A winter storm smacked the Plains and Midwest with heavy, wet snow and blizzard conditions Thursday, and some areas were expected to be buried under more than a foot of snow by Friday.

Hundreds of schools closed, miles of highway were shut down and some airline canceled flights. At least two people were killed when their car overturned on a slick road in North Dakota.

The storm moved into Iowa with rain and sleet but changed over to snow around dawn. Parts of Nebraska, South Dakota, Minnesota and Iowa were hit with a blizzard. In western Iowa, 35 mph wind cut visibility to a quarter-mile or less for three consecutive hours; by midday, as much as a foot of snow covered the town of Atlantic.

"It's like whiteout conditions — you can't really see," said Pat Sinnott, who owns the Pump 'N Munch Too convenience store in Council Bluffs, near the Nebraska border.

She said motorists had been pulling off Interstate 80 and using her phone to call their bosses and say they wouldn't be coming in. A 100-mile stretch of the highway was closed just west of Des Moines to the Nebraska line.

With up to 18 inches of snow expected in parts of northwest Iowa, authorities were warning people to stay off the roads.

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

In North Dakota, a vehicle went out of control on the slick roads Wednesday, hit a ditch and rolled over, killing a couple on their way home from Texas.

The storm was expected to track northeast to La Crosse, Wis., later Thursday, with heaviest snowfall expected along a line north and west of the storm, forecasters said.

In suburban Milwaukee, part of a supermarket roof collapsed after a morning snowfall. Joe Foltz, who works at the Pick n' Save supermarket, said he heard a crackling shortly before the collapse.

"We thought maybe milk crates crashed on the floor," Foltz 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."

In Superior, Wis., Angela Jones decided to stay home with her two children after their day care center closed and a blizzard warning was posted.

"It is snowing and blowing. The wind is blowing really hard," said Jones, 31. "The flag out there is whipping around. I am glad I didn't have to go out in this."

As much as 20 inches of snow could fall in her area of northwest Wisconsin through Friday morning, while closer to 8 inches of snow mixed with sleet was expected across the east-central part of the state, the National Weather Service said.

The storm was part of a larger line of thunderstorms and snowstorms that stretched from Minnesota to Louisiana. Tornadoes swept through southern Missouri Thursday morning, killing a 7-year-old girl, authorities said.

In Nebraska, where up to 9 inches of snow had fallen on the Omaha area Thursday morning, the storm closed schools and universities and forced the cancellation of several events. In parts of the city, snow was falling as fast as 2 inches an hour.

More than 140 school districts canceled classes Thursday in Minnesota even before the heavy snow arrived.

By Friday, snowfall totals were expected to be a foot or more in southern and central Minnesota. In northeastern Minnesota, the totals could hit two feet. The weather service warned of blowing snow and possible blizzard conditions in the countryside.

"We're going to get pummeled," National Weather Service meteorologist Byron Paulson said.