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.

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

The deadliest tornado in recorded U.S. history occurred 100 years ago. The Tri-State Tornado touched down in southeastern Missouri on March 18, 1925.

America faces another tornado-thunder assault after killer cyclones left more than 30 dead in a weekend of chaos. A massive clean-up across six states will be hampered by another storm whipping up to smash the Midwest.