Two children, aged just three and eight, were crushed to death on Saturday after a tree toppled onto the back of their family auto in Lufkin, Angelina County.
The weather service said preliminary information showed an EF-3 tornado touched down with winds of 140 miles per hour (225.3 kph).
The Richland County Emergency Management Agency reported about a half-dozen homes were damaged and at least six people were taken to a hospital to be treated for storm-related injuries.
The National Weather Service warned on Saturday that a "major severe weather outbreak" was likely, bringing hail, thunderstorms, damaging winds, flash flooding and tornadoes.
A microburst "is a localized area of strong straight line winds that descends from a thunderstorm and is less than 2.5 miles in diameter", the weather service said.
At least seven people, including three children, have been killed after a series of devastating tornadoes swept through the South.
There was widespread damage in Alto, a town of about 1,200, and the school district canceled classes until its buildings can be deemed safe.
Monroe County Sheriff Cecil Cantrell said there are reports of trees down in the south part of Monroe County.
Temperatures will fall into the lower 50s this afternoon, the weather service said.
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A deadly storm system is threatening millions of Americans as severe weather now bears down on the East Coast and parts of the Midwest.
A wave of severe storms moved across the Tennessee Valley early Sunday morning, leaving behind damage in many communities.
According to the Post, a tornado that struck the Central Texas town of Franklin destroyed 55 homes, and injured over a dozen people.
Almost 2,300 United States flights were cancelled by Sunday evening, more than 90 per cent of them at airports in Chicago; Houston, Texas; Charlotte, North Carolina; Pittsburgh; Columbus, Ohio and a dozen major airports on the Eastern Seaboard, according to FlightAware.com.
But "the storm is so vigorous that tornadoes are still possible after dark", he said.
The tornado downed trees near the Mississippi State University campus. "Scattered power outages can be expected", the National Weather Service says in its wind advisory.
Mississippi's Monroe County was also hit hard, as multiple homes as well as the local fire station were destroyed, reported CNN.
"Damaging winds and some hail are also associated with these storms pushing through the Southeast this morning", Brink said.
"Damage to roofs, windows, and vehicles will occur".