Over the past 24 hours, more than 40 reported tornadoes have swept across eight states in the Midwest and South -- and the dangerous severe weather threat continues. The eight states with reported tornadoes in the past day are Alabama, Missouri, Arkansas, Mississippi, Louisiana, Illinois, Indiana and Tennessee.

A monster storm sweeping through parts of the U.S. whipped up tornadoes and dust storms and sparked wildfires, killing at least 34 people across six states, according to reports. The massive storm system with winds reaching 80 mph was forecast to sweep through an over 100 million-strong area from the Canadian border to Texas,

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.