Environment Canada has issued a yellow rainfall warning as 20 to 40 millimetres is forecasted, and thunderstorms are possible.
A special weather statement was upgraded to a warning for Sarnia-Lambton on Tuesday afternoon.
The national weather agency said the saturated or frozen ground will have little ability to absorb the rainfall.
Water is likely to pool on roads and in low-lying areas. There is a potential for flooding.
The rain is expected to begin Tuesday night and continue through Wednesday.
Residents are asked not to drive through flooded roadways, to avoid low-lying areas, and to watch out for slippery areas near rivers, creeks, and culverts.
