Ontario endured one of Canada's largest tornado outbreaks on record when 14 twisters touched down on the evening of Aug. 2, 2006. Largest Tornado Outbreaks on Record Canada U.S. Sizeable tornado outbreaks are relatively rare in Canada. The largest outbreak on record occurred in southern Ontario on Aug. 20, 2009, when 19 twisters struck the region. Advertisement AdvertisementAdvertisement AdvertisementThe world's largest tornado outbreak struck the United States in late-April 2011, producing 349 confirmed tornadoes from Texas to New York.