A rare and powerful tornado in Mississippi left storm chasers and forecasters shocked at the devastation it caused.
At least 25 people have died in the state and search and rescue efforts continue through Sunday.
The tornado appeared huge as it approached the small town of Rolling Fork, with some calling it a "corner tornado".
The National Weather Service estimates the storm lasted more than an hour.
"I still can't get over what I saw," said Stephanie Cox, an Oklahoma storm chaser who witnessed the tornado as it headed down the Mississippi River.
Ms Cox told the BBC she was initially unable to determine the size or strength of the storm. But then he heard a huge roar, he said, followed by lightning that set off what he described as a "monstrous" tornado.
"I've never seen or heard one so violent, making that roaring noise that sounds like a train horn coming straight at you," he said.
The NWS estimates the tornado, which began to hit western Mississippi Friday night after forming over the Mississippi, was 59 miles (94 kilometers), three-quarters of a mile wide and about an hour and 10 minutes.
It evolved from a supercell storm, a rotating storm with separate updrafts and downdrafts. It is caused by warm, unstable air near the ground and the change in wind speed and direction with altitude.
These storms are among the rarest, but among the most destructive, according to the NWS.
It is also known that supercell thunderstorms can last longer than normal.
"Conditions were perfect for a long storm, and that's not often the case," said Lance Perrilloux, NWS meteorologist in Jackson, Mississippi.
“It caused the tornado devastating damage over a long distance,” he said.
Ms Cox and others have described it as a "wedge tornado", an unofficial term used to describe tornadoes that appear wider than their length as they approach.
These types of tornadoes are known to be destructive because their width causes damage over a larger area.
Homes and buildings in Rolling Fork were swept away after the storm, and vehicles were thrown and destroyed.
Samuel Emmerson, a member of the University of Oklahoma Radar Research Group, said the tornado threw debris up to 9 km.
Preliminary results registered the tornado as a four on the Enhanced Fujita Scale (EF), meaning it had a three-second gust of 166 to 200 mph, Perrilloux said. He added that the tornado intensified after passing through the Rolling Fork. It then descended approximately 76 miles (122 kilometers) northeast to the town of Black Hawk, Mississippi, where it was lowered to three on the EF scale. Tornadoes also hit parts of Alabama.
Another contributing factor to the devastation of the Mississippi is the timing of the storm.
It hit the town of Rolling Fork around 8:00 p.m. local time (02:00 GMT), with the NWS not issuing a tornado warning until about 20 minutes earlier.
Studies have shown that nighttime tornadoes can be twice as deadly as daytime tornadoes, in part because they are difficult to predict.
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A rare and powerful tornado in Mississippi left storm chasers and forecasters shocked at the devastation it caused.
At least 25 people have died in the state and search and rescue efforts continue through Sunday.
The tornado appeared huge as it approached the small town of Rolling Fork, with some calling it a "corner tornado".
The National Weather Service estimates the storm lasted more than an hour.
"I still can't get over what I saw," said Stephanie Cox, an Oklahoma storm chaser who witnessed the tornado as it headed down the Mississippi River.
Ms Cox told the BBC she was initially unable to determine the size or strength of the storm. But then he heard a huge roar, he said, followed by lightning that set off what he described as a "monstrous" tornado.
"I've never seen or heard one so violent, making that roaring noise that sounds like a train horn coming straight at you," he said.
The NWS estimates the tornado, which began to hit western Mississippi Friday night after forming over the Mississippi, was 59 miles (94 kilometers), three-quarters of a mile wide and about an hour and 10 minutes.
It evolved from a supercell storm, a rotating storm with separate updrafts and downdrafts. It is caused by warm, unstable air near the ground and the change in wind speed and direction with altitude.
These storms are among the rarest, but among the most destructive, according to the NWS.
It is also known that supercell thunderstorms can last longer than normal.
"Conditions were perfect for a long storm, and that's not often the case," said Lance Perrilloux, NWS meteorologist in Jackson, Mississippi.
“It caused the tornado devastating damage over a long distance,” he said.
Ms Cox and others have described it as a "wedge tornado", an unofficial term used to describe tornadoes that appear wider than their length as they approach.
These types of tornadoes are known to be destructive because their width causes damage over a larger area.
Homes and buildings in Rolling Fork were swept away after the storm, and vehicles were thrown and destroyed.
Samuel Emmerson, a member of the University of Oklahoma Radar Research Group, said the tornado threw debris up to 9 km.
Preliminary results registered the tornado as a four on the Enhanced Fujita Scale (EF), meaning it had a three-second gust of 166 to 200 mph, Perrilloux said. He added that the tornado intensified after passing through the Rolling Fork. It then descended approximately 76 miles (122 kilometers) northeast to the town of Black Hawk, Mississippi, where it was lowered to three on the EF scale. Tornadoes also hit parts of Alabama.
Read Also : How Armenian is Kim Kardashian?Another contributing factor to the devastation of the Mississippi is the timing of the storm.
It hit the town of Rolling Fork around 8:00 p.m. local time (02:00 GMT), with the NWS not issuing a tornado warning until about 20 minutes earlier.
Studies have shown that nighttime tornadoes can be twice as deadly as daytime tornadoes, in part because they are difficult to predict.