Significant rainfall is in the forecast for much of Simcoe County, Muskoka and surrounding areas.
Environment Canada says the rain will be heavy at times, with up to 50 millimetres starting Tuesday night or early Wednesday morning and continuing through to Wednesday evening.
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Precipitation will likely start with freezing rain or ice pellets before shifting to rainfall.
Thunderstorms are also possible, according to the national weather agency.
Local police ask motorists to "keep an eye on this developing weather system" and to watch for pooling and possible flooding in low-lying areas.
The heaviest rain is expected Wednesday morning through the afternoon.
Residents are urged to be cautious with children and pets near rivers, creeks and culverts.
