Tornadoes killed more than 30 people across several states in the U.S. Midwest and southeast on Saturday night, part of a monster storm strong enough to topple tractor-trailers and wipe out hundreds of buildings.

Meanwhile, the severe weather threat will linger through Sunday from Florida northward into the upper Ohio River Valley, forecasters warned.

Millions of Americans across the country are on alert for a severe weather outbreak as violent, long-track tornadoes with damaging winds of up to 80 mph and large hail are expected across the Midwest and South with the cross-country storm moving east.