Understanding the difference between a tsunami "watch" and "warning"

A Tsunami Watch is automatically declared by the Pacific Tsunami Warning Center (PTWC) for any earthquake magnitude 7.5 or larger (7.0 or larger in the Aleutian Islands) if the epicenter is in an area capable of generating a tsunami. Civil Defense is notified, and the local media is provided with public announcements. PTWC then waits for data from tide gauge stations to confirm whether or not a tsunami has been generated.

PTWC also requests reports on wave activity from tide-gauge stations near the earthquake epicenter. If the stations observe no tsunami activity, the Tsunami Watch is canceled. If the stations report that a tsunami has been generated, a Tsunami Warning is issued. A warning may be issued automatically if an earthquake powerful enough to create a tsunami occurs nearby. The emergency broadcast system alerts the public of the danger, and evacuation begins. Remember, tsunamis travel at 500 miles per hour; as soon as a warning has been issued you should evacuate immediately.