Heavy downpours are expected in the afternoon as a humid air mass moves across southern Ontario, the weather office said.
The advisory comes after the area was hit with heavy rain on Tuesday afternoon.
Wednesday’s system could see rainfall amounts of 20-40 millimetres in an hour or less along with wind gusts up to 70 km/h and even small hail, Environment Canada said.
Drivers can expect some difficulty behind the wheel as low visibility and ponding water on some roads is expected.
The rain should clear away on Wednesday evening.