Kona lows, hurricanes, flash floods — what swimmers need to know
After any significant storm, stay out of the ocean for at least 72 hours and until the water is visibly clear. After major events (Kona lows, tropical storms), wait even longer. Brown or murky water means contamination is present regardless of whether an advisory has been posted.
A Kona low is a slow-moving subtropical low-pressure system that forms west or southwest of the Hawaiian islands. Unlike typical trade wind showers that affect windward (east) sides, Kona lows bring heavy rain to the normally dry leeward (west and south) sides of the islands.
This is particularly significant for beach water quality because leeward coasts — places like Wailea, Kaʻanapali, the Kohala Coast, and Poʻipū — typically have the cleanest water due to minimal rainfall. During a Kona low, these areas receive unusual amounts of rain, overwhelming drainage systems that aren’t designed for heavy flow.
Hawaiʻi’s hurricane season runs June 1 through November 30, with peak activity from July through September. Even storms that don’t make direct landfall can bring heavy rain and large surf that severely impact beach water quality.
Hawaiʻi’s steep terrain and narrow valleys create conditions where flash floods can develop rapidly, sometimes within an hour of heavy rain in the mountains. Flash floods are the most common weather-related hazard in Hawaiʻi.
Even when it’s sunny at the beach, heavy rain in the mountains above can send a wall of brown, debris-laden water down streams that empty near popular beaches. This is particularly common on Oʻahu’s windward side, Maui’s north shore, and Kauaʻi’s north and east coasts.
If you’re at the beach and the ocean water suddenly turns brown near a stream outlet, get out of the water immediately. A flash flood upstream is pushing contaminated runoff to the coast. This can happen even on an otherwise sunny day.
Active runoff, brown water plumes, potentially dangerous debris. Stay completely out of the water.
Water may appear clearer but bacteria levels can still be elevated. DOH testing may be in progress. Continue to avoid.
Open-ocean beaches with good flushing may be recovering. Enclosed bays and areas near streams may still be contaminated. Check DOH advisories and visual conditions.
Most beaches should be clearing. Verify water is visually clear and check that DOH advisories have been lifted. Some areas near major rivers may take longer.
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⚠️ Important Disclaimer
Safe to Swim Hawaii is an independent passion project — not affiliated with DOH or any government agency. Information is for educational purposes only. They are not real-time measurements.
Always verify conditions with the Hawaii DOH Clean Water Branch before entering the water.
When in doubt, don't go out. 🤙