Tornadoes have killed at least 20 people in the southern US states of Alabama, Georgia and Missouri.
Victims included at least eight people killed when a tornado ripped through an Alabama school building, and two dead when a hospital was hit in Georgia.
The US federal government has said it is ready to help any areas affected by the storms.
Parts of Alabama and Georgia remain under tornado warnings, the National Weather Service has said.
The US Storm Prediction Center (SPC) had earlier issued a rare "major severe weather outbreak" warning for the eastern third of the US.
Meanwhile further north, several mid-western states were hit by heavy snow and blizzards, which closed hundreds of miles of major roads.
Two people died when their car overturned on a slippery road in North Dakota, and another was killed while shovelling snow in Nebraska, the Associated Press reported.
The mayor of Enterprise, Alabama - Kenneth Boswell - said eight teenagers were killed at the local high school.
One other person in Enterprise died in the tornado.
Alabama state emergency official Yasamie Richardson said miscommunication due to confusion at the scene had led to an earlier figure of 15 deaths being reported.
At least 40 people were injured as the tornado, travelling at over 50mph (80km/h), swept through Enterprise.
Antoinette Konz, of the local Montgomery Advertiser newspaper, told the BBC that the tornado had struck just as high school pupils were about to be sent home early.
"We talked to several parents who had just pulled up to the school and walked in the door and that's when everybody told them to 'Get down, get down, get down - it's coming'.
"And the next thing they knew, everything was gone."
Eyewitnesses said the roof and walls of the sports hall had collapsed onto staff and pupils sheltering from the storm.
Alabama Governor Bob Riley has declared a state of emergency and sent 100 National Guard troops to the town.
"Enterprise has suffered major and widespread damage," he said.
Mr Riley was expected to visit the high school on Friday to assess the impact of the tornado.
There was a tenth death in the town of Millers Ferry, in another part of the state.
A suspected tornado hit the Sumter regional hospital in Americus, southern Georgia, killing two, the state's emergency management agency said. It is not clear whether the dead were patients.
Rescuers at storm-damaged Enterprise high school
"Sumter County has apparently lost all its ambulances and rescue vehicles," agency spokesman Buzz Weiss said.
A third person was killed in Taylor County, just to the north, Mr Weiss added.
Six more were reported dead in the south-west of the state, he said. No further details were given.
A seven-year-old girl died as some of the first tornadoes touched down in Missouri around dawn on Thursday.
Tornadoes were also reported in Arkansas and Kansas, damaging houses, mobile homes and service stations, and an electricity substation in Linn County on the Missouri-Kansas border.
Linn County Sheriff Marvin Stites said some people had been forced to leave their homes and seek shelter elsewhere, but that "we're still small enough where neighbour takes care of neighbour".
The severe weather formed a band of storms stretching from Minnesota in the north to Louisiana on the Gulf Coast.
It moved from the west, where it had already dumped several feet of snow from Washington state to Colorado, and was fuelled by moist air being sucked up from the Gulf of Mexico.
Big snowstorms also hit Nebraska and Iowa, with whiteout conditions causing roads across Nebraska to be shut down.
TORNADOES REPORTED IN THE US
People killed by tornadoes in southern US
Enterprise, Alabama: Eight dead at school, one elsewhere
Millers Ferry, Alabama: One dead
Americus, Georgia: Two killed when tornado struck hospital
Taylor County, Georgia: One dead
South-west Georgia: A further six people reported dead
Missouri: Seven-year-old girl killed