Ensemble hydrological forecasting for flood warning in small catchments in Saxony, Germany

Reliable warnings and forecasts of extreme precipitation and the resulting floods are an important prerequisite for disaster response. Especially for small catchments, warning and forecasting systems are challenging due to the short response time of the catchments and the uncertainties of the meteorological forecast products. Thus, ensemble forecasts of precipitation are an option to portray these inherent uncertainties. By this contribution, we present our operational processing scheme for ensemble hydrological forecasting. We use the COSMO-D2-EPS product of the German Weather Service, which provides an ensemble of 20 members each three hours, for lead times up to 27 hours. Each member is evaluated regarding specific extreme precipitation thresholds for predefined hydrological warning regions. If these thresholds are exceeded in a specific region, rainfall-runoff models for the associated catchments are started to propagate the meteorological uncertainty into the resulting runoff, followed by statistical post processing and visualization. In addition, a communication and training concept based on a series of workshops with the locally responsible civil protection forces to deal with the uncertainties in the forecast is associated. Results are presented by a re-analysis of the flood in the upper Weiße Elster catchment in May 2018 in the Vogtland region of Saxony. Rainfall amounts larger than 140mm in 6 hours led to the highest flood warning levels in the region. Analysis show that such extreme amounts of precipitation are only predicted by one member of the COSMO-D2-EPS ensemble forecast. The deterministic COSMO-D2 model run does not show this, which underlines the benefit and the potential of the ensemble predictions, but also the need for a suitable communication of the uncertainties.