Comparing flood forecasting and early warning systems in northwestern Europe

Abstract. This study compares operational Flood Forecasting and Early Warning Systems (FFEWSs) in transboundary river basins in Northwestern Europe, covering parts of Luxembourg, Germany, The Netherlands and Belgium. This region was hit by an extreme flood event in 2021 with more than 200 fatalities. Due to the high death toll and the large number of people who did not receive warnings, the FFEWSs were heavily criticized afterward. This study shows strong improvements in FFEWSs after the flood event in all countries. Interviews with experts across the region reveal that warning thresholds are optimized, some regions include new warning thresholds for extreme events (e.g. dark purple), and flood crisis management plans are improved. In addition, all countries now use mobile phone-based alerts using cell broadcasting technology. The assessment of the warning systems shows strong differences between and within countries, with different flood warning levels and warning colour codes in use. The interviews also reveal that the adoption of operational impact-based forecasts remains a challenge, while these are crucial for translating hydrological forecasts into effective actions. For example, the interviewees stress the need for operational flood inundation forecasts, which are currently only provided in Flanders. Our study has four concrete recommendations: (1) investigate the benefit of streamlined warning levels and colour codes between and within different countries; (2) assess the added value of more extreme warning levels for catastrophic events, such as the purple levels in Luxembourg and some German regions; (3) accelerate the development of operational impact-based forecasting systems, and (4) implement a structural evaluation of warning communication chains. These challenges must be addressed to reduce the gap between early warning and early action during impactful flood events.