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 highlights that the second supercell developed in an atmospheric environment with an unusually weak-to-moderate potential instability. However, numerical simulations revealed a high-Θe tongue over the Adriatic Sea, which was lifted by a southerly flow above the cold pool generated by the thunderstorm outflow, associated with an initial supercell. This raised the moist layer to 1–2km above mean sea level in the area affected by the record-breaking hailstorm. The vertical profile was characterized by an intense south-westerly 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 CAPE seems unnecessary for the occurrence of giant hailstorms in the region.

This hypothesis is then assessed performing a statistical analysis p for the 2018–2023 period in NE Italy, using hail reports from the Pretemp database and observed high-rez Udine soundings. The results show that hail size has a much lower correlation with potential instability compared to kinematic parameters of the mid-troposphere and water-vapor transport. While thermodynamic parameters have better skill in predicting the occurrence of hail (or hail > 2 cm), the kinematic parameters (like water-vapor transport at 600 hPa, VT600= Wind x q at 600 hPa) of the mid-troposphere are better predictors for very large (> 5 cm) and giant (> 8 cm) hail events.