Assessing the ability of a new seamless short-range ensemble rainfall product to anticipate flash floods in the French Mediterranean area

Abstract. Flash floods have dramatic economic and social consequences, and efficient adaptation policies are required to reduce their impacts, especially in a context of global change. Developing more efficient flash flood forecasting systems can largely contribute to these adaptation requirements. The aim of this study was to assess the ability of a new seamless short range (0–6 h) ensemble quantitative precipitation forecast (QPF) product, called PIAF-EPS and recently developed by Météo-France, to predict flash floods when used as input of an operational hydrological forecasting chain. For this purpose, eight flash flood events that occurred in the French Mediterranean region between 2019 and 2021 were reanalysed, using a similar hydrological modeling chain to the one implemented in the French “Vigicrues-Flash” operational flash flood monitoring system. The hydrological forecasts obtained from PIAF-EPS were compared to the forecasts obtained with different deterministic QPFs from which PIAF-EPS is directly derived (i.e. the AROME-NWC numerical weather prediction model, and the deterministic PIAF product). The verification method applied in this work uses scores calculated on contingency tables, and combines the forecasts issued on each 1 km² pixel of the territory. This offers a detailed view of the forecast performances, covering the whole river network and including the small ungauged rivers. The results confirm the added value of the ensemble PIAF-EPS approach for flash flood forecasting, in comparison to the different deterministic scenarios considered.