Umbrellas and rain jackets will be needed across much of Simcoe Muskoka on Wednesday, according to Environment Canada.
The weather agency has issued a rainfall warning for parts of Muskoka and Gray Bruce.
Rain will be heavy at times on Wednesday into Thursday, with 20 to 40 millimeters possible.
Scattered showers and a risk of a thunderstorm are expected for much of Simcoe County until early afternoon.