CAPTAIN COOK, Hawaii — At the edge of an ancient lava flow where jagged black rocks meet the Pacific, small off-the-grid homes overlook the calm blue waters of Papa Bay on Hawaii’s Big Island — no tourists or hotels in sight. Here, one of the islands’ most abundant and vibrant coral reefs thrives just below the surface.

Yet even this remote shoreline far from the impacts of chemical sunscreen, trampling feet and industrial wastewater is showing early signs of what’s expected to be a catastrophic season for coral in Hawaii.

Just four years after a major marine heat wave killed nearly half of this coastline’s coral, federal researchers are predicting another round of hot water will cause some of the worst coral bleaching the region has ever seen.

“In 2015, we hit temperatures that we’ve never recorded ever in Hawaii,” said Jamison Gove, an oceanographer with the National Oceanic and Atmospheric Administration. “What is really important — or alarming, probably more appropriately — about this event is that we’ve been tracking above where we were at this time in 2015.”

Researchers using high-tech equipment to monitor Hawaii’s reefs are seeing early signs of bleaching in Papa Bay and elsewhere caused by a marine heat wave that has sent temperatures soaring to record highs for months. June, July and parts of August all experienced the hottest ocean temperatures ever recorded around the Hawaiian Islands. So far in September, oceanic temperatures are below only those seen in 2015.

Forecasters expect high temperatures in the north Pacific will continue to pump heat into Hawaii’s waters well into October. “Temperatures have been warm for quite a long time,” Gove said. “It’s not just how hot it is — it’s how long those ocean temperatures stay warm.”

Ocean temperatures are not uniformly warm across the state, Gove noted. Local wind patterns, currents and even features on land can create hot spots in the water.

“You have things like two giant volcanoes on the Big Island blocking the predominant trade winds,” making the island’s west coast, where Papa Bay sits, one of the hottest parts of the state, Gove said. He said he expects “severe” coral bleaching in those places.

“This is widespread, 100% bleaching of most corals,” Gove said. And many of those corals are still recovering from the 2015 bleaching event, meaning they are more susceptible to thermal stress.