SALEM, OREGON: One moment, Jayson Thomas was on the Oregon beach with his 3-year-old son. The next, they were gone, swept away by a "sneaker wave" as his wife looked on.

The man and his boy were but the latest to be lost to a sneaker wave, which are prevalent in the Pacific Northwest. A leading expert says there needs to be greater awareness to prevent future tragedies.

In fact, Tuba Ozkan-Haller of Oregon State University has just finished the first year of a three-year research project to devise a sneaker-wave early warning system, a project funded by the National Science Foundation. She hopes the warnings will be sent out by the National Weather Service.

A sign along the trail to the beach warns of sneaker waves and high surf, though it was not immediately clear if warning signs are posted where Thomas and his family were.

The seas off Cape Blanco were not particularly rough on Sunday afternoon when Thomas, his wife and their son, who lived near Eugene, were on the beach, Ozkan-Haller noted.

But appearances can be deceptive.

"People make up their minds about how safe an area is pretty quickly, after watching the beach for five minutes," Ozkan-Haller said Tuesday in a telephone interview from Corvallis. "That doesn't give you the information you need to assess that an area is safe."

While the weather might be fine, a storm far out to sea, even across the Pacific, often generates such a wave. As it moves through the broad surf zone and over the gentle slope approaching Oregon's coast, one wave can catch up with another, combining forces and allowing it to run up further on the beach, said Ozkan-Haller, who is with OSU's College of Earth, Ocean, and Atmospheric Sciences.

Six years ago, a sneaker wave knocked two high school students from Eugene off a rock near Yachats, sending them into the turbulent waters where they drowned.

Stormier winter weather produces more sneaker waves, and victims are often bundled up in winter clothing, which weighs them down when they're soaked, Ozkan-Haller said.

When the sneaker wave came on Sunday, the tide was high "so that means the dry beach was very narrow and there was not much space to run away," she said.

The Oregon State Police said Thomas' wife, Charity Woodrum, who is studying astrophysics at the University of Oregon, was an arm's distance away when her husband and her son were struck by the wave. Witnesses indicated the two were the only ones who were hit, Oregon State Police Capt. Bill Fugate said.

No one, even in a survival suit, would be expected to survive the high seas and cold water temperatures this long, Fugate said.

The US Coast Guard searched Sunday and Monday with two helicopters and a 47-foot motorised life boat while state police and other rescuers used ATVs. They found only Thomas' jacket and a child carrier he had been wearing.

The Coast Guard, state police and local sheriff's office will continue to search for the bodies, Fugate said.

Messages of prayer and condolences were filling Thomas' Facebook page on Tuesday, which featured a photo of him and his son on another beach.

One woman wrote: "as a person who has before lost a dear friend to the same waters, my heart breaks for this precious family."

Rick Warren, a host at Boice Cope Park not far from Cape Blanco, said: "The beaches here are awesome, but they can be dangerous ... You never turn your back on the ocean."

"They're called sneaker waves because people essentially don't watch them," he said. "People go to the beach, not paying attention, looking for agates and walking."

Ozkan-Haller recommends that when people go to the beach in Northern California, Oregon and Washington state, which because of the nature of the coastline are susceptible to sneaker waves, they study the wave action and ensure escape routes aren't blocked by rocks or cliffs.

"The more people learn and have a healthy respect for the ocean, the more that we can avoid these occurrences," Ozkan-Haller said.