Kona Manta Night Snorkel: Is the Sea Calm?

How the call is made

The rules, published

No score and no hidden weighting. Each site gets one of three steps for the 6 to 9 p.m. window, from the worst signal available.

1. An active High Surf Advisory or Warning, or a Flash Flood Warning, for Kona is a rough night. The National Weather Service decides this, not the tool.
2. The NWS Surf Zone Forecast for west-facing leeward shores gives feet for the day. A high end of 4 feet or more is moderate and 7 feet or more is large.
3. The wave model (PacIOOS SWAN) peak height from 6 to 9 p.m. is compared with every evening at that site from 2016 to 2025. Rougher than 70 percent of them is moderate and rougher than 90 percent is large, but only if the wave is also at least 0.4 m (moderate) or 0.8 m (large). Garden Eel Cove is so sheltered that its roughest evenings are still small, and the floors stop a small sea being called rough.
4. Wind, rain and runoff. Wind blowing onto the shore at 15 mph or more raises the step, and 25 mph or more makes it rough. Rain in the evening window at 0.5 mm or more raises it, and 4 mm or more makes it rough. More than 10 mm in the 24 hours before raises it, because runoff can cloud the water.
5. Long-period swell of 14 seconds or more nudges a calm night to moderate, because surge outruns the wave height.
6. If the forecast and the model are two steps apart the answer is CHECK, not a guess.

Honest limit: the cutoffs are starting rules, not validated against real cancellations, and confidence never rises above MODERATE. The wave model reads one nearby water cell per site and is a weak judge on its own: checked against ten years of NWS high-surf advisories it separated advisory days only modestly on this coast. Tides, currents and water clarity are not in the call, and a snorkeling guide notes that moderate currents can occur even when the sea looks fine.