Hawaiʻi: tsunamis, hurricanes, flash floods, vog and wildfire

A distant tsunami starts with an earthquake somewhere else around the Pacific. Honolulu's Department of Emergency Management (DEM) gives travel times of about 5 hours from an Alaska earthquake and up to 14 hours from a Chile earthquake. That is enough time for official warnings, sirens and an orderly evacuation. A local tsunami is different. Honolulu DEM says waves from a local tsunami can reach Oʻahu in as little as 30 minutes, and that the most likely source is an earthquake on or near Hawaiʻi Island.

On the coast closest to the source the warning time can be close to nothing. A USGS account of the November 29, 1975 Kalapana earthquake says the tsunami reached Punaluʻu 84 seconds after a seismic sensor there recorded strong shaking. Waves reached about 20 feet at Halape, where campers were caught between rockfall and the sea. Two people died and 28 were injured. USGS adds that large earthquakes on the island's south flank happen on average more than once a century, and that because far more people now live there, a repeat would probably cause far more casualties.

⚠ If you feel strong shaking near the coast, protect yourself until it stops. Then go inland or uphill right away, on foot if you need to. Do not wait for a siren, a phone alert or an official order. The state's evacuation maps do not account for a locally generated tsunami, so go farther than the mapped zone if you can.

Honolulu DEM says more than 220 people have died from tsunamis in Hawaiʻi since 1946, 6 of them on Oʻahu, and that past tsunami flooding in Hawaiʻi has been more than 30 feet deep.

Every county publishes tsunami evacuation zone maps, and the state links to all of them from its Hawaiʻi Emergency Management Agency (HIEMA) page. On Oʻahu, enter your address in the Oahu Hazard Explorer. Signs on roadsides and in parks also mark where zones begin and end. Look up your home, your work, your children's schools and anywhere you regularly go to the beach.

Honolulu DEM uses two zones. The red Tsunami Evacuation Zone is used for most tsunami warnings. The yellow Extreme Tsunami Evacuation Zone is only for a rare, very large event, such as a magnitude 9 or larger earthquake in the eastern Aleutians, which could flood much more land. Leave the extreme zone only if an Extreme Tsunami Warning is issued. In an ordinary warning, people in the yellow zone should stay put and keep the roads clear.

HIEMA describes the zones as the minimum safe evacuation distance, not a guarantee of safety. The maps are based on about 100 years of past tsunami impacts (Honolulu DEM).

⚠ A tsunami is a series of waves, roughly 10 minutes to an hour apart, and they can keep coming for hours. The first wave may not be the biggest. Do not go back to the coast until officials cancel the warning. Honolulu DEM notes that the cancellation and the all-clear to return are two separate messages.

HIEMA says Hawaiʻi has the largest single integrated outdoor warning siren system in the world. The sirens are tested statewide together with the Emergency Alert System on the first business day of every month at about 11:45 a.m. If you hear a siren at any other time, treat it as real. Check radio, TV or your phone for official instructions.

According to NWS Honolulu, sirens sound for tsunami warnings and evacuations but not for a tsunami watch. Because of that, a phone alert or the radio may be your only notice that a watch is in effect.

Visitors should sign up for the alerts on whichever island they are staying, because the alerts are run by each county.

HIEMA's "2 Weeks Ready" guidance is built on the fact that Hawaiʻi gets most of its food and supplies by ship. A hurricane or tsunami that damages the ports or the supply chain could leave the islands without outside supplies for some time. HIEMA, Honolulu DEM and NOAA's Central Pacific Hurricane Center all recommend keeping 14 days of supplies, which is longer than the few days often suggested on the mainland. For water, HIEMA's figure works out to 14 gallons per person (one gallon a day for two weeks).

Honolulu DEM also recommends one go bag for each person in the household, and says to take your 14-day kit with you if you go to a hurricane refuge area. If roads, power or phones fail, choose a contact person who lives off the island. Local phone networks can fail along with everything else.

