"Freezing rain is expected and delays and cancellations are possible", airport officials said in a tweet.
Environment Canada on Wednesday issued a special weather statement for the Sudbury area, saying people can expect a "wintry mix of precipitation Thursday into Friday".
Forecasters said freezing drizzle is expected Wednesday night and into Thursday morning.
Freezing rain changing to rain or drizzle late in the evening then to a few flurries overnight.
On Wednesday afternoon, the warning was updated to a freezing drizzle advisory.
Toronto and surrounding areas could be in for another icy and wet day on Thursday, Environment Canada says.
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Take extra caution if you're driving Wednesday morning on bridges, exit ramps and overpasses.
St. Lawrence College and Queen's University cancelled evening classes and all branches of the Kingston Frontenac Public Library closed at 2 p.m., as did the Boys and Girls Club of Kingston and area.
Canada Post has suspended mail delivery for the rest of the day in Southern Ontario and the GTA due to unsafe weather conditions. By Friday afternoon, the temperature will drop to -5 C, though it will feel more like -19.
Periods of freezing drizzle are expected to affect the region tonight into Thursday.
The TTC is replacing streetcars with buses along the 506 Carlton and 512 St. Clair routes due to the weather.