Generating ensembles of spatially coherent in situ forecasts using flow matching

Abstract We propose a machine‐learning‐based methodology for in situ weather forecast postprocessing that is both spatially coherent and multivariate. Compared with previous work, our Flow MAtching Postprocessing (FMAP) represents the correlation structures of the observation distribution better, while also improving marginal performance at stations. FMAP generates forecasts that are not bound to what is already modeled by the underlying gridded prediction and can infer new correlation structures from data. The resulting model can generate an arbitrary number of forecasts from a limited number of numerical simulations, allowing for low‐cost forecasting systems. A single training is sufficient to perform postprocessing at multiple lead times, in contrast with other methods, which use multiple trained networks at generation time. This work details our methodology, including a spatial attention transformer backbone trained within a flow‐matching generative modeling framework. FMAP shows promising performance in experiments on the EUMETNET Postprocessing Benchmark (EUPPBench ) dataset, forecasting surface temperature and wind‐gust values at station locations in western Europe up to five‐day lead times.