What exactly is a tornado emergency?

DETROIT (AP) — The first-ever tornado emergency in Michigan was issued Tuesday, one of many called across the country in recent weeks as severe weather tears through states in the Plains and Midwest. As massive storms barrel East, homes and property have been destroyed and thousands of people have gone without power.

The National Weather Service called an emergency for areas of Michigan on Tuesday evening amid forecasts of a damaging tornado and hail. At the time, other spots in Michigan and portions of Illinois...