The system currently is just over the Plains of US right now, providing some flurry activities in through Wisconsin, the Dakotas, et cetera. The system, along with it, is going to bring warm and moist air from the Gulf of Mexico, and it is going to move north, into the arctic air mass. The Arctic air mass currently dominates much of Eastern U.S. and all of Canada. As this system collides with the air mass, a storm forms. 

It looks as if the Arctic Air Mass will retreat further back towards the north as this system approaches. With this retreat, we can see some warm air invading Southern Ontario, and hence changing the snow over to rain and perhaps freezing rain if the temperature permits.

Currently, Southern Ontario will start with heavy snow beginning tomorrow afternoon. Throughout the evening, temperature is going to rise steadily, while heavy snow continues to fall. By the late morning hours, freezing rain and rain may creep into Southern Ontario. By this time, 10 to 15 cm of snow should have already fallen. The rain will continue throughout the afternoon, finally changing back to flurries by evening hours. The flurries can accumulate up to 3 cm.

The preliminary snow - rain line, I'd say right now, can be drawn west from Sarnia, continuing east through London, bending slightly north to Stratford and the Kitchener-Waterloo areas, continuing east through Hamilton, along the Highway 407 corridor through Burlington, Peel Region, and southern York Region, east continuing on the Highway 401 corridor out to Cornwall. North of this line, you can expect heavy snow. Depending on the track of the storm, the line will either shift north or shift south. Hence, with the movement of the line, communities currently denoted as on the snow-rain line right now can experience a final snowfall amount much different than the current forecasted amount.