SAN FRANCISCO (Reuters) - A California mother and father and their 16-year-old son were swept out to sea over the weekend after a deadly chain of events set off when the teenager jumped into frigid waters to save the family dog from turbulent Pacific Ocean waves.

The dog escaped on his own from the water off the Northern California coast. But Howard Kuljian, 50, and Mary Scott, 54, of Eureka died while their 16-year-old son, Gregory Kuljian, remained lost at sea, said Deputy Ariel Gruenthal of the Humboldt County Coroner's office.

"The dog was able to get out somehow," said Dana Jones, a state parks and recreation district superintendent. "It's very sad, and we just always have to be aware when we're around the ocean that nature is sometimes out of control."

The tragedy began on Saturday afternoon while the family, including an 18-year-old daughter who was unharmed, was walking with Gregory's girlfriend along a steep beach at Big Lagoon, about 270 miles north of San Francisco, Jones said.

Howard Kuljian threw the dog a stick, she said, and a wave, possibly as high as 10 feet, pulled the animal into the water. The son went in first to try to rescue his dog, Jones said.

"Then the father went in to save the son. The mother was swept in at that point," she said. "The waves are big and powerful, and that's a very steep beach. The waves pound the beach. When the waves are pounding like that, you don't have a chance to breathe."

A bystander summoned help while Olivia Kuljian, 18, and Gregory's girlfriend, Lily Loncar, 16, watched in horror, Gruenthal said.

Rescuers found the bodies of Howard Kuljian and Scott close to the shore, Jones said. The U.S. Coast Guard searched by air and sea for Gregory, but fog, darkness and the impossibility of survival prompted them to quit on Saturday evening, said Lieutenant Bernie Carrigan of the Coast Guard.

He estimated the water temperature at between 55 and 57 degrees, so cold that hypothermia would rapidly set in, though a dog's coat would protect against it.

"It's kind of a reminder to never turn your back on the ocean," Carrigan said. "It's neat to see that kind of power. It's also dangerous."