sustains powerful
F2 damage at 34 Houston Rd. 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 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 long, making it the longest track tornado in Ontario since the
Williamsford and Arthur tornadoes of 1996. 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 . The first touched down west of Barrie, staying on the ground for . This tornado impacted rural areas between
New Lowell and
Edenvale. The second tornado touched down near
Moonstone, while the third cut a wide, 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. 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. 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 . The second struck the Maple neighbourhood in the city's northeast, causing a path of damage. Both of these tornadoes have been rated F2. 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. ==Confirmed tornadoes==