Winter Storm Ulmer brought blizzard conditions to parts of Colorado, Wyoming, Montana, Nebraska and South Dakota.
Warnings extended to North Dakota and northwest Minnesota, where the late-winter storm is expected to hit Thursday.
'This is a very epic cyclone,' said Greg Carbin, chief of forecast operations for the National Oceanic and Atmospheric Administration's Weather Prediction Center. 'We're looking at something that will go down in the history books.'
Parts of seven states were under blizzard warnings, and 20 states were under some level of high wind alert, Carbin said. Tornadoes were also possible on the storm's eastern edge.
In Colorado, a state patrol officer was hit and killed by a car as he was helping another driver who slid off Interstate 76 near Denver.
Corporal Daniel Groves, 52, was outside his patrol car when he was struck. He died at a hospital.
Hundreds of drivers were stranded on Colorado highways, including 500 in the Colorado Springs area alone.
Gov Jared Polis activated the National Guard to help find and rescue snowbound drivers.
Scores of motorists took refuge at truck stops in eastern Wyoming while blowing snow forced portions of major highways to close in Colorado, Nebraska and South Dakota.
Bria McKenzie, 22, said she and her mother, brother and sister were stuck in their car for more than two hours on a hilly road in Colorado Springs.
The snow was blinding and numbing, and the wind was whipping so hard she didn't feel safe walking to a hospital just down the road, she said.
'We thought about it, well maybe we'll run over to the hospital - at least we'll be warm,' she said.
'But we were soaking wet already just from trying to keep the windshield clear and from trying to push our car and help other people push their cars.'
'It was just like every second you were out there, it felt like parts of you were just freezing.'
McKenzie said they were eventually rescued by her father in his pickup truck.
A total of 1,386 flights were canceled at Denver International Airport on Wednesday, which closed all runways at 12.30pm.
The runways remained closed at 5.30pm and most airlines canceled all their departing flights for the rest of the day.
'Safety is the number one priority of our entire airport community here at DEN,' Denver International Airport CEO Kim Day told the Denver Channel.
'At this time, conditions have deteriorated and our major three carriers have all proactively canceled their afternoon and early evening flights, and our airfield is closed.'
'When visibility improves,' crews will work quickly to re-open runways.'
Day said that most concessions remained open at the airport to 'ensure the comfort of passengers' who were stuck inside.
Wind gusts up to 80 miles per hour had been recorded at the airport, which was expecting 7 to 11 inches of snow, earlier on Wednesday.
The raging storm could rival one that hit the region in 1979 in terms of the extent of heavy snowfall, according to meteorologist Richard Emanuel with the National Weather Service in Cheyenne, where wind was gusting up to 60mph with heavy snow.
According to the National Oceanic and Atmospheric Administration, the storm has officially undergone a phenom called bombogenesis, which occurs when a 'bomb cyclone' rapidly or explosively intensifies over a 24-hour period.
This type of storm system usually, but not always, accelerates and strengthens over the ocean as its central pressure drops at least 24 millibars in 24 hours.
'They typically do get strong systems this time of the year in that part of the country, but this one is maybe a notch stronger than what you typically see,' said meteorologist Marc Chenard of the Weather Prediction Center in College Park, Maryland.
'Pretty much through much of the Plains there's going to be a threat for potential power outage issues,' he added.
More than 100,000 electric power customers in the Dallas-Fort Worth area were left in the dark early Wednesday after a line of rain squalls associated with the system moved through the area.
Xcel Energy said high winds caused about 184,000 homes and businesses to lose electricity, mostly in the Denver area. Zero visibility made conditions difficult for repair workers and it could take days to restore power to everyone, according to Xcel spokesman Mark Stutz.
The storm was expected to drop up to 22 inches of snow in Wyoming, 14 inches in South Dakota and a foot in Colorado.
Many schools and government offices closed for the day in places where more than a foot of snow and winds as high as 100mph were possible, which would be the equivalent to the wind speed of a Category 2 hurricane.
A wind gust of 92mph was recorded in the mountains northwest of Denver.
Greg Giannini needed to get gas for his generator, because the lights were flickering at his home and he was afraid he was going to lose power. Even though he has four wheel drive, his car got stuck at the pumps and he was trying to dig out to get home as the blizzard swirled around him. He was stuck near North Gate Blvd. in Colorado Springs, Colorado
The culprit was a sudden and severe drop in ground-level air pressure in Colorado, the most pronounced dive since 1950, Carbin said.
It was caused by a combination of the jet stream and normal conditions in the wind shadow of the Rockies. Air rushed into the low-pressure area and then rose into the atmosphere.
'It's like a vacuum cleaner, really,' Carbin said. And when that much air rushes higher into the atmosphere, it causes severe weather.
Government workers in Denver, Wyoming and South Dakota were told to stay home and many colleges also closed their campuses.
Hundreds of miles of interstate and smaller highways were closed in Wyoming and western Nebraska because of whiteout conditions.
A 250-mile stretch of Interstate 80 from Cheyenne to Rock Springs in southern Wyoming was closed, along with a 300-mile section of Interstate 25 from Buffalo, Wyoming, to the Colorado border.
About 100 miles of Interstate 80 was closed in western Nebraska.
Severe thunderstorms in North Texas damaged buildings and flipped over small planes parked at an airport.
Flooding forced evacuations in northeast Nebraska and western Iowa, including a retirement home in Pierce, Nebraska.
Residents of Avoca, Iowa, were filling sandbags to keep floodwaters from spreading, and the American Red Cross set up a shelter for anyone who was displaced.
The storm is expected to rapidly intensify overnight on Wednesday.
Winter storm warnings for heavy snow were issued throughout the region, including areas of Arizona, New Mexico and Utah.
A tornado in New Mexico ripped roofs from buildings in the small town of Dexter, about 200 miles southwest of Albuquerque.
Authorities said five people were hurt, but none of the injuries were life-threatening. A dairy euthanized about 150 cows injured by the tornado.
Authorities also blamed high winds for a train derailment in the state after approximately 25 freight cars went off a trestle over a mostly dry river bed.
The New Mexico State Police said no injuries resulted from the wreck Wednesday near Logan, about 184 miles east of Albuquerque.
State Police photos posted on Twitter showed shipping containers strewn across the river bed, with a jumbled pile of containers on the slope above one bank of the Canadian River.
Authorities have blamed high winds for a train derailment in eastern New Mexico where approximately 25 freight cars went off a trestle over a mostly dry river bed. The New Mexico State Police said no injuries resulted from the wreck Wednesday near Logan, about 184 miles east of Albuquerque
Union Pacific spokeswoman Raquel Espinoza said the derailed cars were the tail end of a mixed-freight train consisting of two locomotives and 73 rail cars.
A high wind warning issued earlier by the National Weather Service said the storm moving through the area would produce 'one of the strongest wind events in years for West Texas and southeast New Mexico.'
Rivers in Wisconsin are expected to reach flood stage over the next few days as thunderstorms and showers melt away snow that has accumulated this winter.
Forecasters said the storm could last through Friday.
But by Thursday, the storm system may weaken as it moves over the Tennessee River Valley, bringing mostly rain from Michigan southward to the Gulf Coast and some remaining snow only in the far northern parts of the country, experts said.