Environment Canada issued a special weather statement for the Barrie area, with thunderstorms and heavy rainfall expected Monday.
The national weather agency expects 30 to 50 millimetres of rain, with some areas possibly getting more.
Environment Canada reports the showers may last until Tuesday night.
The heavy rainfall has also prompted the Lake Simcoe Region Conservation Authority to warn about the possibility of flooding.
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The conservation authority says the rain may flood low-lying areas.
Officials said that all watercourses in the area might experience higher than normal water levels and flows.
As a result of the rain, the conservation authority said local streams and rivers would become dangerous, especially in the vicinity of culverts and bridges.
Residents are asked to remind children to stay away from all watercourses.