Seasoned truckers pulled over to wait for conditions to improve.
"You can't see marker lines. You don't know where the road is. It's not a good idea to be out there," said Perry Blaoun, who parked his rig overnight at Sioux Falls, South Dakota.
Travel was not only difficult, it was dangerous, reports CBS News Correspondent Cynthia Bowers. In the mountains of New Mexico a military transport carrying high explosives slid off the road, forcing the evacuation of a neighborhood near Gallup and tying up truck traffic for hours.
At least six deaths were attributed to the storm as it swirled from the Southwest and Rockies across the Plains, including a crash near Onawa, Iowa, which killed three members of one family and injured two other people.
CBS News Affiliate KMTV reports the driver of a tractor-trailer on Interstate 29 north of Onawa Monday lost control of the vehicle, slid through the median, and hit a van in oncoming traffic.
One man died Monday in Nebraska when he slipped on ice and hit his head, a traffic death in Minnesota was blamed on heavy fog, and a weekend traffic death in Texas was blamed on the storm.
By Tuesday, the bulk of the storm had plowed eastward, spreading rain across the Great Lakes and along the East Coast.
A foot-and-a-half of new snow sat on the ground at Mitchell and Huron, South Dakota, and a foot had fallen at Loup City, Nebraska. Mitchell declared a snow emergency, banning all non-emergency vehicles from the streets.
South Dakota authorities closed about 130 miles of Interstate 90 from Sioux Falls to Chamberlain on Tuesday.
"The wind is making visibility bad, and blowing snow and drifting," said South Dakota Highway Patrol Capt. Warren Anderson.
During the weekend, the storm piled 14 inches of snow on western Oklahoma. The National Weather Service estimated that 30 to 35 inches fell in some mountain canyons in north central New Mexico with more than 40 inches in two days at Colorado's Wolf Creek Pass.
About 125 Nebraska schools canceled classes Tuesday or opened late because of the heavy snow accumulation and wind gusting up to 35 miles per hour. Gusts to 52 miles per hour whistled through Rapid City, South Dakota.
"Given the amount of snow that fell and the amount of winds we'll have, it'd be a real bad day to send kids to school," said Daniel Nietfeld, a National Weather Service meteorologist
Schools also closed in parts of South Dakota, Minnesota, Wisconsin and Iowa, and the Iowa Legislature canceled Monday's sessions.
A Northwest Airlines spokesman said the airline canceled about 240 flights, most of them traveling to and from Minneapolis-St. Paul International Airport.
Blizzard and winter storm warnings remained in effect Tuesday in parts of South Dakota, and in Minnesot forecaster Chris Scott said freezing rain in some areas would change to a mixture of snow and ice.
"With the snow on top of the ice, it could be a real mess," he said. "We're looking at pretty significant ice accumulations. It could be a dangerous situation."
The storm was caused by a strong mass of warm air from the Gulf of Mexico that collided with a shallow layer of cold air, weather service forecaster Bill Harrison said.
"It's a very unusual amount of moisture feeding this far north for January," he said.
Snowdrifts stood up to 7 feet high in parts of New Mexico and Interstate 25 was closed early Tuesday because of blowing snow and ice between Las Vegas and Raton, state officials said. Some rural highways also remained closed Tuesday in northeastern New Mexico.
"They cleared the roads (Monday), but the wind just blew it all over again," said Sharon Roybal of Rainsville, New Mexico.
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