What is a tsunami advisory and how is it different from a tsunami warning?

A tsunami triggered by a submarine blowout in Tonga is expected to affect areas of the west coast of North America on Saturday, with people urged to stay away from beaches, marinas and ports.

The National Weather Service (NWS) has issued a tsunami advisory for coastal areas of California, Oregon, Washington, Hawaii, British Columbia and Alaska.

An advisory is the second most severe tsunami warning issued by the NWS. A tsunami warning is the most severe warning of its kind, according to the US National Oceanic and Atmospheric Administration (NOAA) Tsunami Warning System.

An advisory is issued when a tsunami “with the potential to generate strong currents or waves dangerous to those in or very near the water is imminent, forecast or occurring”.

The threat of this tsunami “could continue for several hours after initial arrival, but significant flooding is not expected for areas under advisory”.

Local officials are expected to take action in affected areas, including closing beaches, evacuating ports and marinas and “repositioning vessels in deep waters when it is safe to do so”.

The US tsunami warning system says people in areas covered by an advisory should stay out of the water and away from beaches and waterways.

The NWS may also update an advisory, cancel it, or turn it into a warning depending on the situation. A warning is more serious than a tsunami watch, which can also be issued.

Tsunami warnings

A tsunami warning is issued when a tsunami has the “potential to generate widespread flooding” and is occurring or imminent.

“Warnings alert the public that dangerous coastal flooding accompanied by strong currents is possible and may continue for several hours after initial arrival,” the NWS says.

“Warnings alert emergency management officials to take action for the entire tsunami hazard area. Appropriate actions for local officials to take may include evacuation of low-lying coastal areas and repositioning vessels in deep waters when it is time to do so safely. then.”

In the event of a tsunami warning, the inhabitants of the affected areas must move “to the heights or inland”.

Tsunami warnings were issued by a number of countries on Friday, starting with the peaceful nation of Tonga, where the submarine volcano Hunga Tonga Hunga Ha’apai erupted, causing tsunami waves.

Warnings have now been issued in Australia, New Zealand, Fiji and Vanuatu. An earlier warning for American Samoa has been rescinded.

An NWS advisory issued at 6:32 a.m. PT said: “Areas in the advisory should not expect widespread flooding.

“Tsunamis are a series of dangerous waves several hours after the initial arrival time. The first wave may not be the greatest.”

An image from the Tonga Meteorological Service shows the eruption of the underwater volcano which caused tsunami waves. A tsunami advisory has been issued for some coastal areas in western North America.
Tonga Meteorological Services/Government of Tonga

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