Dynamic and statistical analysis of giant hail environments in northeast Italy

On July 24, 2023, the new European record for hail size was set in northeast (NE) Italy with a 19-cm wide hailstone recorded during a supercell outbreak. During this event, severe storms were triggered in the Alps, moved eastward, intensified rapidly in the foothills, and generated damaging hailstorms in the plains. A detailed analysis of the available observations and numerical simulations highlights that the atmospheric environment was unusual. A high-Theta(e) tongue over the Adriatic Sea was lifted by a southerly flow above the cold pool generated by the thunderstorm outflow associated with an initial supercell. This raised the most unstable layer to 1-2 km above mean sea level in the area affected by the record-breaking hailstorm. The vertical profile was characterized by moderate potential instability (likely a value of MUCAPE 1200 J center dot kg(-1) fed the storms) and an intense southwesterly flow in the mid-troposphere. Additionally, there was anomalously high water-vapor transport in the layer 2-5 km above mean sea level. Consequently, high instability seems unnecessary for the occurrence of giant hailstorms in the region. This hypothesis is assessed with a statistical analysis performed for the 2018-2023 period in NE Italy using hail reports and observed soundings. The results show that hail size has a much lower correlation with potential instability (R similar to 0.3) compared with kinematic parameters of the mid-troposphere and water-vapor transport (R similar to 0.5-0.6). Furthermore, thermodynamic parameters have better skill in predicting the occurrence of hail or large hail (> 2 cm), while kinematic parameters of the mid-troposphere are better predictors for very large (> 5 cm) and giant (> 8 cm) hail events.