Uncertainty quantification and spatial biases assessment in precipitation forecasts: a methodology for real-time flood forecasting applications

Meteorological forecasts are crucial for mitigating flood impacts, but they are always affected by uncertainty, particularly regarding the accurate prediction of the location for intense convective precipitation. This issue is especially critical for flood forecasting in small watersheds, where even slight displacements in the predicted rainfall position can lead to significant flow forecast inaccuracies. This study i) proposes a methodology to assess spatial biases of rainfall forecasts produced by a convection permitting meteorological model, ii) identifies whether the model has “preferential” misplacement directions in forecasting convective events and iii) suggests a method to cope and deal with this uncertainty in hydrological predictions. In this study, 64 significant convective rainfall events have been analyzed by comparing the Quantitative Precipitation Forecast (QPF) from the MOLOCH meteorological model with observed rainfall fields, covering a large portion of the Lombardy Region (northern Italy). The model’s average displacement error is quantified using the Fractions Skill Score (FSS), yielding a value of 20 km. By deriving each rainfall event’s displacement vector through pattern matching, a systematic misplacement tendency has been identified, with a consistent forecast shift by the model towards the northeast direction in the “hydraulic node of Milan” study area. The bidimensional rainfall displacement probability density function is then obtained through Kernel Density Estimation (KDE). This distribution can be used as a generator of shifted rainfall forecasts, substantially creating an ensemble from a high-resolution deterministic model, aimed at taking into account the uncertainty associated with possible QPF misplacements. The methodology can be generalized and applied to any river basin and limited-area meteorological model.