European record-breaking hailstorms in northern Italy on 19 and 24 July 2023

During 12-25 July 2023 northern Italy was affected by six severe storm outbreaks, that caused about 4.0 billion of euros of economical damages, at least 280 injuries and 3 deaths. The European record of the largest hailstone was broken two times: the first one in Carmignano del Brenta (Veneto region) on 19 July 2023 (16 cm-large hailstone), the second one in Azzano Decimo (Friuli Venezia Giulia – FVG - region) on 24 July 2023 (19 cm-large hailstone). In the evening of 24 July 2023 an exceptional outbreak took place with three violent supercells that swept Northeastern (NE) Italy, all generating giant hail. A radar analysis showed that those storms were characterized by an impressive Bounded Weak Echo region in the reflectivity field, and by a couplet of inbound and outbound velocities in the medium level, highlighting the presence of a mesocyclone. The synoptic configuration of the two days was similar, characterized by an intense and moist southwesterly flow in the Mediterranean region, located between a sub-tropical ridge and a trough over western Europe. On the other hand, surface conditions in the NE Italy were different during the two events, with much higher values of equivalent potential temperature (theta-e) on 19 July 2023 (about 350 K) than on 24 July 2023. During this latter event, before the record-breaking hailstorm at 2100 UTC, just 330 K of theta-e were observed in the plain. An atmospheric sounding in the evening of 24 July 2023 showed that also the convective potential instability was quite low in the FVG region, while the vertical wind shear and the storm relative helicity were very strong. We hypothesized that the intense and moist southwesterly flow was an essential feature for the development of the violent supercells, while surface thermodynamic conditions were not detrimental.