A wintry blast of rain and snow is forecast to track across parts of Ontario and southern Quebec, slickening road conditions just in time for the peak year-end holiday travel season.

In southern Ontario, the rainfall will be replaced by heavier snowfall this afternoon, according to Environment Canada. As much as 50 centimetres of snow is expected to fall in parts of central Ontario by Saturday.

The heaviest overnight snowfall was expected in the National Capital Region, where sleet and freezing rain were concerns for motorists earlier Thursday evening. By 5:30 a.m. this morning, school buses were reportedly cancelled in Ottawa.

CBC Ottawa weather specialist Ian Black tweeted shortly before 10 p.m. that ice pellets had ceased, but that snow was falling "with authority" around Carleton Place.

Environment Canada issued weather warnings for parts of Ontario and Quebec earlier on Thursday, forecasting a messy mix of rain, wet snow and freezing rain. This blast of winter is due to a low-pressure system moving northeast from Colorado, also called a "Colorado low," through Illinois toward Ontario and Quebec.

Some parts of northern Ontario were under winter storm warnings. Further south, and into eastern Ontario, many areas were under freezing rain warnings that were lifted early Friday morning. Winter storm warnings were also in effect in some parts of eastern Quebec.

"An intensifying Colorado low currently over Chicago will track across southern Ontario tonight," Environment Canada said in a weather statement updated Thursday afternoon. "Snow ahead of this storm will arrive over areas near Georgian Bay this evening and towards the St. Lawrence Valley near midnight."

This weather system has already been wreaking havoc in the U.S. Midwest, sweeping across several states early Thursday, shutting schools and creating treacherous roadways. In the U.S., the storm led airlines to cancel about 1,000 flights ahead of the Christmas holiday — relatively few compared to past big storms, though the number was climbing.

In Iowa, a 25-car pileup left at least one person dead, The Associated Press reported. Along with Thursday's fatal accident in Iowa, the storm was blamed for road deaths in Nebraska, Kansas and Wisconsin. In southeastern Utah, a woman who tried to walk for help after her car became stuck in snow died Tuesday night.

"This [system] of low pressure has been firing off severe weather along a cold front for the southeastern U.S., with snow in the northern plains, and it will be snow for northern Ontario," said CBC meteorologist Jay Scotland.

A wintry mix of precipitation was expected from Muskoka to Brockville, while cities farther north were expected to be cool enough for snow and heavy snow, said Scotland. As much as 15 to 30 centimetres of snow is forecast ranging from the Nickel Belt to Timmins, Ont., he added.

In turn, this could result in some snow for the Greater Toronto Area and lake effect snow squalls near the shores of Georgian Bay and Lake Huron.

The blast of winter has already been affecting some Ontario travellers, with more than 80 flights cancelled midday Thursday at Toronto's Pearson airport.

Even without weather woes, air travellers are advised to come early Friday because it's Pearson's busiest day of the holiday season, with roughly 105,000 people expected to pass through the airport.

'Calm before the storm' in Quebec

"Snowfall could be as high as five centimetres an hour through the overnight hours tonight in Ottawa," said Scotland Thursday.

Quebecers were advised to brace for major blast of snow on Friday. As the winter storm approaches, dense snow, possibly mixed with ice pellets, was expected to fall over southern Quebec and continue through to Saturday, Environment Canada said.

The snow could be between 15 and 40 centimetres, but in the southernmost regions it will turn to rain on Friday, it added.

"Quebec, it's the calm before the storm," said Scotland.

With files from The Associated Press