The agency is forecasting rainfall upwards of 20 millimetres today — heavy at times — across a large band of southern Ontario from Windsor, through the Greater Toronto Area to Ottawa.
It says the frozen ground has limited ability to absorb the rainfall.
A provincial flood watch across southern Ontario noted no major flooding was expected but some localized flooding was possible in floodplains, low lying areas and places with poor drainage.
Environment Canada says rainfall could hit 35mm over some locations.
The unseasonal conditions are expected to continue into next week, with Environment Canada forecasting a high of 13C Tuesday in parts of southwestern Ontario before daytime temperatures dip below freezing again on Thursday.