Rain, heavy at times, has prompted a special weather statement for Sarnia-Lambton, Chatham-Kent, Windsor-Essex, Grey-Bruce, and Huron County.
A rainfall warning is in effect for Perth County.
Environment Canada said showers are expected to push into the region through Tuesday afternoon as a moisture laden system moves into the Great Lakes Basin.
The national weather agency said showers are expected to become more widespread and heavy at times Tuesday night, along with the risk for thunderstorms as a more unstable air mass moves in.
Showers will continue into Wednesday morning, and some areas could receive in excess of 45 millimetres before tapering off.
We're advised heavy downpours can cause flash floods and water pooling on roads.