Severe thunderstorms could wash away the sunny start to Thursday, according to Environment Canada.
The federal weather agency has issued a severe thunderstorm watch for Toronto and parts of Southern Ontario.
A cold front is expected to sweep through Southern Ontario on Thursday afternoon.
That front could bring showers and potential severe thunderstorms, with strong winds being the main threat, Environment Canada said.
Wind gusts of 90 km/h are possible.
Halton, Peel, York and Durham regions, as well as Hamilton and Niagara are also under the watch.
Environment Canada said hail is also possible.