Examination of a Remarkable Great Lake-Spawned Tornadic Supercell: The 2011 Goderich Ontario F3 Tornado Event

On the afternoon of 21 August 2011, a tornado formed over Lake Huron associated with an intensifying supercell. It moved onshore and ripped through the very centre of historic downtown Goderich, with post-storm surveys revealing low-end F3 damage. There were 37 injuries and one fatality. This tornadic supercell was remarkable on a number of counts. First, it occurred in an area where commonly used severe weather forecast parameters suggested only a minimal risk of supercells or tornadoes (e.g., MLCAPE, SRH, SCP, and STP were not noteworthy). In spite of this nominal threat, it caused surprisingly severe damage along a 20 km path, making it the strongest tornado in Ontario in 15 years. Second, the tornado and its parent supercell developed and intensified over the lake and well behind a cold front, both of which are rarely seen in the Great Lakes area. The most severe damage was close to the shoreline. Third, there have been only two known tornadoes in the history of the larger Goderich area along the shore of Lake Huron, and both were F0. Being located on the eastern shore of Lake Huron, this area is typically affected by stable marine air when surface-based storms develop inland. Mesoscale analyses at 10 min intervals indicate that cold pools associated with earlier and ongoing convection combined and led to the development of a strong baroclinic zone, roughly from west to east across Lake Huron. The Goderich storm initiated, intensified, and became tornadic along this ‘convective bridge' across the lake. On the synoptic scale, the event occurred as a strong cyclonic upper-level jet began to arrive from Michigan. We compared this pattern to the upper-level jet pattern associated with 59 past F3 or greater Canadian tornadoes using reanalysis data. It was found that 85% of these tornadoes occurred in the exit region of a strong upper-level jet, demonstrating that there was at least one ‘typical' aspect to this event. Despite the unusual nature of this event, the Ontario Storm Prediction Centre was successful in issuing both a severe thunderstorm watch and a tornado warning before the storm made landfall, mainly due to enhanced situational awareness.