Environment Canada has updated its winter storm watch to a warning, with heavy snow expected to start Wednesday evening in the Barrie, Orillia, and Midland areas.
The warning is also in effect for Coldwater, Orr Lake, Lagoon City, and Washago.
The national weather agency said significant snowfall is expected to begin Wednesday night and last into Thursday.
Environment Canada expects snowfall of 15 to 20 centimetres in some areas, with heavy and blowing snow reducing visibility at times.
The national weather agency said the heavy snow is expected to start Wednesday evening and will taper off into a lighter snow and then turn into a mix of freezing drizzle.
Another round of snow with a mix of ice pellets is also in effect in the area for Thursday afternoon and evening, the weather agency said.
Environment Canada said there might be reduced visibility due to heavy snow and blowing snow for people going outside or driving.
The storm is the result of a Colorado low bringing a wintry mix of snow and ice pellets to much of southern Ontario.
The weather agency warned that heavy precipitation might lead to hazardous winter travel conditions, which may also be mixed with freezing rain in some areas.