Back-to-back storm systems could drop upwards of 80 mm of rain by Tuesday.
Environment Canada says showers and thunderstorms are expected to start affecting the area from Sunday afternoon with rainfall amounts of 10 to 20 mm expected by Monday afternoon, noting that some areas could see rainfall totals of 20 to 40 mm due to thunderstorms.
Another round of precipitation is expected to move in from Minnesota on Monday evening and will likely persist into Tuesday.
Rainfall amounts in this second round of precipitation still remain uncertain but an additional 20 to 40 mm seems possible at this stage.
The agency adds that there still remains uncertainty in precipitation timing and amounts.
Environment Canada has issued a special weather statement for the region.
To date, 130.1 mm of rain has fallen this May.
Well above the May monthly average of 80.8 mm, but below the record of 215 mm, which was set in 2007.