Two of Toronto's largest school boards say all elementary and secondary classes may be conducted virtually Monday as the city braces for what Environment Canada calls a "significant snowfall" overnight.

The weather agency has issued a snowfall warning predicting that between 25 and 35 centimetres of snow will fall between Sunday night and Monday night.

In a message to parents Sunday night, the Toronto District School Board (TDSB) said a decision will be made about whether to cancel school buses and close schools by 6 a.m. Monday morning. If that happens, all kindergarten to Grade 12 classes will move online.

"While we had shared last month that if buses were cancelled, it would be a typical "snow day" (no live learning), as the entire system has already been learning remotely for almost two weeks and given the disruptions to students' learning, we feel it would be best to extend remote learning for one additional day in kindergarten to Grade 12," the statement said.

"This is especially important as we anticipate an increased number of students and staff absences as a result of COVID-19 and related isolations over the upcoming weeks."

The Toronto Catholic District School Board (TCDSB) issued a similar statement Sunday evening, saying its decision is also expected by 6 a.m. Monday.

"Given the unique circumstances with school staff and students already learning/working remotely for the last two weeks, we felt it would be best to continue with remote learning for students in kindergarten to Grade 12 for one additional day, to avoid any further disruption to learning," the TCDSB statement said.

The TCDSB said child-care facilities may remain open, and it asked parents to confirm directly with their providers.

Ontario schools are otherwise expected to return to in-person learning Monday after two weeks of virtual learning following the holiday break.

City of Toronto forecasters expect the volume of snow to be "heavy, disruptive."

Winds gusting up to 60 km/h are only expected to worsen visibility.

The snow may fall as fast as three to five centimetres per hour on Monday morning, making for a potentially treacherous commute.

Environment Canada says a low pressure system tracking south of the Great Lakes will generate snowfall amounts of 25 to 40 centimetres in some areas of southern Ontario by Monday night.

The agency's winter storm watch says the heaviest amounts are forecast for much of eastern Ontario and the Niagara Peninsula, as well as Simcoe County north of Toronto. The Greater Toronto Area and Hamilton could see accumulations of 15 to 25 centimetres by Monday night.

"There may be a significant impact on rush-hour traffic in urban areas," the agency said, advising people that "rapidly accumulating snow will make traffic difficult."

Vincent Sferrazza, maintenance and operations director with Toronto's transportation services, said road salting machines started applying liquid brine to roads on Sunday afternoon in anticipation of the storm. The salt and water solution is meant to prevent roads from freezing over and to stop snow from sticking to the ground, making it easier for snowplows to clear it away.

He said the city's fleet of more than 1,000 machines — from salt spreading trucks to road, sidewalk and bike lane plows — as well as the crews that operate them, stand ready to be deployed as soon as the snow hits the ground.

"It will be all hands on deck," Sferrazza said. "All pieces of equipment will be activated."

The Don Valley Parkway and the Gardiner Expressway will be serviced first, as will major arterial roads, including University Avenue, Bay Street and Yonge Street, followed by collector and local roads, bike lanes and sidewalks.

Sferrazza advised residents planning to travel on Monday to give themselves extra time to get to their destination, as roads and sidewalks won't be in the best condition for travelling.

"If you see all of our equipment out on the road and on the sidewalks, please stay clear of them," he said. "They have a very important job to do. They're effectively making the road and sidewalk safe so that you can use them."

People are being advised to exercise caution while driving, especially as students are expected to return to in-person learning on Monday morning.

However, even that return is in jeopardy for some school boards, given Omicron-related staffing shortages.