Environment Canada said a "major ice storm" is likely over parts of southern Ontario tonight.
The weather agency noted that road conditions in the Toronto area are expected to deteriorate in the evening.
"As temperatures hover near the freezing mark, treated roads may tend to be merely wet while untreated roads may become slippery," Environment Canada said.
In the Ottawa area, a winter storm warning is in effect — but the worst of the weather is expected to arrive later tonight and into the morning. On Thursday afternoon, the temperature in the Ottawa area was hovering around 4 C under mostly cloudy skies. Precipitation is expected to start falling around 3 a.m. Friday, according to CBC Ottawa climatologist Ian Black.
"For southern Quebec, it will be more of a snow event," CBC weather specialist Craig Larkins said on Thursday evening.
The weather office issued snowfall warnings for the southern regions of Quebec to begin Thursday night until Friday.
Between 15 and 25 centimetres of snow and ice pellets are expected in Montreal and surrounding areas, with the wintry blast expected to continue tomorrow.
Warnings about the storm covered much of central and eastern Ontario on Thursday, as it moved in from the United States.
By 7 p.m. ET, about 26 per cent of scheduled daily flights had been cancelled at Toronto's Pearson airport. The airport is urging travellers to check their flight information before arriving at the airport.
Around noon, Pearson experienced ice pellets and about one centimetre of snow had accumulated, tweeted CBC meteorologist Jay Scotland.
Ice pellets, heavy rain and strong wind created hazardous driving conditions for the morning commute, especially near and west of the Greater Toronto Area extending to Lake Huron, reports Scotland.
School buses were cancelled earlier in the day, including in Peel, Halton and York regions just outside Toronto. But the Toronto District School Board said all student transportation was operating as normal and schools remained open.
A heavy rainfall warning was in effect for much of southwestern Ontario, with the Windsor, Chatham, Sarnia areas likely to be soaked by about 50 to 80 millimetres by Friday.
A freezing rain warning covers the GTA and the Hamilton-Niagara region, where 10 to 20 centimetres of freezing rain are possible.
Environment Canada said another concern in Toronto and parts of southern Ontario was the risk of "widespread power outages beginning this evening from downed tree limbs and power lines" because of ice accumulation and the possibilities of wind gusting 60 km/h.
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