Story highlights Expected low of 6 below zero worries mother of missing 10-year-old: "It's my baby girl"

More sleet, snow will hit Washington on Tuesday

Below-average temperatures predicted for days

While rest of nation shivers, Florida stays warm

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.

Photos: Photos: Ice storm spreads across U.S. Photos: Photos: Ice storm spreads across U.S. Ice storm spreads across U.S. – Snow covers buses for the Baltimore County Public Schools in Towson, Maryland, on Tuesday, December 10. Snow and ice snarled travel across parts of the United States, hitting major airline hubs in the Northeast. Hide Caption 1 of 30 Photos: Photos: Ice storm spreads across U.S. Ice storm spreads across U.S. – A mirror reflects light snow on a sidewalk in New York on December 10. Hide Caption 2 of 30 Photos: Photos: Ice storm spreads across U.S. Ice storm spreads across U.S. – A man makes his way across a parking lot during a snowstorm in Philadelphia on December 10. Hide Caption 3 of 30 Photos: Photos: Ice storm spreads across U.S. Ice storm spreads across U.S. – Snow trucks plow along Route 1 in Plainsboro, New Jersey, on December 10. Hide Caption 4 of 30 Photos: Photos: Ice storm spreads across U.S. Ice storm spreads across U.S. – A car drives up a snow-covered street in Brooklyn, New York, on December 10. Hide Caption 5 of 30 Photos: Photos: Ice storm spreads across U.S. Ice storm spreads across U.S. – A woman walks past Independence Hall during a snowstorm in Philadelphia on December 10. Hide Caption 6 of 30 Photos: Photos: Ice storm spreads across U.S. Ice storm spreads across U.S. – Baltimore Gas & Electric lineman Jim Thompson repairs a power line that was damaged by a tree limb in Towson, Maryland, on Monday, December 9. Hide Caption 7 of 30 Photos: Photos: Ice storm spreads across U.S. Ice storm spreads across U.S. – An American Airlines jet is de-iced at O'Hare Airport in Chicago on December 9. Wintry conditions have caused numerous flight cancellations and delays. Hide Caption 8 of 30 Photos: Photos: Ice storm spreads across U.S. Ice storm spreads across U.S. – A woman strolls past the Capitol Building in Washington on December 9, after an overnight ice storm hit the region. Federal agencies opened two hours late because of the weather. Hide Caption 9 of 30 Photos: Photos: Ice storm spreads across U.S. Ice storm spreads across U.S. – Emergency personnel respond to an accident on southbound Interstate 43 in Mequon, Wisconsin, on Sunday, December 8. Hide Caption 10 of 30 Photos: Photos: Ice storm spreads across U.S. Ice storm spreads across U.S. – Juan Carlos tries to free his vehicle from the snow on December 8 in Wilmington, Delaware. Hide Caption 11 of 30 Photos: Photos: Ice storm spreads across U.S. Ice storm spreads across U.S. – The Detroit Lions and Philadelphia Eagles face off during heavy snow in Philadelphia on December 8. Hide Caption 12 of 30 Photos: Photos: Ice storm spreads across U.S. Ice storm spreads across U.S. – A towing company employee works where a semitrailer jackknifed off Interstate 94 in Racine County, Wisconsin, on December 8. Hide Caption 13 of 30 Photos: Photos: Ice storm spreads across U.S. Ice storm spreads across U.S. – Vehicle traffic on I-35 in Sanger, Texas, is at a dead stop due to icy road conditions on Saturday, December 7. Hide Caption 14 of 30 Photos: Photos: Ice storm spreads across U.S. Ice storm spreads across U.S. – Motorists struggle to cross the icy bridge leading to Minnis Drive in Haltom City, Texas, Saturday, December 7. Large ice ruts made passage difficult. Ice remains Saturday after low temperatures refroze sleet from Friday's storm that had begun to melt. Hide Caption 15 of 30 Photos: Photos: Ice storm spreads across U.S. Ice storm spreads across U.S. – Moisture from low-lying clouds coats trees in Virginia's Shenandoah National Park with a layer of ice and frost on December 7. Hide Caption 16 of 30 Photos: Photos: Ice storm spreads across U.S. Photos: Ice storm spreads across U.S. – A fountain stands frozen in Chandler's Cove at South Lake Union in Seattle on Friday, December 6. Hide Caption 17 of 30 Photos: Photos: Ice storm spreads across U.S. Photos: Ice storm spreads across U.S. – A pedestrian walks down the street after a heavy snowfall on December 6 in Carbondale, Illinois. Hide Caption 18 of 30 Photos: Photos: Ice storm spreads across U.S. Photos: Ice storm spreads across U.S. – Icicles hang from mailboxes in Owensboro, Kentucky, on December 6. Hide Caption 19 of 30 Photos: Photos: Ice storm spreads across U.S. Ice storm spreads across U.S. – A worker at Wilson Air Center de-ices an airplane before it takes off from Memphis International Airport in Memphis, Tennessee, on December 6. Hide Caption 20 of 30 Photos: Photos: Ice storm spreads across U.S. Ice storm spreads across U.S. – As freezing rain falls, Tyshun Lindsey, 15, rings a bell to get drivers' attention while selling bags of pecans in Memphis on Friday, December 6. Hide Caption 21 of 30 Photos: Photos: Ice storm spreads across U.S. Ice storm spreads across U.S. – A man clears a path for Christmas tree shoppers in Indianapolis on December 6. Hide Caption 22 of 30 Photos: Photos: Ice storm spreads across U.S. Ice storm spreads across U.S. – A tree, split in two, lays on two homes on December 6 in Dallas in this photograph taken by CNN iReporter Earl Wallace IV Hide Caption 23 of 30 Photos: Photos: Ice storm spreads across U.S. Ice storm spreads across U.S. – A gust of wind collapses Joseph Mezo's umbrella as he walks to work on December 6 in light sleet and ice conditions in Dallas. Hide Caption 24 of 30 Photos: Photos: Ice storm spreads across U.S. Ice storm spreads across U.S. – Matthew Eller catches air after sliding down a hill and jumping a ramp in Arlington, Texas, on December 6. Hide Caption 25 of 30 Photos: Photos: Ice storm spreads across U.S. Ice storm spreads across U.S. – A bronze statue is covered in ice December 6 at Grand Prairie Municipal Airport in Grand Prairie, Texas. Hide Caption 26 of 30 Photos: Photos: Ice storm spreads across U.S. Ice storm spreads across U.S. – Traffic moves slowly along an ice-covered highway in Dallas on December 6. Hide Caption 27 of 30 Photos: Photos: Ice storm spreads across U.S. Ice storm spreads across U.S. – Jeff Chupp walks his ice-covered bicycle along Great Southwest Parkway in Grand Prairie on December 6. Hide Caption 28 of 30 Photos: Photos: Ice storm spreads across U.S. Ice storm spreads across U.S. – Stephen LeFlore scrapes ice from his windshield in McAlester, Oklahoma, on Thursday, December 5. Hide Caption 29 of 30 Photos: Photos: Ice storm spreads across U.S. Ice storm spreads across U.S. – An American Airlines employee hands out cots to stranded airline passengers at Dallas-Fort Worth International Airport on December 5. Hide Caption 30 of 30

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.