(CNN) -- Frigid weather that gripped much of the United States created a crisis in northwest Nevada, where rescue teams have been searching a mountainous area for two adults and four children.
James Glanton, 34, and Christina MacIntee, 25, are missing, along with a 10-year-old, two 4-year-olds and a 3-year-old, the Pershing County Sheriff's Office said. The six set out Sunday for Seven Troughs mountain range and haven't been seen since.
Fears for their safety grew as the temperature was expected to hit 6 below zero Tuesday night.
Amanda Fitzpatrick, mother of the 10-year-old, Shelby Fitzpatrick, told CNN's Piers Morgan in a telephone interview that she'd joined the search. Rescue teams have tried to stay positive, she said.
"It's been extremely hard, probably the hardest 24 to 36 hours of my life," she said. "It's my baby girl."
The relationship between the two adults and the other children was unclear.
At least 15 people have died because of the weather, mostly in traffic accidents. Eight died in Oklahoma alone, including a 6-year-old who fell through ice on a creek in Tulsa and men who died in house fires in Westville and Tulsa, the state Department of Emergency Management reported on Monday.
Temperatures across the country are expected to stay very low, usually 10 to 20 degrees below normal, for the rest of the week in regions struggling after days of wintry weather, according to the National Weather Service.
Dallas is still trying to shake off the effects of a weekend ice storm and had about 20,000 customers without power on Monday, according to power company Oncor. Anchorage, Alaska, has been warmer than St. Louis and Denver.
"It's very unusual," CNN meteorologist Dave Hennen said. "This literally spreads across the entire U.S., and we're 12 days from the official start of winter."
Even if snow leaves, the cold will remain. More sleet, snow and freezing rain will smack Washington on Tuesday morning. The storm will move off the East Coast in the afternoon and night, the National Weather Service said, but the mercury won't rise above freezing until Friday. The forecast is about the same for Philadelphia and New York City, though those cities won't see temperatures above 32 until days later.
Portland, Oregon, should have more snow and freezing rain this week; Chicago, too.
The nation's airports appear to be getting back to normal. The website Flightaware.com says only 304 flights have been canceled for Tuesday, up from 1,700 on Monday and 2,600 on Sunday.
Florida is pretty much the only place in the country to escape the cold, with Punta Gorda, a town on the Gulf Coast, reporting Sunday's national high temperature of 87 degrees. Mimi Huddleston, a bartender at Harpoon Harry's, has a message for the rest the country, and to her credit, it's not "nyah nyah."
"We live in paradise," she said Monday. "Snowbirds" from the North who come in for a drink are always talking about the weather back home. "They say it's too cold for them and they like it here."
The country's coldest spot on Monday was Daniel, a community of about 150 people in western Wyoming. It registered 29 degrees.
Rachel Grimes of the Sublette County Chamber of Commerce said people are busy "recreating" on skis and snowmobiles. "We normally don't get cold weather like this until after the holidays," she said. "The wind is blowing today, so it feels colder."
Tuesday's storm in the East could drop up to 5 inches of snow in Virginia before moving out to sea, the National Weather Service said. Much of the Plains and Rocky Mountains will stay very cold through Wednesday, with the lowest temperatures probably found in the higher elevations of the Great Basin eastward through the Dakotas and into Minnesota.
Travel will remain hazardous in spots.
In Arizona, a Saturday night snowstorm stranded 300 vehicles along Interstate 15. Rigs jackknifed and passenger cars slid into rigs, causing chain-reaction crashes and an enormous backup, Arizona Department of Public Safety Officer Bart Graves said. Authorities shut the interstate for more than 12 hours to clear it.
"We had travelers running out of gas. They provided them food, water and blankets," Graves said.
Some residents in the Dallas suburb of Plano had to deal with an unusual danger: sheets of ice cascading from buildings to the sidewalks and streets.
"The apocalypse has started," one man said shortly before layers of ice fell onto cars.
Late Sunday night in New York, there was a 20-car pileup on the Bronx River Parkway. Forty people were injured, none seriously, authorities said.
Along Interstate 95 outside Stamford, Connecticut, Paul Lee captured frightening video of cars sliding and spinning across ice.
Freezing rain is expected to fall from central Virginia to southeast New York on Monday. Some parts could see up to a quarter-inch of ice.
CNN's Indra Petersons, Judson Jones, Ed Lavandera, Dave Alsup and Emily Minner contributed to this report.