(CNN) Huge swaths of the South, Midwest and East Coast are under the gun for severe weather as states still recovering from last weekend's tornadoes hunker down for more.

On Thursday, "the greatest threat for tornadoes will be along the Gulf Coast states of Mississippi and Alabama," CNN meteorologist Michael Guy said.

The National Weather Service said Thursday afternoon that a large and damaging tornado had been observed in southern Mississippi.

The massive storm system won't leave the South quietly, either. It'll fan out to the Ohio Valley region and then the East Coast, threatening 100 million people with severe weather Thursday and Friday.

"Severe thunderstorms may produce damaging gusts, a few tornadoes (possibly strong), and isolated large hail today across the lower Mississippi Valley through the central Gulf Coast region," the National Weather Service's Storm Prediction Center said.

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