Emerging off-season heat events in Europe

Off‑season hot extremes have emerged as a distinct and insufficiently studied component of European climate variability. Recent unprecedented episodes, such as the major April 2023 pre‑season heat wave in Spain and the early‑April 2024 hot spell in Central Europe with temperatures exceeding 30 °C nearly two months before the conventional summer season, highlight their growing relevance. While summer heat waves are well documented, systematic assessments of pre‑ and post‑season hot events remain scarce. We introduce a new framework for identifying these episodes (days) across Europe for 1961–2024, based on an off-season heat threshold that incorporates local annual maximum temperature, temperature amplitude, and the sharpness of the seasonal cycle. The framework also provides an objective definition of transitional seasons, including their timing and length, enabling consistent detection of hot extremes outside the core summer period. Using ECA&D station data, ERA5 reanalysis, and the E‑OBS dataset, we map the climatology of off‑season hot events by characterising their frequency and intensity. We also quantify regional differences in the off‑season heat‑threshold values and in the timing and length of transitional seasons. Last but not least, we investigate long‑term changes in the frequency and intensity of such events.