A special weather statement is in effect for the Barrie, Orillia, Collingwood and Midland areas.
Environment Canada issued the warning Wednesday, saying heavy rain is expected to hit the region Thursday morning.
The weather agency says a few isolated thunderstorms are also possible in the afternoon.
Officials say some areas could see between 15 and 25 millimetres of rain by late Thursday evening.
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Environment Canada says the heavy rain and mild temperatures will produce potentially significant snow melt and localized ponding on streets and other low-lying ares.
According to the weather agency, the heavy rain and mild temperatures are associated with a strong Colorado low which will transition across northern Ontario Thursday.