A methodological framework for the evaluation of short-range flash-flood hydrometeorological forecasts at the event scale

Abstract. This paper presents a methodological framework designed for the event-based evaluation of short-range hydrometeorological ensemble forecasts, in specific context an intense flash-flood event characterized by high spatiotemporal variability. The proposed adopts point view end users charge organization evacuations and rescue operations at regional scale. Therefore, local exceedance discharge thresholds should be anticipated time accurately localized. A step-by-step approach is proposed, including first rainfall forecasts. step helps us to define appropriate spatial temporal scales flood anticipation rising limb (discharge thresholds) then analyzed large number ungauged sub-catchments using simulated flows zero-future forecasts as references. Based on this second step, several gauged are selected, which detailed forecast hydrographs finally achieved. methodology tested illustrated October 2018 flash affected part Aude River basin (southeastern France). Three nowcasting research products recently Météo-France evaluated compared. results show that, provided that larger percentiles considered (75th percentile instance), these correctly retrieve area where accumulations were observed but have tendency overestimate its extent. hydrological indicates threshold exceedances better localized if compared naive scenario price significant increase false alarms. Some differences performances between three also identified. Finally, even based low documented events remains challenging due limited statistical representation available data, herein contribute draw conclusions about usefulness newly developed ensembles forecasting purpose their limits possible improvements.