Tsunami Warning vs Advisory vs Watch: What Each Alert Means
A tsunami warning means a tsunami that may cause widespread flooding is expected or occurring. A tsunami advisory means strong currents or waves dangerous to people in or very near the water are expected or occurring. A tsunami watch means a tsunami is possible, and a tsunami information statement means an earthquake happened but in most cases there is no threat. Two NOAA tsunami warning centers issue these alerts for US coasts.
What do the four US tsunami alert levels mean?
The National Weather Service (NWS) defines each level and the action that goes with it. The table follows the NWS tsunami alerts page.
| Alert | What it means (NWS) | Action the NWS gives |
|---|---|---|
| Tsunami Warning | A tsunami that may cause widespread flooding is expected or occurring. | Follow instructions from local officials. Evacuation is recommended. Move to high ground or inland, away from the water. |
| Tsunami Advisory | A tsunami with potential for strong currents or waves dangerous to those in or very near the water is expected or occurring. Beach and harbor flooding may occur. | Stay out of the water and away from beaches and waterways. Follow instructions from local officials. |
| Tsunami Watch | A distant earthquake has occurred. A tsunami is possible. | Stay tuned for more information. Be prepared to take action if necessary. |
| Tsunami Information Statement | An earthquake has occurred, but there is no threat, or it was very far away and the threat has not been determined. | In most cases there is no threat of a destructive tsunami. No action is suggested. |
Alerts change as data arrive. The NWS says warnings, advisories and watches may be updated or canceled as information becomes available. Advisories, watches and information statements may be upgraded if the threat turns out to be greater than first thought. The same three words appear in weather alerts, and our guide to watch vs warning vs advisory covers how the NWS uses them for storms and floods.
How do the warning centers decide which alert to issue?
The first message after an earthquake is based on preliminary earthquake data: location, depth and magnitude from seismic networks. According to the tsunami.gov FAQ, that is all the information available in the first few minutes, so the centers use preset criteria to decide whether to send a message and which alerts to include.
Speed depends on the seismic network. The FAQ says the centers can issue messages within five minutes where networks are dense, and within 10 to 15 minutes where they are sparse.
The FAQ gives the usual magnitude criteria. Tsunami warnings are typically issued after coastal earthquakes of magnitude 6.5 or greater for the US and Canadian Atlantic and Gulf coasts, and magnitude 7.1 or greater for coasts along the Pacific Ocean and Caribbean Sea. As forecasts and sea-level readings come in, a warning generally applies where the forecast or observed tsunami is more than 3.3 feet (1.0 meter) or the impact is unknown. An advisory applies where it is about 1 to 3.3 feet (0.3 to 1.0 meter).
Which warning center covers your coast?
| Center | Location | Areas served |
|---|---|---|
| National Tsunami Warning Center (NTWC) | Palmer, Alaska | The continental United States, Alaska and Canada |
| Pacific Tsunami Warning Center (PTWC) | Honolulu, Hawaii | The Hawaiian Islands, the US Pacific and Caribbean territories and the British Virgin Islands. PTWC is also the primary international forecast center for the Pacific and Caribbean. |
NOAA says both centers are staffed 24 hours a day, 7 days a week, and each one backs up the other. The Palmer center took its current name on October 1, 2013. If you live in a US territory in the Pacific or Caribbean, such as Guam, American Samoa, Puerto Rico or the US Virgin Islands, tsunami.gov says to rely only on the PTWC messages written for your region.
What is a tsunami threat message?
International messages work differently from US alerts. PTWC sends tsunami threat messages to officially designated national authorities in the Pacific Tsunami Warning System and the Caribbean early warning system, known as CARIBE EWS. A threat message goes out when a potential or confirmed tsunami is forecast to affect some or all coasts in those systems. The national authorities then decide on public alerts for their own countries. For the public, tsunami.gov’s guidance on a threat message is to seek more information and follow instructions from national and local authorities. The full list of message types is on the tsunami.gov message definitions page.
What should you do if you are at the coast?
The NWS says a natural warning can come before any official alert: strong or long shaking at the coast, a sudden rise or fall of the ocean, or a loud roar from the ocean. In that case, the NWS says a tsunami could arrive within minutes and advises not waiting for an official warning or for instructions from local officials. Its guidance, from the NWS page on what to do during a tsunami:
- In an earthquake, protect yourself: drop, cover and hold on.
- If you are on the beach or near the water and feel an earthquake, move quickly off the beach to high ground or inland as soon as you can do so safely.
- Follow instructions from local officials.
- Stay out of the tsunami hazard or evacuation zone until local officials say it is safe. The first wave may not be the last or the largest, and the danger may last for hours or days.
How do tsunami alerts reach you?
The NWS says tsunami warnings are broadcast on local radio and television, marine radio, Wireless Emergency Alerts, NOAA Weather Radio and NOAA websites such as Tsunami.gov. Local officials may also use outdoor sirens, text messages and telephone notifications. Our page of the latest NTWC and PTWC tsunami messages collects the official bulletins in one place, but it is not an alert service. The guide to how emergency alerts reach you explains phone settings and other channels.
Bottom line
A tsunami warning means move to high ground or inland as local officials direct. An advisory means stay out of the water and away from beaches, a watch means be ready, and an information statement usually means no threat. NTWC in Palmer covers the continental US, Alaska and Canada, and PTWC in Honolulu covers Hawaii and the US territories. At the coast, the NWS treats strong or long shaking as a warning in itself.
FAQ
Is a tsunami watch the same as a tsunami warning?
No. A watch means a tsunami is possible, usually after a distant earthquake, and the NWS advises staying tuned and being prepared. A warning means a tsunami that may cause widespread flooding is expected or occurring.
How long does a tsunami warning last?
It lasts until the warning center cancels or downgrades it. NWS guidance is to stay out of the hazard zone until local officials say it is safe, because the danger can last hours or days and the first wave may not be the largest.
Why was there only an information statement after a big earthquake?
Information statements report an earthquake or tsunami of interest. In most cases they mean there is no threat of a destructive tsunami. Many large earthquakes do not move the seafloor enough to create a dangerous tsunami; our guide to what causes a tsunami explains why.
Where can I see the latest earthquakes behind a tsunami message?
Tsunami messages name the preliminary magnitude and location of the earthquake. Our live list of USGS earthquakes worldwide shows the USGS data for the same event, which may be updated after the first message.