Environment Canada predicts between 50 and 75 millimetres of rain from Southern Lake Huron to the Ottawa and Prescott regions starting Saturday night and ending Tuesday morning.
But the organization does admit the total amount of precipitation remains up in the air.
“Several disturbances with associated significant rainfall are expected to move across southern Ontario in the next few days,” according to a special weather statement.
“There is still uncertainty over exactly how much rain each disturbance will bring. However, indications at this time suggest the total rainfall amounts … could be very high.”
CBC Ottawa meteorologist Teri Loretto believes Ottawa will get about 50 millimetres of rain before Wednesday.
Environment Canada has also issued a rainfall warning for areas of western Quebec like Maniwaki, Chelsea and Upper Gatineau. Like other areas of the province, there could be between 25 and 45 millimetres of rain.
Environment Canada also believes rain could switch to snow in the Ottawa-Gatineau region on Tuesday, with Algonquin Park seeing more snowfall than surrounding areas.