The Devastating Convective Wind Event of 24 July 2023 in La Chaux-de-Fonds, Switzerland : Causes, Probable Mesoscale and Storm-Scale Mechanisms at Play and Nowcasting Implications

On July 24^th 2023, a low-topped high-precipitation (HP) supercell thunderstorm struck the city of La Chaux-de-Fonds in northwest Switzerland and caused over 135 million Swiss Francs of material damages, tore down approximately 22,000 trees, damaged 3,000 buildings throughout the city, resulted in one fatality, 45 injuries and inflicted deep psychological trauma on its inhabitants. A MeteoSwiss automated ground station at the local airport in the direct path of the storm recorded a validated maximum 1-sec convective wind gusts of 217 km/h. After an extensive post-event analysis utilizing multiple data sources (including but not limited to numerical weather model data, satellite/radar imagery/algorithms, a damage survey, high resolution aerial photography, video footage and ground station data), it was determined that both a microburst and tornado were responsible for the devastating wind damages rated IF2 on the International Fujita Scale. It is hypothesized that this hybrid outcome was a result of a phasing of specific meteorological parameters and phenomena at various spatial and temporal scales. The data seem to show that the remnants of a rear-inflow jet (RIJ) present from an earlier bow-echo phase of the storm coupled with a descending reflectivity core (DRC) associated with the supercell’s rear-flank downdraft (RFD) collapsed to the ground as it was approaching the city. This phasing at the storm-scale generated a powerful wet microburst within the RFD which seems to have helped initiate a tornadic circulation just to its north as the overall HP mesocyclonic circulation literally engulfed the city and surroundings, traversing it in a few minutes. The rapid evolution of this event stresses the importance of disposing of and developing new nowcasting tools and techniques aimed at increasing warning lead times for these type of convective wind events, even if by only a few minutes.  This unique case within the Jura topography also highlights the complex interactions that can take place as dynamic convective storms, including supercells, impringe on this mountain chain and helps explain perhaps why the Jura region has been known to have spawned several strong and even violent tornadoes in the past. This case will hopefully motivate further observational and perhaps high-resolution convection-allowing numerical modeling studies aimed at better understanding what makes this region particularly prone to occasional tornadoes and help determine what role the complex Jura topography may actually play.