Tornado warning:

A warning means that a tornado has been sighted or a developing tornado is reported by trained spotters or indicated on Doppler radar.

A warning is issued when the tornado has begun happening. Since tornadoes are small, a half-mile wide tornado is considered huge, a tornado will miss many more buildings that it hits in the area warned. Still, a tornado warning means be ready to take shelter immediately if there are any indications a tornado is approaching.