"Significant freezing rain" is on the way, Environment Canada said in a weather warning.
The poor weather will start with rain and possible thunderstorms, but "a more wintery change is on the way," the agency said.
Environment Canada's website says a blast of cold air is making its way toward the region, where it will combine with a low-pressure system that is currently over Missouri.
That combination will create "a messy mix of ice pellets and freezing rain" by Thursday morning.
The inclement weather is set to pose problems for drivers making their way into work on Thursday morning, particularly for areas near the Greater Toronto Area and west of it, all the way to Lake Huron.
Environment Canada says up to 10 millimetres of freezing rain will fall across the region, potentially exceeding 20 millimetres in some cases.
Temperatures are expected to hover around the freezing mark on Thursday.
Air Canada says flights landing or taking off from Pearson International Airport on Thursday may be cancelled or delayed as a result of the weather. The airline is advising its passengers to check the status of their flights before heading to the airport.
WestJet is advising customers to check flights leaving from or landing in Toronto, Hamilton or Kitchener, Ont., on Thursday.
The sudden turn in the weather has prompted the City of Toronto to issue an extreme weather alert to ensure that people living on the streets take appropriate shelter.
The city issued a news release on Wednesday afternoon to reiterate the fact that extra shelter beds are available when an extreme weather alert is in effect.
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