Analysis of the 18 July 2005 Tornadic Supercell over the Lake Geneva Region

On the afternoon of 18 July 2005, a particularly intense supercell thunderstorm struck the Lake Geneva region of Switzerland. The storm initiated just southwest of Lyon, France, and tracked over 300 km toward the northeast before losing its supercell characteristics in the Swiss Alpine foothills around the town of Interlaken. During its 3-h lifespan, the storm’s forward translation averaged 60–80 km h−1. At the height of its severity, this supercell was responsible for hail the size of golf balls, a microburst with measured wind gusts of up to 160 km h−1, and two confirmed tornadoes. Miraculously, nobody was killed nor seriously injured. However, the material losses were considerable, including ravaged vineyards, damage to buildings and vehicles, and sections of forests that were completely destroyed. A postevent analysis was undertaken utilizing radar imagery/algorithms, satellite images, lightning and surface data, eyewitness reports, and a damage survey. This case provides a unique look at a supercell evolving within an Alpine environment and helps confirm prior research concerning certain storm features and signatures that have been observed in North American Great Plains supercells and elsewhere. Concerning tornadogenesis, this paper helps confirm via observations and simulations hypotheses proposed in several previous papers that low-level wind flow modified through channeling by mountains can periodically provide a locally favorable wind shear environment for tornadogenesis. For this particular case, inflow winds that channeled around mountain features appear to have been instrumental in the formation of the second tornado, since important topographical obstacles prevented any significant low-level gradient wind shear from operating on the eastern end of Lake Geneva, where this tornado occurred.