Environment Canada says a series of low pressure systems will lead to significant rainfall across parts of southern Ontario this week.
A special weather statement was issued for most of the region on Monday.
Forecasters say between 20 and 40 millimetres of rain is possible starting Tuesday night and lasting through Wednesday.
Higher amounts are possible in areas that receive thunderstorms.
Heavy downpours may cause flash floods, water pooling on roads, and localized flooding in low-lying areas.
Environment Canada said the saturated, or still frozen, ground will have little ability to absorb the precipitation.
Rainfall warnings may be issued for some areas as the event draws nearer.
