Box jellyfish warning dates for every month. South shore Oʻahu beaches are most affected 8–12 days after each full moon.
Warning windows are days 8–12 after each full moon. 2026 has 13 full moons (including a blue moon on May 31), so there are 13 warning windows. These dates are approximate and most reliable for south-facing Oʻahu beaches.
| Month | Full Moon | Warning Window | Status |
|---|---|---|---|
| January | Jan 3 | Jan 11 – 15 | - |
| February | Feb 1 | Feb 9 – 13 | - |
| March | Mar 3 | Mar 11 – 15 | - |
| April | Apr 1 | Apr 9 – 13 | - |
| May | May 1 | May 9 – 13 | - |
| May/June | May 31 | Jun 8 – 12 | - |
| July | Jun 29 | Jul 7 – 11 | - |
| August | Jul 29 | Aug 6 – 10 | - |
| September | Aug 28 | Sep 5 – 9 | - |
| October | Sep 26 | Oct 4 – 8 | - |
| November | Oct 26 | Nov 3 – 7 | - |
| December | Nov 24 | Dec 2 – 6 | - |
| Dec/Jan | Dec 23 | Dec 31 – Jan 4 | - |
Hawaiian box jellyfish (Alatina alata) use lunar and tidal cycles to time their reproductive behavior. About 8 to 12 days after each full moon, changing tidal patterns push the jellyfish from deep water toward shore, where they release eggs and sperm in the shallows.
This pattern has been documented by the Waikīkī Aquarium and University of Hawaiʻi researchers for decades. It is most consistent on Oʻahu's south-facing shores, where the reef structure and current patterns concentrate the jellyfish near popular swimming beaches.
The cycle is predictable enough that lifeguards post purple warning flags when jellyfish are expected. However, jellyfish can occasionally appear outside the predicted window, especially if strong currents or onshore winds push them in earlier.
South-facing beaches on Oʻahu see the most consistent jellyfish arrivals during the warning window:
North shore beaches and neighbor islands can also see box jellyfish, but the timing is less predictable. Always check for purple warning flags at lifeguarded beaches.
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When box jellyfish are present on south-facing beaches, these ocean activities keep you safe and entertained.
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⚠️ Important Disclaimer
Safe to Swim Hawaii is an independent passion project — it is not affiliated with the Hawaii Department of Health or any government agency. Jellyfish warning dates are estimates based on the known lunar cycle pattern and may not exactly predict jellyfish presence at any specific beach. “No DOH Alerts” means no advisory is currently posted — it does not mean the water was tested and found safe. DOH only monitors a fraction of Hawaii’s beaches, and some areas have no regular testing at all.
Always verify current conditions with the Hawaii Department of Health Clean Water Branch and check for purple warning flags at lifeguarded beaches before entering the water.
This site does not recommend or advise anyone to swim at any beach. We share government data and geographic analysis so you can make your own informed decisions. By using this site you accept full responsibility for your own safety. See our Terms of Use for full details.
When in doubt, don’t go out. 🤙
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