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, 19 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, and 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, growing in size and intensity as it approached. The tornado reached F2 status as it inflicted severe damage to homes and to 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 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.^[1][2]

The next significant tornado to touch down occurred near the town of Thornbury along the shore of Georgian Bay. The tornado developed to the south of there, passing through Clarksburg and Blue Mountain, before moving out over the water. It was later rated at F2. As this cell moved out over Georgian Bay, more cells formed and intensified ahead of and within the main line as it moved eastward.

One supercell developed to the southwest of Barrie, and produced a family of four tornadoes as it tracked to the northeast for 125 kilometres (78 mi). The first touched down west of Barrie, staying on the ground for 12.6 km (7.8 mi). This tornado impacted rural areas between New Lowell and Edenvale. The second tornado touched down near Moonstone, while the third cut a 1 km (3,300 ft) wide, 10 km (6.2 mi) long path through the town of Gravenhurst, both of them rated F0. The final tornado of this family, an F1, touched down at Ril Lake. To the north of there, an F0 tornado affected Dollars Lake, to the northeast of Britt, while another F1 tornado hit at Arnstein near North Bay.^[1] To the east, F0 damage occurred from a tornado at Redstone Lake near Haliburton, where a woman was injured by flying glass and the roofs of cottages were damaged. Nearby in Haliburton County, another tornado inflicted F1 damage to trees at Haliburton Forest.^[1]

To the south, the most prolific tornadoes in the Greater Toronto Area in recent memory tore through sections to the north and east of Toronto. An isolated cell produced a tornado that swept across the Royal Riding Academy, in a rural area east of Newmarket. To this storm's west, another tornadic cell developed and ultimately produced the most damaging tornadoes in an urban area of Ontario since the Barrie tornado outbreak of 1985. Although originally thought to be a single tornado, two separate F2 tornadoes struck the city of Vaughan. The first hit the Woodbridge neighbourhood in the city's southwest, traveling for 3.5 km (2.2 mi). The second struck the Maple neighbourhood in the city's northeast, causing a 2.7 km (1.7 mi) path of damage. Both of these tornadoes have been rated F2.^[1]

Following the Vaughan tornadoes, the storms moved into sections of Central Ontario and Eastern Ontario. There, an F0 tornado struck near Orono, an F1 touched down in the Bancroft/Carlow/Mayo area, and a final confirmed F1 tornado struck at Rice Lake.^[1]

In the hours after the tornadoes touched down, a state of emergency was declared in the city of Vaughan^[14][15] and in West Grey, including the town of Durham.^[16] In addition, two days after the tornado that affected the Blue Mountain area, a state of emergency was declared there.

Damage from the tornadoes in Grey County alone is conservatively estimated at over 30 million dollars.^[17] With damage at Vaughan estimated to be well over ten million dollars, the cost of this event surpasses that of the 1996 Southern Ontario tornadoes which caused insured losses of eight million dollars, and the Hamilton, Ontario tornado of 2005 which caused twenty five million dollars in damage at Lawfield Public School.^[2] As such, the outbreak is the costliest in Ontario since 1985 directly related to tornado damage.

Given their intensity and urban setting, particularly the tornadoes at Vaughan, the outbreak garnered media attention unlike any other in recent years. Cable news network CP24 set a viewing record with over 1.2 million people tuning in.^[18] The public interest in the tornadoes was mirrored by that of politicians; a review of the tornado warning process and changes to the dissemination of severe weather warnings in Canada will be made accordingly.^[19][20]

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• Heat wave of 2006 derecho series, record-setting number of tornadoes, 17 confirmed, in one outbreak sequence that touched down in Southern Ontario, until the 2009 Southern Ontario tornado outbreak surpassed it with 18 tornadoes from the same system.
• Southern Ontario tornado outbreak of 2005, among the highest recorded storm-related insurance claims in Canada, mostly because of record rainfall in the Toronto area from a dying tornadic supercell.
• 1985 Barrie tornado outbreak, the deadly and violent tornado outbreak that occurred in Southern Ontario, 13 confirmed tornadoes in all with 12 fatalities.