Two youths died in a collision in Listowel, Ont. Wednesday, and police blamed icy driving conditions.

Their car lost control and veered into the path of an oncoming tractor-trailer. The drivers have not been identified.

Ontario Provincial Police say the blinding storm caused hundreds of collisions as drivers began their evening commute. Motorists are asked to stay off the roads, or drive at the safest speed possible.

The snow is expected to become freezing rain overnight and high winds could reach up to 70 km/hr.

In Toronto, several schools and bus routes were shut down in the morning, and at least 200 flights cancelled at Pearson International Airport.

Forecasters said the city can expect more terrible weather Friday just before the morning commute, with freezing rain turning back into heavy snow.

Schools were also closed across several American states on Thursday, and hundreds of planes kept grounded, as massive snowstorms hit the Plains and Midwest.

Treacherous road conditions have killed two people in North Dakota, and officials even kept snowplows from clearing streets in western Minnesota because of heavy winds and snow.

The victims were driving through the state after visiting Texas, when their vehicle lost control on a slick road and rolled over.

Iowa was hit with freezing rain in the morning, but it soon changed to heavy snow as a blizzard swept through the western part of the state.

The state's governor, Chet Culver, has issued a disaster declaration and warned drivers to stay home.

"There's a real chance for people to get themselves stranded in some real treacherous conditions," Jim Saunders, a spokesman for the Iowa Department of Public Safety, told The Associated Press.

Another blizzard struck eastern Nebraska. It's expected to bring 38 centimetres of snow by Friday.

Heavy snowfall is likely responsible for the collapse of a supermarket roof in suburban Milwaukee.

Pick n' Save employee Joe Foltz told AP he heard a crackling sound right before the roof caved in.

"We thought maybe milk crates crashed on the floor," he said. "About 10 minutes later, it started going down. ... So I rushed everybody out of the emergency exit door and, thank God, we got everybody out."

Tornadoes hit southern U.S.

The same weather system created tornadoes in the southern U.S., killing a seven-year-old girl in Missouri and at least seven people in Alabama.

Earlier reports had put the number as high as 16.

Five of the dead in Alabama were trapped under a collapsed roof at a high school, according to state officials.

State Emergency Management Agency spokesperson Yasamie Richardson told AP the victims "are in relation to the high school but whether they are all students or some students and teachers we're not sure."

Student Martha Rodriquez, 15, left the school moments before the storm. She returned later to find the collapsed roof.

"The stadium was destroyed and there were cars tipped over in the parking lot and trees were ripped out. There were trees and wood everywhere. It was just horrible," she said.

Another two people died in Alabama from separate storms, and at least 40 people have been injured by tornadoes throughout the state.

With files from The Associated Press

"Eventually all of this will move through into Atlantic Canada," said CTV weather reporter Anwar Knight.

Environment Canada says heavy snow is falling over Quebec and western New Brunswick and will move through the rest of the Maritimes as the evening progresses.

Up to 20 centimetres of snow are expected in some areas.

Most of eastern Ontario is still under a snowfall warning and road conditions are poor in many places because of snow, ice and slush. Police advise motorists to use extreme caution.

The storm knocked out power for at least 75,000 people in southern Ontario, as was blamed for at least two deaths.

On Thursday, two children -- seven-year-old Anita Hiebert and her six-year-old brother Pancho -- were killed in a traffic accident on an icy highway near London, Ont., in Listowel. Their mother is in critical condition in a London hospital.

Hydro One Spokeperson Al Manchee told CTV.ca that more power outages are expected as the storm moves east.

"We've got in the order of about 75,000 to 80,000 out across the province on our systems," said Manchee.

There is still no timeframe as to when power will be restored, but some customers could remain in the dark until early next week.

"Bad road conditions are hampering restoration," said Manchee.

Thousands of Toronto residents are also without electricity Friday as ice pulled down wires and snapping trees severed lines.

"We've got 225 wires down as of 8:30 a.m.," Toronto Hydro spokesperson Thelma Hatzis told CTV.ca. "That's in various pockets (of the city) and that's not including feeders that are out."

She added that Toronto Hydro had crews out all night in anticipation of the storm.

"We realize it's an inconvenience for (our customers) ... but our crews have been working really, really hard to get that power back on for those folks," Hatzis said.

The weather also delayed or cancelled flights at Toronto's Pearson International Airport. So far 100 flights have been cancelled for Friday. Airline officials are advising passengers to call ahead Friday to check the status of flights.

Classes have also been cancelled for thousands of Ontario students.