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 moveing east.

The threat of tornadoes in parts of the U.S. proved deadly as whipping winds moved east into the Mississippi Valley and Deep South on Saturday, causing at least 20 fatalities in four states, injuries and widespread damage.