Which of the following best describes a "tornado warning"?

A tornado warning specifically indicates that a tornado has been sighted in the area or has been detected by radar. This alerts individuals that a dangerous situation is occurring and that immediate action should be taken to protect themselves, such as seeking shelter. The distinction is critical because a warning signifies an immediate threat, whereas other options refer to conditions where danger is possible or involve different scenarios altogether. The precision of a tornado warning is vital in facilitating timely responses from the public, ensuring safety when a tornado is imminent or already happening.