Analogs-based attribution of the July 24th, 2023 extreme hail storms in northeastern Italy

Between July 19 and 24, 2023, a multi-day outbreak of severe convective storms impacted Europe, affecting several countries. Northern Italy experienced multiple severe storms during this period, with July 24 marking the most intense day, particularly for hailstorms. On this day, three long-lived hailstorms caused significant damage, injured 119 people, and produced the largest hailstone ever observed in Europe—and the second largest globally—with a diameter of 19 cm. Recent studies highlight positive trends in both the frequency and intensity of convective environments favorable to thunderstorm activity in this region, alongside an increase in reports of large hail events.

This case study examines these trends in the context of the July 24, 2023, event, aiming to determine whether significant changes have occurred that may have increased the likelihood or severity of such an event. We employ a storyline approach based on circulation analogs to analyze the atmospheric conditions leading to this hailstorm.

Results show that similar events are fuelled by much larger CAPE today compared to just a few decades ago, likely linked to the strong upward trend in Mediterranean sea surface temperatures, coupled with a modest decrease in bulk wind shear. Additionally, the data suggest a potential intensification of the dynamics underlying similar configurations over the past 70 years, due to steepening of the horizontal geopotential gradient across the region.