Southern Ontario Tornado Outbreak of 2009

F2 tornado
Max rating1	F2 tornado
1Most severe tornado damage; see Fujita scale

The Southern Ontario Tornado Outbreak of 2009 was a series of severe thunderstorms that spawned numerous tornadoes in Southwestern Ontario, Central Ontario and the Greater Toronto Area (GTA) on August 20, 2009, and was the largest single-day tornado outbreak in Ontario history and the largest in Canadian history. During the afternoon and evening, nineteen tornadoes touched down with four of them producing F2 damage. One F2 struck the town of Durham where significant damage occurred and one person was killed; two F2s struck the city of Vaughan, just north of Toronto, damaging hundreds of homes; another F2 devastated apple orchards and resort areas near Thornbury. The number of tornadoes associated with this event surpasses that of August 2, 2006, which saw 18 documented tornadoes. In addition, it was one of the most destructive and costly tornado events ever in the province, and included one fatality, one of only a handful of deadly tornadoes in Ontario's recent memory. At one point, over 10 million people in Southern Ontario, one-third of Canada's population, were placed under tornado watches and/or warnings as the storms rolled through.

During the afternoon of August 20, 2009, a series of supercell thunderstorms began to develop over southeast Michigan, increasing in number as they moved into western sections of Southwestern Ontario. Beginning at around 3:00pm, the storms rapidly strengthened, with a particularly intense cell forming to the south of Lake Huron. This storm passed into Ontario, travelled to the northeast through Huron County and into Grey County, moving towards the town of Durham. The first tornado touchdown occurred about 14 km (8.7 mi) southwest of the town, and the tornado grew in size and intensity as it approached, reaching F2 status as it inflicted severe damage to homes and an industrial park in the southwest part of the town.

The tornado weakened and continued through more rural areas before striking the town of Markdale, where F0 damage occurred to homes and trees. The total path of this tornado was 36 km (22 mi) long, making it the longest track tornado in Ontario since the Williamsford and Arthur tornadoes of 1996.

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