Analysis of the 18 August 2022 western Mediterranean derecho: Atmospheric dynamics and impacts over the northwestern Italian coast

On 18 August 2022, an exceptionally intense and fast-moving convective system crossed the western Mediterranean and parts of central Europe, causing widespread wind damage, large hail, and storm surges. Wind gusts exceeding 60 m/s were reported and the event exhibited the typical structure of a derecho, extending over 1000 km and resulting in 12 fatalities and over 100 injuries. This study provides a detailed analysis of the event's evolution and its impacts along the northwestern Italian coastal regions. The analysis is based on observational data, reports, and post-event surveys. The atmospheric characteristics conducive to the extreme event are reconstructed through high-resolution numerical simulations using the Weather Research and Forecasting (WRF) model, in a configuration that includes the assimilation of RADAR and lightning data (3DVAR). The simulations realistically reproduced the two main convective pulses that affected Liguria and northern Tuscany, capturing both the timing and the spatial distribution of maximum wind gusts (30-40 m/s) and rainfall. Post-event surveys documented wind damage up to EF2/IF2 intensity and confirmed the occurrence of downburst-like phenomena in Liguria. Soundings and model diagnostics indicated extreme instability (CAPE >4000 J/kg), strong low-level shear (> 40 m/s), and a dry mid-level layer favorable to intense downdrafts. A concurrent marine heatwave (SST anomalies up to +3 degrees C) likely enhanced low-level moisture fluxes and convective potential. The results emphasize that such marine heatwave-supported derechos represent a growing threat to Mediterranean coastal regions and highlight the need for improved short-term forecasting frameworks.