Environment Canada has issued rainfall warnings for many parts of the province - stretching from Niagara to Windsor.
The federal agency says regions can expect total rainfall amounts between 20 and 45 millimetres between last night and today.
It says localized flooding is possible in low-lying areas and keeping children and pets away from swollen creeks and riverbanks is advised.
"Rain is forecast to spread northeastward early Tuesday. The rain will continue for several hours before tapering to showers from west to east Tuesday afternoon."