Tornado Watch: 

A tornado watch means that conditions are favorable for a tornado to form. When a watch is issued, you should begin to find appropriate shelter and continue to monitor the weather and the media for updates.

Tornado Warning:

A tornado warning means that a tornado has touched down in your area. You should take shelter immediately and continue to monitor the media for updates.

The Storm Prediction Center of the United States Weather Service provides a web site that provides real-time information on the evolution of weather conditions likely to create storms and other extreme weather events in the United States, along with satellite maps and other useful information.

Tornado Alley 

Oklahoma, Kansas and Arkansas together with the northern regions of Texas and Louisiana, is a high risk area for tornadoes and high winds.