The Central Pacific hurricane season runs from June 1 to November 30. The Central Pacific Hurricane Center in Honolulu has warned that it only takes one tropical cyclone making landfall to have a major impact on the state. Honolulu DEM says a Hurricane Watch means a storm is expected within 48 hours, and a Hurricane Warning means one is expected within 36 hours. Different islands can be under different alerts at once.

If you live in a single-wall wood-frame house built before 1995 that has not been retrofitted, plan to evacuate. Your best option is the home of friends or family that meets the criteria above. Hurricane refuge areas are a last resort. Honolulu DEM says they are not built to state evacuation-shelter standards, provide no supplies and may not open for every storm, so check which ones are open before you leave. Storm surge is the leading cause of hurricane deaths. Honolulu DEM says even a Category 1 storm could leave thousands of people on Oʻahu without homes.

Flash floods are a hazard all year. Honolulu DEM says Hawaiʻi averages about 11 flood events a year, mostly in the October to May wet season, and that floods killed more than 140 people on Oʻahu between 1915 and 2019. Streams can rise within minutes when heavy rain falls upstream, even if it is not raining where you are. Most flood deaths happen when people drive or walk through moving water.

⚠ Do not cross a stream, ford or flooded road while it is rising or running fast, whether you are on foot or in a car. Wait until the flooding has completely gone down. If you are on a hike and the water starts rising or turning brown, get to high ground and stay there.

Kīlauea gives off sulfur dioxide gas. USGS explains that this gas reacts in the air over hours to days and forms the fine particles that make up the haze called vog (volcanic smog). The Hawaiʻi Department of Health (DOH) says the current Kīlauea eruption began on December 23, 2024 and has gone through repeated episodes of lava fountaining. Since then, air monitors on Hawaiʻi Island, Maui and Oʻahu have recorded periodic rises in sulfur dioxide and fine particles. Most readings were good or moderate, but stations in the southern part of Hawaiʻi Island have at times recorded unhealthy air.

If ash, Pele's hair (thin strands of volcanic glass) or Pele's tears (small drops of volcanic glass) fall where you live, DOH recommends wearing a well-fitting N95 mask, heavy gloves, long sleeves and pants, shoes and eye protection while you clean up. Do not sweep or use a leaf blower, because that puts the ash back into the air.

USGS says lava has covered more than 90 percent of Kīlauea's surface and about 40 percent of Mauna Loa's in the past 1,000 years. Lava flows downhill from the summit or the rift zones and buries whatever is in its path. Where lava reaches the sea it can build new land called a lava delta. These deltas can collapse without warning and throw hot rock hundreds of meters. USGS has published lava flow hazard zone maps based on past flows. Check your zone before you buy or rent on Hawaiʻi Island, and follow the Hawaiian Volcano Observatory's alert level.

The Lahaina fire of August 8 and 9, 2023 killed 101 people, according to the Phase One report the Fire Safety Research Institute (FSRI) prepared for the Hawaiʻi Attorney General. The Attorney General called it the deadliest fire in North America in 100 years. FSRI's Phase Two analysis made 84 findings and 140 recommendations. It concluded that no single organization, person, action or event caused the devastation. Instead, it found a complex mix of factors: the weather and its effects, how prepared people and agencies were for the risk, decades-old infrastructure, how organizations were structured, and how the incident was managed and coordinated.

Phase Three of the investigation sets out longer-term priorities for the state, including staffing an Office of the State Fire Marshal and having each county fire department complete a community risk assessment.

Honolulu DEM says 99 percent of wildfires in Hawaiʻi are started by people, mostly by accident. It recommends keeping at least 30 feet of defensible space around your home. A Fire Weather Watch usually comes 12 to 48 hours ahead of dangerous fire conditions. A Red Flag Warning means conditions for fire spread are happening now or will be very soon.

⚠ Many neighborhoods in Hawaiʻi have only one or two roads out. If you live in one, leave early. Do not wait for an evacuation order, or for the fire to come into view. Learn your county's evacuation zone or level system ahead of time (Maui County uses Genasys zones, and Honolulu uses three levels), and keep your car fueled during Red Flag days.