Two of the most important terms that weathermen use are also the two terms that people mix up the most, and the confusion could prove lethal. When the atmosphere is ripe for tornadoes, meteorologists will bat around the terms "tornado watch" and "tornado warning" almost every other sentence, but people often get the two confused and don't always know what they mean.
A tornado watch means that conditions are favorable for the development of tornadoes within the next couple of hours. Tornado watches are often issued an hour or two before severe weather begins, and they stay in effect for about six hours or until the thunderstorms pass through.
Tornado watch boxes cover huge areas...usually multiple states at a time. You should be cautious and keep a close watch for rapidly changing weather when a tornado watch is in effect.
A tornado warning means that a tornado is imminent, and you need to immediately seek shelter to protect yourself. Tornado warnings are issued when meteorologists detect strong rotation in a thunderstorm on radar — which indicates that a tornado is likely on the ground — or when spotters actually see a tornado on the ground.
Tornado warnings are often called "polygons." These polygons usually cover one or two counties, and appear on most maps in red.
There is a rare third option — the tornado emergency. Tornado emergencies are issued when a large, violent tornado is confirmed by spotters and is moving into a heavily populated area. Wikipedia keeps a running list of every tornado emergency in the United States since the first one was issued on May 3, 1999.
It's worth noting that the same goes for severe thunderstorm watches and warnings. A watch means that conditions are favorable for large hail and damaging winds, and a warning means that you're in immediate danger and should take appropriate action to protect yourself.
We could have our first extremely dangerous tornado outbreak of the year later this weekend, so now is a good time to go over these terms and make sure you have a plan in case you find yourself under the gun for strong storms.