A storm impacting Rockies and Plains states has met the scientific definition of what's commonly known as a "bomb cyclone." It's the second such blast to hit the region in less than a month. Hundreds of flights have been canceled and some roads in the region are becoming impassable or accidents have already occurred.

The weather phenomenon is difficult to explain in layman's terms. Essentially, air pressure drops rapidly and a storm strengthens explosively.

Weather service meteorologist Mike Connelly in South Dakota said the storm could be "historic" in terms of widespread heavy snow. He expects some records to be set. Wind gusts could reach up to 50 mph.

Blizzard warnings were posted in at least six states from Colorado to Minnesota as the storm developed, CBS News correspondent Adriana Diaz reports. The National Weather Service said up to 2.5 feet of snow could fall in parts of eastern South Dakota and southwestern Minnesota. Some 17.5 million could face extreme weather.

Get Breaking News Delivered to Your Inbox

South Dakota Gov. Kristi Noem has closed state government offices in 52 counties. CBS New York weathercaster said the area has been pummeled by snow. Numerous schools around the state have closed. Numerous traffic crashes were reported in northeastern South Dakota, and the storm knocked out power to thousands of homes and businesses in Sioux Falls.

An unusual but not rare weather phenomenon known as "thunder snow" — snow accompanied by thunder and lightning — was reported in central South Dakota. "It's essentially a thunderstorm, but it's cold enough for snow," Connelly said.

Minnesota Gov. Tim Walz said "the National Guard stands ready" to rescue any stranded motorists. Multiple crashes along a snowy stretch of Interstate 35 prompted officials to close about 10 miles of the highway about 50 miles south of Minneapolis.

Winter storm warnings were issues across multiple states, including Montana, Wyoming and Utah.

"A strong late-season winter storm will move across the North-Central U.S. into the Great Lakes through Friday," the National Weather Service said Wednesday. "This will feature blizzard conditions and some ice on the cold side with severe thunderstorms on the warm side."

A map showing warnings, watches, advisories and alerts on April 10, 2019 National Weather Service

"Further south, high winds and very dry conditions will produce the potential for rapid wildfire growth across the High Plains and Southwest states into at least Thursday," the service said.

The storm last month led to massive flooding in the Midwest that caused billions of dollars in damage.

Watertown, South Dakota, is in the bullseye of the storm, Diaz reports, where the area could get up to 24 inches of snow — on top of the 56 inches that has already fallen this year, nearly triple the norm.

"Bomb cyclone" preparations in Denver

Colorado Gov. Jared Polis has activated the Colorado National Guard for the impending "bomb cyclone," CBS Denver reported. This is the second bomb cyclone to hit Colorado in less than a month.

The National Guard, about 50 soldiers officials say, will help rescue stranded drivers from across the state, the station added.

In addition, Colorado Department of Transportation officials have closed eastbound Interstate 76 from Lochbuie because of Wednesday's blizzard. They say they're prepared to shut down more interstates if needed.

CBS Denver said nearly 100 plows have traveled and cleared the roads since Wednesday morning. Travel on Wednesday night is discouraged.