Twenty to 30 centimetres of snow are expected to accumulate in parts of southern Ontario early Wednesday.Twenty to 30 centimetres of snow are expected to accumulate in parts of southern Ontario early Wednesday. (Canadian Press)

A major storm is expected to hit southern Ontario late Tuesday, bringing up to 30 centimetres of snow in some parts and strong winds.

A low-pressure system that is massing in Texas on Monday will push into the Great Lakes on Tuesday, said CBC meteorologist Johanna Wagstaffe. That system will bring periods of light snow to southwestern Ontario overnight Monday, she said.

The snow will become heavy Tuesday starting around 6 p.m., said Wagstaffe. In Toronto, the heaviest snowfall is expected to occur between 3 a.m. and 6 a.m. Wednesday.

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Winds will produce blowing snow and reduced visibility.

Environment Canada on Monday morning upgraded a special weather statement to a winter storm watch.

"The heavy snowfall and blowing snow will cause whiteout conditions making for extremely hazardous driving conditions. This storm has the potential to create near-paralyzing conditions."

The agency said that 20 to 30 centimetres of snow could accumulate in areas stretching from Windsor in the west to Brockville in the east by Wednesday afternoon, after which the storm is expected to taper off.

"It's possible it could be the most snow we've seen in a storm so far this year, at least for regions outside the snowbelt," said Environment Canada meteorologist Ria Alsen.

The agency is advising those living in the affected areas to watch for further statements about the storm.

Meanwhile, the U.S. National Weather Service has issued a storm watch in Buffalo and a number of counties in upstate New York for Tuesday and Wednesday. The agency predicts snow accumulation of 20 to 40 centimetres and blowing winds. "Very difficult driving conditions developing," the agency said.

The Canadian Automobile Association advises motorists to avoid the roads or give themselves extra travel time on Wednesday. It also suggests they carry a fully charged cellphone, winter survival kit, extra clothing, blankets, non-perishable food and candle, in case they get stranded.

The storm is expected to follow a deep freeze in much of southern Ontario on Monday.

The City of Toronto, meanwhile, called off a daylong cold weather alert Monday morning as temperatures rose from a low of -18 C.

With files from The Canadian Press