Canada: Severe weather forecast in parts of Southern Ontario through Aug. 18 after heavy rainfall and flooding affect Greater Toronto Area Aug. 17

Heavy rainfall is forecast to continue in parts of Southern Ontario through at least Aug. 18 after severe thunderstorms and flooding affected the Greater Toronto Area on Aug. 17, causing road closures, power outages, and flight disruptions at Toronto Pearson International Airport (YYZ). Road closures and detours have notably been reported in Mississauga, Etobicoke, and North York, and power outages have affected thousands of people in the Greater Toronto Area, including Richmond Hill and Markham. Environment Canada also reported that a tornado touched down in the Waterloo Regional Municipality; officials had announced at around 11:00 Aug. 17 that a tornado warning had been issued for the Kitchener-Cambridge-Region of Waterloo area. Lingering disruptions to ground transport and business operations will likely affect the abovementioned locations over the short term.

As of around 18:00 Aug. 17, Environment Canada had issued rainfall warnings for several areas in the Golden Horseshoe region, including the following:

• Caledon

• City of Toronto

• Guelph-Erin-Southern Wellington County

• Halton Hills-Milton

• Innisfil-New Tecumseth-Angus

• Kitchener-Cambridge-Region of Waterloo

• Mississauga-Brampton

• Mount Forest-Arthur-Northern Wellington County

• Newmarket-Georgina-Northern York Region

• Orangeville-Grand Valley-Southern Dufferin County

• Pickering-Oshawa-Southern Durham Region

• Shelburne-Mansfield-Northern Dufferin County

• Uxbridge-Beaverton-Northern Durham Region

• Vaughan-Richmond Hill-Markham

Authorities will likely issue new alerts or update/rescind existing advisories as weather conditions change over the coming hours.

Sustained heavy rainfall could trigger flooding in low-lying communities near rivers, streams, and creeks. Urban flooding is also possible in developed areas with easily overwhelmed stormwater drainage systems. Sites downstream from large reservoirs or rivers may be subject to flash flooding after relatively short periods of intense rainfall. Landslides are possible in hilly areas, especially where the soil has become saturated by heavy rainfall. Additional disruptions to electricity and telecommunications services are possible where significant flooding, landslides, or strong winds impact utility networks.

Floodwaters and debris flows will likely render some bridges, rail networks, or roadways impassable, impacting overland travel around affected areas. Ponding on road surfaces could cause hazardous driving conditions on regional highways. Authorities could temporarily close some additional routes that become inundated by floodwaters.

Further severe weather could also trigger additional flight delays and cancellations at airports in the region. Flooding could block regional rail lines; freight and passenger train delays and cancellations are possible in areas that experience heavy rainfall and track inundation. Localized business disruptions may occur in low-lying areas.