Environment Canada says significant rainfall and mild temperatures are expected on Wednesday.
The agency issued a special weather statement on Monday afternoon, in advance of the weather event expected to impact southern Ontario.
"This rainfall, combined with melting snow and the limited ability of the frozen ground to absorb the rain may lead to localized flooding in low lying areas and water pooling on roads," the notice read.
Between 15 and 25 millimetres is expected Tuesday night into Wednesday.
Areas covered by the notice include Sarnia-Lambton, Chatham-Kent, London and Middlesex County, Elgin County, Windsor and Essex County, Huron County, Perth County, Bruce County, Grey County, and Wellington County.