 2021 western Europe floods. This underscores the vital role of early warning and preparedness in reducing future disaster risk.