Characterising Hourly Extreme Precipitation in Portugal: Spatial–Temporal Variability and Case Studies in Two Wine Regions

Extreme precipitation is a natural hazard with significant socioeconomic implications, namely for sectors such as agriculture, including viticulture. This study provides the first comprehensive analysis of extreme precipitation events in mainland Portugal, based on sub-hourly observations. Using 10-minute precipitation data from 71 weather stations for the period 2000 to 2022, we assess the spatial and temporal variability of these events, including their seasonality, diurnal cycle, and synoptic-scale drivers. The mean seasonal contribution of extreme precipitation to total annual precipitation, defined using thresholds of 10–20 mm h^-1 (yellow warnings) and >20 mm h^-1 (orange and red warnings) following the criteria of the Portuguese Weather Service, is highest in winter, indicating a stronger influence of intense precipitation on annual totals. This contribution decreases in autumn and spring, reaching its minimum in summer. Extreme precipitation events occur most frequently between September and December, with a secondary maximum in April and May, particularly in the Alentejo region. The diurnal cycle exhibits a pronounced afternoon peak, consistent with convectively driven thunderstorms. In spring and summer, extreme events tend to account for a larger fraction of daily precipitation totals. Two extreme events were selected not only as case studies of heavy precipitation, hail and lightning but also as examples of understanding the specific weather conditions and atmospheric dynamics associated with such severe weather patterns. In the first case, the event on 28 May 2018 in the Douro region was associated with a cut-off low, whereas in the second case, the event on 14 September 2021 in the Alentejo region was associated with a frontal system in the final phase of its life cycle. ERA5 instability indices show a good agreement with observed lightning patterns. These results, particularly at the regional scale, provide valuable insights for climate research and socio-economic sectors such as viticulture, where extreme precipitation and hailfall pose significant risks. Acknowledgements: Research funded by Vine & Wine Portugal–Driving Sustainable Growth Through Smart Innovation, PRR & NextGeneration EU, Agendas Mobilizadoras para a Reindustrialização, Contract Nb. C644866286-011. The authors acknowledge National Funds by FCT – Portuguese Foundation for Science and Technology, under the projects UID/04033/2025: Centre for the Research and Technology of Agro-Environmental and Biological Sciences (https://doi.org/10.54499/UID/04033/2025) and LA/P/0126/2020 (https://doi.org/10.54499/LA/P/0126/2020).