High-intensity tornadoes in western Europe occurring under weakly unstable atmospheric conditions (Most Unstable CAPE ≤ 500 J/kg) are more frequent than commonly assumed, representing 58% of all recorded EF2+ tornadoes between 1940 and 2022. These events challenge conventional paradigms of tornadogenesis, which typically emphasize high buoyancy environments. This study explores the atmospheric mechanisms that enable such intense tornadoes in low-CAPE settings by focusing on a well documented case: the EF4-rated Bihucourt tornado that struck northern France on 03 August October 2008. Using a combination of ERA5 reanalysis data and numerical modeling, we identified the synoptic and mesoscale features conducive to tornadogenesis under limited atmospheric instability. Particular attention was paid to the influence of upper-level jet stream dynamics, which enhance vertical wind shear and helicity, especially within the left-exit region of a jet streak. However, synoptic-scale conditions alone fail to fully account for the tornadic potential observed. A key hypothesis explored in this study was the role of external or sustained forcing mechanisms such as local convergence zones, gravity waves, or terrain-induced circulations, which contribute to maintain or enhance mesocyclonic structure within a convective storm. To test these mechanisms, we employed a dual modelling approach: Cloud Model 1 (CM1), a 3D model for simulating mesoscale convective processes, was used to resolve Hautmont storm-scale and boundary-layer processes at high spatial and temporal resolution; meanwhile, the Weather Research and Forecasting (WRF) model provided insight into the broader mesoscale environment surrounding the storm event. Together, these simulations aim to shed light on how strong dynamic forcing can compensate for weak thermodynamic instability, and under what conditions tornadogenesis is favoured. Ultimately, the findings of this study aim to improve the understanding and forecasting of tornadoes in marginal environments, which remain underrecognized in current European operational frameworks.
Lacroix et al. (Fri,) studied this question.