Damaging winds and some tornadoes are the main threats along this line.
A major severe weather outbreak is underway to our southwest.
Population centers in the zone of greatest risk, 4 out of a possible 5 on the Storm Prediction Center's scale, include Monroe and Alexandria, Louisiana.
The same system caused damage in Columbus, Mississippi.
At least one mobile home was destroyed, throwing a man from the mobile home.
About 90 million people are under the gun for destructive weather Sunday, CNN meteorologist Haley Brink said.
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Two children, siblings aged 3 and 8, were killed when a tree fell on the vehicle in which they were sitting in Pollok, Texas, said a spokeswoman for the Angelina County Sheriff's Department. Earlier, two children died in East Texas after a tree fell on their moving vehicle.
Police in Monroe County earlier described the destruction from the storm Saturday evening.
By late Saturday, the storms were expected to organize into a line as they cross the western border of Mississippi.
The Mississippi Emergency Management Agency warned of a confirmed tornado around Saturday night. The children, ages 3 and 8, were killed when a tree fell on a auto Saturday. The children, ages 8 and 3, were dead at the scene, although both parents, who were in the front seat, escaped injury, he said. The 13-year-old boy drowned in a drainage area in West Monroe late Saturday afternoon, the Ouachita Parish Sheriff's Office said.
"It looks like a storm came up through Louisiana and into MS and exploded in Monroe County", Monroe County Sheriff Cecil Cantrell said.
The weather service also says preliminary information showed an EF-3 tornado with winds of 140 mph touched down in Franklin, located about 125 miles (200 kilometers) south of Dallas.