Operational hail forecasting in Greece

Since 2021, the METEO unit of the National Observatory of Athens (NOA) in Greece implemented a multi-model operational hail forecasting system. In our forecasting system we make use of four high-resolution numerical weather prediction models, namely BOLAM, WRF-ARW, MOLOCH, and ICON. BOLAM, WRF-ARW and MOLOCH are operated by NOA and run twice per day at 00Z and 12Z, while ICON is operated by the German Weather Service (DWD) and runs eight times per day. For the purpose of our application here we use the 00Z and 12Z initialization times for all four models and we focus our analysis over the domain of Greece extending from 18˚ to 30˚E and from 34˚ to 43˚ N. Here we apply the newly developed Hail Size Index (HSI) as an offline diagnostic tool in the four models for forecasting the expected maximum hail size. We choose HSI against other commonly used indices (e.g. significant hail parameter) because by construction it is more suitable for estimating hail size in European thunderstorms. The computation of HSI takes into account the atmospheric instability, wind shear, freezing level height, lifting condensation level, equilibrium height and temperature lapse rate. Here we present preliminary results of a validation of our hail forecasting system using observations from the European Severe Weather Database (ESWD) provided by the European Severe Storms Laboratory (ESSL). Our analysis primarily focuses on the predictability of large hail (> 2.5 cm) events which can lead to significant economic losses. In addition, we also attempt to assess the predictability of high accumulation of small hail (< 2.5 cm) events which can still pose a threat to agriculture and travel safety.