A flood watch is being issued for all municipalities in the Grey Sauble Conservation Watershed.
Warmer temperatures on Friday and Saturday have caused the snow to melt and cause pooling in fields and low-lying areas.
The expected warm temperatures on Sunday and forecasted 15 to 25 mm of rain could cause major flooding.
There have been reports of small watercourses and drainage ditches being overwhelmed from the snowmelt, resulting in some flooding situations.
The rivers are also expected to continue rising throughout the night as the rainfall begins.
The flood watch will remain in effect until at least 1 p.m. on March 18.