National Weather Service personnel use information gathered from Doppler weather radar, tornado spotters and other sources to issue severe thunderstorm and tornado watches and warnings.
When a severe thunderstom watch is issued, it means conditions are right for these types of storms to develop, and you should remain alert for approaching storms.

A thunderstorm warning means the storm is occuring in the area that is under the warning. It is important to be alert to these types of storms, because tornadoes can develop during a severe thunderstorm with little warning.
Thunderstorm warnings are issued to local radio and television stations and are broadcast over local National Oceanic & Atmospheric Administration (NOAA) Weather Radio stations serving the area. These warnings are also relayed to local emergency management and public safety officials who can activate local warning sirens to alert the community at large.

When a tornado watch is issued, you need to stay alert for approaching storms. Listen to the radio or television for further developments. Don’t be fooled by clear, sunlit skies behind thunderstorm clouds. Tornadoes generally occur near the trailing edge of a thunderstorm.
If a tornado warning is issued, that means you need to seek shelter immediately. A tornado has been spotted, or doppler radar has indicated a tornado has developed.

When a Tornado Watch is issued: Conditions are right for a tornado to form. Remain alert for approaching storms. When a Tornado Warning is issued: A tornado has been sighted or indicated by radar. Take cover immediately.