The Sequence of Heavy Precipitation and Flash Flooding of 12 and 13 September 2019 in Eastern Spain. Part II: A Hydrometeorological Predictability Analysis Based on Convection-Permitting Ensemble Strategies

On 12 and 13 September 2019, widespread flash flooding caused devastating effects across eastern Spain. Within the framework of the HyMeX program, this study examines predictability of the long-lasting heavy precipitation episode (HPE) conducive to flash flooding. A set of short-range, convection-permitting ensemble prediction systems (EPSs) is built to cope with different sources of meteorological uncertainty. Specifically, the performance of an ensemble Kalman filter, tailored bred vectors, and stochastic model parameterization is compared to more standard ensemble generation techniques such as dynamical downscaling and multiple physics. Results indicate EPSs focusing on sampling model uncertainties related to parameterization of subgrid processes are skillful, especially when deep convection and its interaction with complex orography are important. Furthermore, representation of small-scale thermodynamical aspects is improved through data assimilation, leading to an enhanced forecasting skill as well. Nevertheless, predictability remains relatively low at the catchment scale in terms of exceedance probabilities in cumulative precipitation and peak discharge. The analysis presented herein could serve as a basis for the future implementation of real-time flash flood warning systems based on skillful meteorological EPSs over small-to-medium, semiarid watersheds in eastern Spain and, by extension, over the flood-prone western Mediterranean region.