(CNN) A powerful "bomb cyclone" is whipping the Rockies and Plains for the second time in four weeks, dropping snow that is making travel miserable from Colorado to Minnesota and beyond, delivering a temperature shock to a region that had been enjoying more springlike conditions.
And it wasn't the only major weather threat in Wednesday's forecast, with a risk of wildfires in parts of the Southwest, where dry conditions and wind gusts near hurricane strength could push any spark to disaster.
As for the bomb cyclone, heavy snow dropped in parts of South Dakota early Wednesday, and soon began slamming other states, including Wyoming, Colorado and Minnesota. More than 4 million people are under blizzard warnings from Wednesday into Thursday, including parts of Wyoming, Colorado, Kansas, Nebraska, South Dakota and Minnesota.
The Plains could get more than 2 feet of snow by Friday morning, and South Dakota could be the hardest hit, with more than 30 inches possible. High winds are making travel even more treacherous.
"Travel will be very difficult to impossible" Wednesday evening into Friday morning, a National Weather Service office in Nebraska said, using language nearly mirrored by offices throughout the region.
In South Dakota, more than 10 inches already was reported in several areas northwest of Pierre by Wednesday morning. In the eastern part of the state, the sheriff's departments of Deuel and Hamlin counties asked that people refrain from driving through Thursday.
"Roads are becoming impassable. Unless you are experiencing an emergency, please stay off the roads," the National Weather Service said in a message it relayed on behalf of the two counties.
South Dakota Gov. Kristi Noem ordered state government offices closed in 52 counties Wednesday, and state transportation officials were urging people to stay off highways.
In Denver, several airlines have canceled flights, and passengers are "strongly encouraged" to check in with airlines to confirm that their flights are still scheduled to fly, the Denver International Airport said in an alert on its website.
As the storm moved from west to east, authorities reported hair-raising travel conditions, flight cancellations and traffic pileups. The Minnesota State Patrol responded to 213 crashes by early evening Wednesday.
In Colorado, Gov. Jared Polis activated the state's national guard to help stranded motorists, while the blizzard shut down parts of interstates in South Dakota.
A bomb cyclone is a rapidly strengthening storm that reaches certain criteria in 24 hours.
This one comes about four weeks after a similarly powerful system dumped heavy snow and rain on some of the same territory, leaving hundreds of millions of dollars in livestock and crop damage in Nebraska alone, largely through flooding.
It's rare enough to have one form inland, much less two in a month. More typically, bomb cyclones form off the US East Coast in the form of nor'easters.
The weather changed abruptly in Denver, which on Tuesday reached a near-record high, 78 degrees, for that date. By late Wednesday morning, temperatures were in the mid- to upper 30s.
The Minneapolis/St. Paul area may be spared blizzard conditions but still could see up to 5 inches of snow.
April snow isn't unheard of there. Last year, a mid-April snowstorm brought a record 15.8 inches of snow to the Twin Cities.
The snowfall could lead to additional flooding in the region.
In South Dakota and western Minnesota, rivers will see moderate to major flooding as additional snow will slow the rate at which waters recede.
The Red River is experiencing significant flooding because of melting snow, with this new storm likely prolonging the high water.
A warmer section of the storm, meanwhile, could bring strong rain and other severe conditions -- including damaging winds, large hail and tornadoes -- to parts of Nebraska, Kansas and Iowa on Wednesday.
In Iowa, Gov. Kim Reynolds urged Iowans to make preparations for flooding.
High winds and dry conditions combine for an extreme wildfire threat in parts of New Mexico and West Texas on Wednesday, the National Weather Service said.
Winds gusted as high as 75 mph -- right at hurricane strength -- in New Mexico, according to CNN meteorologist Taylor Ward. Winds reached 60 mph in West Texas.
A slightly lower -- but still critical -- wildfire risk exists for much of the rest of the Southwest and into parts of the Plains.
There was a tweet noting high winds moving through Pueblo, in south central Colorado, from the Pueblo West Cyclone Athletics Twitter account.
Frontier and United airlines will allow passengers scheduled to travel Wednesday or Thursday within the Plains area to change their tickets at no charge.
Frontier said customers whose flights are canceled may request a refund.
More than 740 flights coming into or leaving Denver International Airport were canceled Wednesday, according to flight tracking website flightaware.com.