At least seven people have been killed and dozens injured due to severe weather in southern Oklahoma, Texas and Louisiana.
On Wednesday and early Thursday, severe thunderstorms, flooding and tornadoes were reported in the region.
Five people were killed in "extremely dangerous" twisters in Oklahoma and Texas. In Louisiana, one person died in a storm and another in floodwaters.
Millions of people remain in the path of possible severe weather on Thursday.
More than 100,000 homes across the US south were without power as of Thursday afternoon.
The National Weather Service (NWS) said the "extremely dangerous tornado" that hit the city of Onalaska and other parts of Polk and San Jacinto counties in Texas formed around 18:00 local time (23:00 GMT).
Onalaska is some 90 miles (144km) north of the city of Houston.
The tornado killed three and injured up to 30, local media report, and caused significant damage to homes and businesses and other infrastructure.
Texas Governor Greg Abbott said the state's emergency management agencies are working with local officials to provide assistance.
A "large and dangerous" tornado also struck Marshall County in southern Oklahoma at around 16:30, according to the NWS.
The twister killed two people.
The county's emergency management chief Robert Chaney told CBS News one person's body was found a quarter-mile (0.4km) away from a factory near the town of Madill. Employees were leaving just as the twister hit, he said.
Other tornadoes were also reported throughout the evening in the region, with storms dropping golf and baseball sized hail.
In Louisiana, one woman was killed in Rapides Parish in a severe storm, according to local authorities.
The severe weather in the region damaged homes, uprooted trees and downed power lines.
In DeSoto Parish, one man reportedly drowned after accidentally falling into floodwaters. Officials had to rescue others in their homes.
DeSoto Parish Sheriff Jayson Richardson told the Shreveport Times there was "extreme flooding" in the city of Mansfield.
"Water like I've not seen in many, many years, if ever," he said. "I think we had about 20 or so homes that people were flooded in."
Tornadoes were also reported in Mississippi.
The NWS forecast for Thursday says a storm system that could cause "widespread regions of heavy rains, flash flooding and severe thunderstorms" is moving across the south-eastern part of the country.
Parts of the South are under tornado warnings as of Thursday morning.
Earlier this month, tornados and flooding killed 26 people in the same region. These storms bring the 2020 US tornado death toll to 74, the highest annual total so far since 2011, according to the Washington Post.
As many as 60 tornadoes ripped through Louisiana, Texas and Mississippi on 12 April, while severe storms also hit parts of Alabama, Georgia and the Carolinas.