What is a Tsunami Warning?

The highest level of tsunami alert. Warnings are issued due to the imminent threat of a tsunami from a large undersea earthquake or following confirmation that a potentially destructive tsunami is underway. They may initially be based only on seismic information as a means of providing the earliest possible alert. Warnings advise that appropriate actions be taken in response to the tsunami threat. Such actions could include the evacuation of low-lying coastal areas and the movement of boats and ships out of harbors to deep water. Warnings are updated at least hourly or as conditions warrant to continue, expand, restrict, or end the warning.

What is the difference between a "Fixed Regional Warning" and an "Expanding Regional Warning"?

These types of warnings are issued to the Pacific Ocean only. If PTWC issues a Fixed Regional Warning, only the countries within 1000 km (621 miles) of the earthquake epicenter are placed in a warning. All other countries are advised of the earthquake but are not placed in a warning or watch. If PTWC issues an Expanding Regional Warning, the countries within 3 hours of the predicted tsunami arrival time are placed in a warning, and countries within 3-6 hours of the predicted tsunami arrival time are placed in a watch. Every hour, PTWC will issue new bulletins to update the lists of countries within a warning or watch.

What is a Tsunami Watch?

The second highest level of tsunami alert. Watches are issued by the TWCs based on seismic information without confirmation that a destructive tsunami is underway. It is issued as a means of providing an advance alert to areas that could be impacted by destructive tsunami waves. Watches are updated at least hourly to continue them, expand their coverage, upgrade them to a Warning, or end the alert. A Watch for a particular area may be included in the text of the message that disseminates a Warning for another area.

What is a Tsunami Advisory?

The third highest level of tsunami alert. Advisories are issued to coastal populations within areas not currently in either warning or watch status when a tsunami warning has been issued for another region of the same ocean. An Advisory indicates that an area is either outside the current warning and watch regions or that the tsunami poses no danger to that area. The Center(s) issuing the Advisory will continue to monitor the event, issuing updates at least hourly. As conditions warrant, the Advisory will either be continued, upgraded to a watch or warning, or ended.