(CNN) -- The warnings couldn't have been more dire.
"DO NOT TRAVEL," the National Weather Service in Amarillo, Texas, posted on its website, telling residents not to venture out in what it was calling "a crippling, historic blizzard."
The storm was dumping snow over the Texas Panhandle at a rate of 2 to 3 inches an hour.
Almost all roads in the Panhandle were impassable, and whiteout conditions forced the state Department of Transportation to pull virtually all of its snowplows off roads, Texas DOT spokesman Paul Braun said Monday morning.
On its Facebook page, the weather service posted a video of the wind and snow whipping a U.S. flag outside its Amarillo office.
"If after watching the last video you thought you could still get out and travel, well you haven't seen anything yet!" the Facebook post said. "We encourage everyone to share this video with your Facebook family and friends and keep them safe!"
For some, the warnings didn't come soon enough.
Emergency crews were having trouble reaching drivers who were caught on the roads, Texas Department of Public Safety Trooper Gabriel Medrano said. Cars were in ditches, he said, because drivers couldn't tell where road ended and ditch began.
National Guard units were being sent to help stranded motorists, the Texas DOT said.
Whiteouts were also reported in Oklahoma.
"Roadways and ditches are snow packed with no visibility, and are very slick and hazardous. All travel is discouraged," the Oklahoma Department of Transportation said. Highways were being closed across much of the Oklahoma Panhandle, where as much as 16 inches of snow could fall, the weather service said.
The blizzard is the second major winter storm to hammer the region in a week.
The number of people under blizzard and winter storm warnings, watches and advisories totaled 35 million, according to CNN's weather unit.
"The unprecedented nature of this much snow in this short a period of time will create conditions ... across the entire city that are basically unprecedented for the traveling public," Joe Pajor of the Wichita, Kansas, Public Works and Utilities department, told CNN affiliate KSN. Parts of the area could see up to 26 inches of snow from the storm, the station reported, and the weather service said Wichita would be under a blizzard warning until 6 a.m. Tuesday.
Wichita schools were closed for the third straight school day as the new storm roared in on the heels of one that dumped up to 22 inches of snow on some areas late last week.
Kansas' governor on Sunday extended a state of emergency declaration to include the new storm.
"This storm has the potential to be more dangerous than last week's storm," Gov. Sam Brownback said. "So, we ask you to stay off the road unless it's absolutely critical. If you have to be out, be prepared with a charged cell phone, an emergency kit with food, water, blankets, flares and a shovel."
The Kansas Department of Transportation reported that many highways in the central part of the state, including Interstate 70, were completely covered by snow and ice on Monday morning.
With two storms dumping massive amounts of snow in quick succession, the weather service warned that structures may not be able to withstand the weight.
Schools closures were also reported from Colorado to Texas, as were the closing of many state and local offices.
All flights in and out of Rick Husband Amarillo International Airport in Texas were canceled until noon on Monday.
"Nothing coming in or out until then at the earliest," airport spokesman Daryl Ware said.
Preston Smith International Airport in Lubbock was also closed, CNN affiliate KCBD reported, and Texas Tech University in Lubbock shut down on Monday because of the storm.
The weather service said visibility was less than one-quarter mile in Amarillo.
And major highways in Texas, including portions of Interstate 40 and U.S. Highways 60, 87 and 187, were closed because of blizzard conditions.
"May see 4-6 foot drifts!" wrote National Weather Service meteorologist Rick Smith on Twitter. "Traveling is beyond discouraged!"
Salt truck drivers were on standby overnight in Oklahoma City.
"Well, we're pretty well prepared right now. We have 28 trucks loaded, plows on," Oklahoma City Emergency Management's Mike Love told CNN affiliate KWTV.
"We run our emergency snow route. We'll run that until it's free and clear. And if this stuff comes in like they're saying tomorrow, with these high winds, look forward to some drifting."
In Wichita, Pajor said the city used half of its road-clearing salt and sand in last week's storm.
To the east, Kansas City is expecting 9 to 15 inches of snow Monday night into Tuesday, according to the National
To the west, more than 9 inches of snow was reported in some areas of Denver by early Monday. Airlines in the Colorado capital were working to return to normal operations a day after more than 200 of 1,500 flights had been canceled and hundreds more flights were delayed because of the weather, according to the Federal Aviation Administration.
Up to 19 inches of snow was reported in Jefferson, about 70 miles southwest of Denver.
Forecasts of similar conditions raised alarms throughout the Plains, leading to crowded stores as residents prepared.
Amanda Roberts, an entrepreneur and blogger in Warrensburg, Missouri, where snow is expected to begin Monday night, went to the stores ahead of the storm.
"The snow has everyone stocking up on groceries," she said in a Twitter post. "Fresh produce is basically gone, but I got the last gallon of chocolate milk. I call it a win."
Rain, flooding the issue in Southeast
While millions will see snow -- including Chicago, where 3 to 5 inches of snow and sleet are expected Tuesday -- rain may rule for the next few days in parts of the Southeast.
In Mobile, Alabama, on the Gulf Coast, residents prepared for the possibility of heavy rain and wind gusts as strong as 30 mph by Monday night.
The rain is part of a band affecting five Southeastern states where flash flood watches are in effect from Monday afternoon through Tuesday morning.
Some areas from Louisiana to South Carolina could see up to 4 inches of rain.
Kansas City International Airport set a February 21 record of 9 inches of snow, 4 more inches than the amount that fell the same date in 2010. Monday might bring 6 to 10 more inches, forecasters said.
Kansas City is approaching its February snowfall record of 20.7 inches, set in 1960.
In Kansas, Wichita saw its second-highest storm snowfall total on record last week with 14.2 inches over two days, the National Weather Service said.
The town of Russell in the state's middle lay under a 22-inch layer of white by the time the storm moved off.
CNN's AnneClaire Stapleton and Judson Jones contributed to this report.
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