- NEW: Federal offices in Washington closed as more sleet and snow are expected
- New York and Philadelphia are expected to get several inches of snow
- Temperatures will be about 10 to 20 degrees below normal in many areas
- About 775 flights have been canceled for Tuesday
Wintry weather near you? Share your photos and videos with CNN iReport.
(CNN) -- Frigid weather gripping much of the United States led to a frenzied search in northwest Nevada, where rescue teams are searching a mountainous area for two adults and four children who went to play in the snow and never returned.
James Glanton, 34, and Christina MacIntee, 25, are missing, along with Glanton's two children and MacIntee's niece and nephew. The children are 3 to 10 years old, the Pershing County Sheriff's Office said.
The group set out Sunday for Seven Troughs mountain range and haven't been seen since.
Fears intensified as the temperature plummeted to below zero.
Baltimore Gas & Electric lineman Jim Thompson repairs a power line that was damaged by a tree limb in Towson, Maryland, on Monday, December 9. A massive weather system has brought cold and snow to much of the United States. 
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. 
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. 
Emergency personnel respond to an accident on southbound Interstate 43 in Mequon, Wisconsin, on Sunday, December 8. 
Juan Carlos tries to free his vehicle from the snow on December 8 in Wilmington, Delaware. 
The Detroit Lions and Philadelphia Eagles face off during heavy snow in Philadelphia on December 8. 
A towing company employee works where a semitrailer jackknifed off Interstate 94 in Racine County, Wisconsin, on December 8. 
Vehicle traffic on I-35 in Sanger, Texas, is at a dead stop due to icy road conditions on Saturday, December 7. 
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. 
Moisture from low-lying clouds coats trees in Virginia's Shenandoah National Park with a layer of ice and frost on December 7. 
A fountain stands frozen in Chandler's Cove at South Lake Union in Seattle on Friday, December 6. 
A pedestrian walks down the street after a heavy snowfall on December 6 in Carbondale, Illinois. 
Icicles hang from mailboxes in Owensboro, Kentucky, on December 6. 
A worker at Wilson Air Center de-ices an airplane before it takes off from Memphis International Airport in Memphis, Tennessee, on December 6. 
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. 
A man clears a path for Christmas tree shoppers in Indianapolis on December 6. 
A tree, split in two, lays on two homes on December 6 in Dallas in this photograph taken by CNN iReporter Earl Wallace IV. 
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. 
Matthew Eller catches air after sliding down a hill and jumping a ramp in Arlington, Texas, on December 6. 
A bronze statue is covered in ice December 6 at Grand Prairie Municipal Airport in Grand Prairie, Texas. 
Traffic moves slowly along an ice-covered highway in Dallas on December 6. 
Jeff Chupp walks his ice-covered bicycle along Great Southwest Parkway in Grand Prairie on December 6. 
Stephen LeFlore scrapes ice from his windshield in McAlester, Oklahoma, on Thursday, December 5. 
An American Airlines employee hands out cots to stranded airline passengers at Dallas-Fort Worth International Airport on December 5. 
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Photos: Ice storm spreads across U.S. 
Winter storm creates deadly conditions 
Girl,10, among missing in winter weather 
Snow and ice causing travel nightmares 
Falling ice crushes cars in Texas Amanda Fitzpatrick, mother of 10-year-old Shelby Fitzpatrick, has joined the search in the freezing cold.
"It's been extremely hard, probably the hardest 24 to 36 hours of my life," she told CNN's Piers Morgan. "It's my baby girl."
More snow and ice
The pre-winter wintry weather isn't over yet. Temperatures in storm-stricken parts of the country are expected to stay about 10 to 20 degrees below normal for the rest of the week, the National Weather Service said.
Federal offices in Washington are closed Tuesday as more sleet, snow and freezing rain are expected. And a swath of 2 to 6 inches of snow is expected to fall Tuesday from Virginia to the New England coast, forecasters said.
Philadelphia can expect to get 3 to 5 inches of snow on Tuesday; New York, a little less than that.
The storm will finally leave the East Coast late Tuesday, but the mercury won't rise above freezing until at least Friday, the National Weather Service said.
Elsewhere across the country, Portland, Oregon, and Chicago will see snow and freezing rain this week.
Even St. Louis has been colder than Anchorage, Alaska.
"It's very unusual," CNN meteorologist Dave Hennen said. "This literally spreads across the entire U.S."
Choked off travel
At least the nation's airports are getting back to normal. The website Flightaware.com says about 775 flights have been canceled for Tuesday. That's down from the 1,900 flights canceled Monday and 2,800 Sunday.
But road conditions have been treacherous. At least 15 people have died from weather-related incidents in the past several days, mostly in traffic crashes. Eight died in Oklahoma alone, state officials said.
And the hazardous conditions in northern Nevada have many concerned about the missing children and adults.
"If there's one thing you should learn from the lessons of these people, never let your tank below a half a tank (of gas) in the winter," CNN meteorologist Chad Myers said. "You might need that for the electricity, for the power, for the heat in that car."
CNN's Randi Kaye, Chuck Johnston and Judson Jones contributed to this report.
