Severe thunderstorm and tornado watches are issued by the Storm Prediction Center in Norman OK for the entire country. Watches usually cover a large area, around 25,000 square miles, and are usually in effect for about 6 hours. A watch means conditions are favorable for severe weather and/or tornadoes for the next few hours.
Severe thunderstorm and tornado warnings are issued by local National Weather Service offices for specific storms. A typical tornado warning covers about 350 square miles and severe thunderstorm warnings cover around 900 square miles. Warnings are usually in effect for 30 to 60 minutes. A warning means severe weather or a tornado is occurring, imminent or highly likely in the next few minutes.
Also keep in mind that watches are only issued for widespread, organized outbreaks of severe weather. There are times when there may be a few isolated severe weather events when no watch is in effect, but warnings may need to be issued for those storms.
List of Cooling Centers During Severe Heat CLICK HERE!