Toronto is under a flood warning as the city braces for up to 40 mm of rain.
Environment Canada said the rain, which started falling early Sunday morning, is expected to continue into the night and hit Niagara and areas east of Toronto the hardest.
The low-pressure weather system is predicted to bring 20 to 40 mm of intense rain which could cause river and roadway flooding.
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Toronto and Region Conservation Authority said bodies of water within the GTA should be avoided as the slippery and unstable conditions are considered hazardous.
The rain is predicted to last until Monday morning.