A special weather report for Toronto has been issued by Environment Canada, as well as for the parts of Peel, York, Halton and Durham regions.
In the statement, Environment Canada has said that these regions are likely to receive as much as 20-40 mm of rain in the city and thunderstorms are expected.
The federal weather agency said that rainfall is expected to continue until Saturday.
In addition, for areas west of the GTA, including London, Stratford, and Sarnia, a severe thunderstorm warning has been issued.
Residents are advised to be cautious around all shorelines, rivers and streams in the GTA as higher water levels are expected, said the Toronto and Region Conservation Authority.