Tornado watches and warnings are issued by the National Weather Service or the National Severe Storms Lab, which constantly monitor atmospheric conditions over the United States.
Tornadoes are very destructive, so it's important to know when one may form so you can take shelter. Forecastors at the National Weather Service are always on the lookout for developing storms. Even though nobody knows how tornadoes form, they do know the conditions when they are most likely to form.
What's the different between a tornado watch and a tornado warning?
TORNADO WATCH
A tornado watch means that conditions are favorable for producing a particular weather event, but that it has not formed yet. Watches alert the public that they need to pay closer attention than usual to the weather, just to be safe. A tornado watch is issued when atmospheric conditions could cause tornadoes to form, although none have formed yet.
TORNADO WARNING
A tornado warning means that a particular weather event has formed and that it threatens the area under the warning. A tornado warning means that a tornado is actually present and moving in the warn vicinity. Residents should take immediate shelter in a tornado warning.
Source: