Observed and projected changes in extreme precipitation intensity and temporal patterns: a case study of the Misa River Basin (Central Italy)

Abstract. Extreme hydrological events such as floods are intensifying under climate change, yet uncertainties in local-scale rainfall projections impede reliable adaptation strategies. This study examines future rainfall extremes and their daily timing in the Misa River basin (Central Italy) using the Weather Research and Forecasting (WRF) and Consortium for Small-scale Modeling in Climate Mode (COSMO-CLM) models, together with rain gauge observations, comparing historical periods (1979–2008 and 1981–2008, respectively) with a future period (2039–2068) under RCP8.5. Mean-state precipitation is projected to decrease annually and in summer, while November average daily precipitation increases. In contrast, extreme precipitation shows a different pattern: both models project more frequent extremes relative to their own historical baselines, with a historical 100-year event becoming 10–20 times more frequent by mid-century. However, the models diverge when evaluated against observed historical flood events: WRF shows a broadly uniform response, with return periods for past events reduced by nearly 50 % and annual exceedance probability doubling, while COSMO shows an event-dependent response, with events remaining within the 95 % confidence bound. Sub-daily event magnitudes, including average peak intensity and nine-hour maximum accumulation, decrease in both models. Overall, both models agree that extremes intensify in a model-relative sense, but this agreement breaks down against observed benchmarks and sub-daily magnitudes, reflecting model-chain spread rather than uniform intensification. These results suggest that historically based design standards may underestimate future rainfall hazards under some projections, underscoring the need to account for model-chain uncertainty in flood risk planning across the Misa River basin.