By Hollie Silverman and Joe Sutton, CNN
(CNN) -- Millions of people were stuck in homes without electricity in sub-freezing temperatures Monday afternoon as the demand for power outpaced the ability to provide it.
More than 4 million customers in the United States had no power as of Monday afternoon because of the deadly storm system. At least 3.6 million outages were reported in Texas, where rolling blackouts started overnight because utility companies were dealing with problems with natural gas supplies and frozen wind turbines.
The city of Galveston said up to 95% of households were powerless early Monday afternoon.
Nearly 145 million people in the United States were under some sort of winter weather alert Monday, with icy roads, power outages and dangerously low temperatures threatening to snarl traffic and paralyze cities from coast to coast.
At least 13 people have died in weather-related vehicle accidents since cold temperatures took hold of the country. Nine died in three incidents 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 accidents Monday.
The bad weather was widespread, with more than a third of the continental US 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.
The severe winter weather has sparked emergency declarations in at least seven states, including Alabama, Oregon, Oklahoma, Kansas, Kentucky, Mississippi and Texas.
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."
Air traffic disrupted
Air traffic was halted, at least temporarily, at a number of airports.
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.
Delays were reported at Dallas/Fort Worth International, Chicago O'Hare International, Austin-Bergstrom International and other airports, the FAA said.
More than 3,700 flights with US destinations or departures had been canceled Monday morning, according to the flight tracking website FlightAware.com.
Icy conditions on roadways
The Houston area has been hit especially hard. City police responded to more than 130 traffic accidents Sunday night, Police Chief Art Acevedo said in a tweet.
A 10-car pileup on Interstate 45, south of downtown, was just one of many incidents on icy roads. Acevedo urged people to avoid traveling.
"On patrol in center of the city, about every business is closed. No reason to be driving in these conditions ... Button down & stay home the icing is going to get worse as the day progresses," Acevedo said in another Twitter post.
Houston Mayor Sylvester Turner, who declared a state of emergency earlier in the day, echoed the chief's orders.
"Please stay off the roads tonight and through tomorrow. This is serious! The roads are dangerous!"
On top of the danger icy roads pose, the mayor also cautioned residents that the weather could cause rolling blackouts.
Icy roads in Texas have already proved deadly. A pileup in Fort Worth on Thursday, involving more than 130 cars, killed nine people and injured dozens more, with at least 65 people seeking treatment at local hospitals following the crash.
Shelters opened and power outages widespread in Texas
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.
"This is typically done through rotating outages, which are controlled, temporary interruptions of electric service. This type of demand reduction is only used as a last resort to preserve the reliability of the electric system as a whole," ERCOT said in a statement.
The council had previously asked consumers and businesses to reduce their electricity use as much as possible through Tuesday.
Houston and the surrounding areas are under their first-ever wind chill warning. Every county in Texas, Oklahoma and Arkansas is currently under a winter storm warning.
Cold snap from coast to coast
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.
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.
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.
"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."
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.