More than 40 people have been killed over the past five days by severe storms in the South and Midwest.
Flash floods, tornadoes and now snow have destroyed hundreds of homes and businesses and snarled transport links.
The National Weather Service has issued tornado warnings for Texas, Arkansas, Louisiana, Oklahoma and Mississippi.
The governors of Missouri, Oklahoma, and New Mexico have declared states of emergency.
In Illinois, three adults and two children were swept away in their car by the flood waters. In Texas, at least 11 people were killed by powerful tornadoes with winds of up to 200 miles an hour.
Hundreds of homes in Texas were reduced to rubble and cars were blown off the road. Five people died in their cars on a highway passing the city of Garland. Pedro Barineau of the local police said they had no chance.
“There were sirens that were going off, I mean multiple times and in multiple areas, notifying people of a tornado. However, people on the highway that were driving through they had no idea that the sirens were going off and the tornado was on them in a matter of seconds,” he said.
Texas Governor Greg Abbott said his office had declared Dallas County and three nearby counties a disaster area. He also warned people to be wary of snow in western parts of the state and of rivers spilling their banks.
Parts of New Mexico, where a state-wide emergency has been declared, are expecting up to 2ft of snow. The New Mexico city of Roswell recorded 12.3 inches of snow on December 27 – an area record for a single day.
Oklahoma Governor Mary Fallin declared a state of emergency on December 27 in anticipation of blizzard conditions and an ice storm.
The bad weather forced the cancellation of nearly 1,500 flights on December 27, according to tracking service FlightAware.com. About half of the cancelled flights were in Dallas, a major US flight hub.
While extreme weather in the United States around Christmas is not unknown, meteorologists say that unseasonably high temperatures in some areas contributed to the severity of the storms.
The forecast for the eastern US is for high temperatures to continue – Washington DC pushed close to 70F on December 27.
At least 8 people are reported to have been killed in Texas in new tornadoes, raising the death toll to 26 in a week of storms across the South and Midwest.
Five people died when their cars were reportedly blown off a highway in Garland, near Dallas. Another three bodies were found in nearby towns.
Texas and Oklahoma could suffer a “historic blizzard”, bringing up to 16in of snow, officials say.
The storms across the South have been unusually powerful for winter.
Reports from Texas said churches were destroyed, cars mangled and trees toppled across a wide area.
Photo CBS News
Garland police believed that “winds from a tornado that passed through” the town late on Saturday were the cause of car accidents, Melinda Urbina from the Dallas County Sheriff’s office said.
At least five people were reported dead in the cars.
Melinda Urbina said the winds “tossed the cars around”, and the vehicles were later found below Interstate 30, about 15 miles north-east of Dallas.
She also urged local residents to stay off the roads.
Police officers in Garland are now trying to determine whether they were any other casualties at the crash site.
“We’re dealing with darkness out here,” police spokesman Mike Hatfield was quoted as saying by the Dallas News website.
“All of the street lights and highway lights are out,” he added.
Two people were found dead at a petrol station in Copeville, and a third was killed in Blue Ridge, reports in local media said.
The storms have damaged a number of buildings in the area.
Kevin Taylor, a church pastor in Glenn Heights, south of Dallas, described to WFAA how his church began collapsing around him.
“Doors began to turn inward, when I saw that I figured the glass was going to shatter and hit me in the face, so I broke and ran down the hallway and by the time I got just a few feet everything collapsed and went dark and fell on top of me,” he said.
He added: “By the grace of God I’m here though.”
At least 30,000 people were reportedly without power, and there were reports of burst gas lines.
The National Weather Service confirmed that several tornadoes had touched down near Dallas and other towns in northern Texas.
The deaths in Dallas come as much of the south-central region of the US has been hit by severe weather in the past week.
At least six people have been killed and scores more injured ahead of the Christmas break after a strong storm has hit the South and Midwest of the US.
The storm has been described by forecasters as particularly dangerous.
Three people were killed as tornadoes moved through northern Mississippi, officials said, along with two in Tennessee and one in Arkansas.
There have been reports of at least 20 tornadoes of varying severity.
The high winds have also caused significant damage to homes.
Authorities in parts of Mississippi – where a seven-year-old was among those killed – are conducting a house-by-house search-and-rescue operation after the state was hit by multiple tornadoes.
Planes at a small airport in the north-west of the state were overturned and an unknown number of people were injured.
“I’m looking at some horrific damage right now,” Clarksdale Mayor Bill Luckett was quoted by the AP as saying.
“Sheet metal is wrapped around trees; there are overturned airplanes; a building is just destroyed.”
Mississippi’s I-55 was closed in both directions as the tornado approached, the state’s Highway Patrol said.
The bad weather is also due to hit – or has hit – the states of Kentucky, Alabama, Indiana, Illinois, Missouri and Alabama.
The national Storm Prediction Center in Oklahoma has released a “particularly dangerous situation” warning for the first time since June 2014, AP reports, when two massive tornadoes destroyed a rural Nebraska town, killing two people.
The possibility of bad weather just before Christmas in the US is not unusual, officials at the center say.