(CNN) In Texas, the governor called out the National Guard to help. In Kansas, the governor asked residents to do all they could to conserve power. In Kentucky, state officials said people need to be cautious as more frigid winter weather is coming.
Nearly 154 million people in the United States were under some sort of winter weather alert Monday, according to the National Weather Service. Wind chill warnings or advisories are in effect for about 68 million people. Icy roads, power outages and dangerously low temperatures were making life miserable.
The cold air is so widespread that you could travel nearly 2,000 miles from the Rio Grande on the Mexican border to the St. Lawrence River on the Canadian border entirely in winter storm warnings or watches.
More than 5.1 million customers across the country had no power as of Monday afternoon because of the deadly storm system. At least 4.2 million outages were reported in Texas, where rolling blackouts started overnight.
The severe winter weather has sparked emergency declarations in at least seven states, including Alabama, Oregon, Oklahoma, Kansas, Kentucky, Mississippi and Texas.
Texas Gov. Greg Abbott and the Texas Military Department have deployed the National Guard to conduct welfare checks on residents, according to a news release from his office.
LIVE UPDATES ON THE WINTER STORM
State officials are also sending resources to assist local communities in clearing roadways and assisting health care personnel and power grid workers, the release said.
Officials have opened 135 warming centers across the state.
"Due to the severe weather and freezing temperatures across our state, many power companies have been unable to generate power, whether it's from coal, natural gas, or wind power," Abbott said.
The city of Galveston said up to 95% of households were powerless early Monday afternoon.
At least 15 people have died in weather-related vehicle accidents since cold temperatures took hold of the country. Nine died in three vehicle crashes in Texas on Thursday, one person died in a wreck in Oklahoma on Sunday and three people were killed in Kentucky, including two in separate traffic accidents Monday.
Authorities in Tennessee attributed two deaths Monday to the storm, but gave no details.
Bad weather was widespread, with more than a third of the continental United States recording below-zero temperatures Monday.
The mercury dropped to 5 degrees in Dallas, 6 below zero in Oklahoma City and 32 below zero in Kansas City, Missouri -- the coldest for those cities since 1989. Snow fell in Brownsville, Texas, where measurable snow has occurred only twice on record since 1898.
Texas has borne the brunt of the cold weather. Officials in Harris County -- the state's most populous county, which includes Houston -- warned its 4 million residents to stay indoors because the cold weather will be around for a while.
"The safest place to be is in your home, even if you lose power," Francisco Sanchez, the county's deputy emergency management coordinator, told CNN affiliate KPRC Monday morning. "It's going to get colder before it gets warmer. These conditions will not improve until Tuesday night or Wednesday morning."
Kansas Governor Laura Kelly pleaded Monday with residents to conserve power.
"I can't stress this point enough. We all must cut back on natural gas and electricity usage now to ensure we have enough available to make it through these sub-zero temperatures," she said. "How we respond over the next 48 to 72 hours is critical."
Kansas Corporation Commission Chairperson Andrew French echoed Kelly's sentiments, as many Kansans experienced their first series of rolling blackouts Monday afternoon.
"We are right on the edge of whether curtailments of power are needed or not, and so to the extent folks can conserve safely, we would certainly encourage them to try to cut back on that usage of natural gas and electricity (over) the next 48 to 72 hours which will be the critical period," French said.
"What we're facing is three winter storms in seven days," said Kentucky Transportation Secretary Jim Gray in a Monday morning news conference.
Kentucky is experiencing its second storm of three. More snow is expected later Monday.
"We had what amounted to an intermission, actually, between the winter storms this weekend," Gray said. "That enabled our highway crews to get a bit of rest and make some headway in clearing fallen limbs and trees, for example, and restocking our salt supplies."
Gov. Andy Beshear pleaded with residents to stay off the roads and to be aware of the dangers of carbon monoxide poisoning, saying "those are casualties we don't want to see. We did not make it through almost a year of a pandemic to lose people to a snow or an ice storm."
Air traffic was halted at a number of airports.
Abiline Regional Airport in West Central Texas, George Bush Intercontinental and William P. Hobby in Houston, Lafayette Regional and Baton Rouge Metropolitan in Louisiana and Jackson-Medgar Wiley Evers International in Mississippi were closed Monday, the FAA said.
Austin-Bergstrom International Airport and San Antonio International Airport said Monday morning there would be no more flights for the day. Later airport officials announced there would be no outbound or incoming flights at either airport Tuesday.
About 3,900 flights with US destinations or departures had been canceled Monday morning, according to the flight tracking website FlightAware.com. Another 3,600 have been called off for Tuesday.
Houston rushed to open warming facilities for its homeless population.
City Councilwoman Letitia Plummer told CNN a line formed early for a place inside the George R. Brown Convention Center.
"We are leading in evictions around the country and because of that, our homeless numbers are increasing. These are people at the convention center that wouldn't normally be there," Plummer said, adding people have been "self-evicting."
Plummer said the city has opened six additional warming facilities, each housing 50-60 homeless, and that none of them are full at the moment. The city is working to open more warming facilities to ensure no one in need is turned away, she said.
The Electric Reliability Council of Texas (ERCOT) tweeted that it started rolling outages early Monday.
The council had previously asked consumers and businesses to reduce their electricity use as much as possible through Tuesday.
Below-freezing temperatures are forecast to affect more than 245 million people in the lower 48 states over the next seven days, with more than 50 million Americans expected to experience temperatures below zero.
There is the potential for more than 240 cold temperature records to be broken by Tuesday evening, and some records have already been shattered.
The heaviest snow in the East is expected to fall from the Mississippi Valley, through the Ohio Valley and Great Lakes. A total of 6-12 inches is expected by Tuesday evening from Arkansas to upstate New York.
Oklahoma City has gone a record five days without climbing over 20 degrees Fahrenheit -- they are not expected to top that temperature until Thursday, for a stretch of nine days.
"This cold snap is forecast to result in record low temperatures that are comparable to the historical cold snaps of Feb 1899 & 1905," according to the National Oceanic and Atmospheric Administration.
Dangerous wind chills have been recorded in eastern Colorado and western Kansas, according to the National Weather Service in Pueblo, Colorado. Wind chills ranging from 42 degrees below zero near Yuma, Colorado, to 25 degrees below zero near Norton, Kansas, were reported late Sunday evening.
Along with the unusual, widespread cold are snow events that could also break records.
Seattle has already reported more than 11 inches of snow over the weekend, the most since January 1972, almost 50 years ago. More than 50 inches of snow has fallen in parts of Wyoming over the past few days.
Cities in the South, including Dallas and Oklahoma City, have the potential for their biggest snowfall in a decade, and between two snowstorms this week, have their snowiest weeks on record.
But not every place was cold. Miami hit a record high heat index of 91 on Sunday.
Correction: An earlier version of this story had the wrong day for a pileup in Fort Worth. It was Thursday.