Denver International Airport canceled hundreds of flights, including several to and from Helena, as a late winter storm brought blizzards, floods and a tornado across more than 25 states Wednesday.
"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."
It could develop into the worst storm of its type in more than three decades, he said.
Blowing snow forced portions of major highways to close in Colorado, Nebraska, South Dakota and Wyoming.
The vast storm stretched at least 1,000 miles, said meteorologist Ryan Maue of weathermodels.com.
"It looks beautiful on satellite as long as you are not caught on it," he said.
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. When that much air rushes higher into the atmosphere, it causes severe weather.
Meteorologists call the rapid change in pressure a "bomb cyclone" or "bombogenesis."
Forecasters had warned about the storm for days, Maue said.
"Everybody saw it coming. Some people underestimated it so they're stranded and that's unfortunate," he said.