Environment Canada has issued a special weather statement warning that significant rainfall could affect much of southern Ontario beginning tonight and continuing through Wednesday.
Forecasters say rainfall totals between 20 and 40 millimetres are possible as a series of low-pressure systems moves through the province.
In some locations, higher amounts may occur if thunderstorms develop within the weather system.
The rainfall is expected to begin overnight and continue through Wednesday as the weather systems track across southern Ontario.
Meteorologists say one of the biggest concerns is the condition of the ground.
In many areas, the soil remains either saturated from recent precipitation or frozen following winter conditions.
That means the ground will have limited ability to absorb incoming rainfall.
As a result, the risk of water pooling on roads and flash flooding could increase in some areas.
Heavy downpours may cause localized flooding in low-lying locations, particularly near waterways or drainage systems that are already under strain from recent runoff and snowmelt.