Violent tornadoes ripped through parts of the U.S., wiping out schools and toppling semitractor-trailers in several states.

Extreme weather warnings remain in place for Florida up through Pennsylvania Sunday before massive storm moves eastward to Atlantic Ocean

An "outbreak" of tornadoes and severe thunderstorms is expected through Saturday night in Alabama, Mississippi, Louisiana, and other southern states.

In some extreme cases, thunderstorms can spawn a derecho, a long-lived windstorm with gusts over 100 mph, strong enough to cause widespread destruction. If a severe thunderstorm warning mentions damaging winds or a derecho, take cover immediately—just as you would for a tornado.

At least 20 people have died, including 12 in Missouri, as a storm outbreak has produced tornadoes, high winds and fires in the South and threatens to cause flooding and further storms throughout the weekend.