Reading Hawaiʻi's Ocean Conditions

Swell has a season and a direction

Every island has beaches facing north, south, east and west, and each one reacts differently to the same ocean. Once you know which way a beach faces, you can usually guess whether it will be calm.

Hawaiʻi's surf comes from storms far away. Between about October and April, deep winter storms near Alaska and the Aleutians push waves thousands of miles toward the islands, and they land with the most force on north- and west-facing sand. From roughly May to September, the swell source shifts to winter storms south of the equator, so it is the south-facing beaches that see surf. On top of that, the northeast trades that dominate Hawaiʻi's weather roughen up the windward side on most afternoons.

Where locals check before they go

Clues you can see from the sand

• Count a few sets. Waves arrive in groups. Wait through at least two or three sets before judging the size.
• Listen. A heavy, booming sound from waves hitting the sand usually means dangerous shorebreak.
• Look at the sand line. A steep drop right at the water's edge means waves will break hard and close to shore.
• Watch for gaps. A calm-looking channel between breaking waves can be a rip current carrying water out.
• Check the rocks. If waves are washing over ledges and tide pools, stay off them.
• See what the lifeguard flags say, and watch where experienced locals are, and are not, swimming.

Rain, jellyfish and sirens

After heavy rain

Big rainstorms flush mud, debris and bacteria down streams and storm drains and into the shallows, and the water near a stream mouth can stay unhealthy well after the sky clears. If the ocean looks brown or murky, skip it; when health officials have posted an advisory, recent reports quoted guidance to give it two to three days after the rain stops. Rain also raises the risk of leptospirosis in rivers, streams and ponds, so keep children and anyone with open cuts out of fresh water after storms.

Jellyfish

Box jellyfish have a habit of drifting into south- and west-facing shallows roughly a week and a half after each full moon. When they show up, lifeguards put up warning signs, and the Waikīkī Aquarium posts a forecast calendar of likely days. On the windward side, onshore breezes sometimes push blue Portuguese man-of-war onto the sand; even stranded ones can sting. If you are stung, leave the water and ask a lifeguard for help.

Tsunami sirens

On the first business day of every month, just before noon (11:45 a.m.), you will hear the statewide sirens sound for around a minute as a routine test. Sirens on any other day are not a test: get off the beach, head inland or uphill, and follow instructions from the Hawaii Emergency Management Agency and local news.