Significant rainfall is expected on Tuesday night, continuing through Wednesday all across the Durham Region, according to Environment Canada.
Rainfall amounts of 20 to 40 mm are expected. Locally higher amounts are possible in areas that receive thunderstorms.
The saturated or frozen ground will have little ability to absorb this rainfall, the agency said.
A series of low pressure systems will lead to significant rainfall across parts of southern Ontario. Heavy downpours may cause flash floods and water pooling on roads. Localized flooding in low-lying areas is possible, it said.