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

A new methodological framework for the event-based evaluation of high resolution short-range hydro-meteorological ensemble forecasts is presented. It is specifically designed to address the questions of spatial and temporal scales at which ensemble forecasts should be evaluated, according to the characteristics of a flash-flood event. We adopt the point of view of end-users in charge of organizing evacuation and rescue operations. For this purpose, the potentially local exceedance of discharge thresholds need to be anticipated in time and accurately localized in space. A step-by-step approach is proposed, involving, first, an evaluation of the rainfall forecasts to define the spatial and temporal scales for the event-based evaluation. Second, a spatial analysis of the anticipation lead-times of hydrological responses is performed, focusing on the flood rising limbs, with the evaluation carried out against a reference forecast based on simulated flows. Based on this second step, several gauged sub-catchments are selected, at which a detailed evaluation of the hydrological forecasts is finally conducted.

This methodology has been tested and illustrated on the October 2018 flash-flood which affected part of the Aude River basin (south-eastern France). Three ensemble rainfall nowcasting research products recently developed by the French meteorological service (Météo-France) coupled with rainfall-runoff models (GRSDi and CINECAR) are evaluated and compared. The originality of the method is in the evaluation of the whole hydro-meteorological forecast chain by defining criteria corresponding to the users. The evaluation may seem limited (a single event and a limited number of outlets) but flash floods justify the implementation of such an evaluation framework.

Even if evaluating ensemble hydro-meteorological forecasts based on a limited number of documented flood events remains dependent on the statistical representativeness of the available data, the evaluation framework proposed herein helps drawing rapid and robust conclusions about the usefulness of newly developed rainfall ensemble forecast approaches and about their limits and improvement possibilities. It also contributes to pull together the community towards better framing post-event evaluations, so that we can put together events and evaluations from different parts of the world to collectively enhance our capacity to forecast, take decisions and increase preparedness for floods.