There are several levels of advisory involving tornadoes, but only two are actual warnings. Here they are in order of increasing severity:
Note that a PDS tornado watch is a variant of tornado watch, and a tornado emergency is a variant of tornado warning.
In the U.S. tornado warnings are issued by local offices of the National Weather Service.
Yes. In most cases a warning is issued before a tornado strikes.
Usually a watch is issued first, and then warnings are issued as tornadic storms develop. However, occasionally a tornado warned storm may develop without a tornado watch.
Last year in 2011, there were over 1800 tornado reports (of which about 1700 were confirmed), but the number of tornado warnings was higher as not all warnings correspond to a formal report. So the number of warnings issued in 2011 was likely well over 2000. However, 2011 was an extremely active tornado season. An average tornado season sees about 1200 tornadoes in the U.S., which would likely work out to about 1800-2000 warnings.
It is impossible to predict when a place will be hit by a tornado. Warnings for specific locations can seldom issued more than 30 minutes in advance.
Tornadoes warnings are broadcast via TV, radio, and smartphone apps. In some places, sirens will sound if a tornado warning is issued.
In the United States, tornado warnings are issued by local offices of the National Weather Service. Tornado watches are issued by the Storm Prediction Center in Norman, Oklahoma, a subdivision of the National Weather Service.
All tornado watches in the United States are issued by the Storm Prediction Center in Norman, Oklahoma. Tornado warnings are issued by your local National Weather Service office. Hopefully, they are announced and broadcast by everyone with those capabilities.
A tornado watch usually does not warrant school closure as even when one is issued the chances of a tornado striking any given location are very low. Tornado warnings, which indicate a more imminent threat, are issued on too short a time scale for school closures.
In the United States warnings are issued by local offices of the National Weather Service while regional outlooks are issued by the Storm Prediction Center.
Yes. Tornado warnings are now a common part of severe weather alerts.
If a tornado is spotted or detected, or if a rotation is detected that is likely to lead to a tornado, that is when a warning is issued. A watch simply means that conditions are favorable for tornadoes, but does not mean that there is an imminent threat. So a tornado is more likely to strike when a warning is issued.