Severe thunderstorms capable of producing about 30 millimetres of rain are expected in Toronto into Saturday evening. There is also the possibility of hail and strong winds.
Severe thunderstorms can produce tornadoes, Environment Canada said, warning people to be prepared for severe weather and take cover if threatening weather approaches.
Environment Canada issued a severe thunderstorm watch for all of southern Ontario, including the city of Toronto, and parts of southern Quebec on Saturday.
More than 50 millimetres of rain are possible in some areas outside of Toronto.
Renfrew, Pembroke and Barry's Bay in Ontario was upgraded to a severe thunderstorm warning late Saturday afternoon, as was Pontiac in southern Quebec.
Despite warnings being in place for Saturday, CBC Toronto meteorologist Jay Scotland remained optimistic for better weather for Mother's Day on Sunday.