Steven Dubois / The Associated Press

PORTLAND — After days of waiting on standby, an Oregon National Guard helicopter has joined the search for a climber missing on Mount Hood.

The helicopter, which has night vision capability, lifted off late Thursday afternoon as clouds finally gave way to sun. The weather forecast calls for clear skies today, and the Clackamas County Sheriff’s Office says the search for Kinley Adams will continue through the weekend.

Rescuers believe the 59-year-old Salem dentist is on the upper part of the mountain, but they had been unable to search that area this week because of poor visibility, storms and avalanche danger. The improving weather also allowed ground crews to start checking higher elevations, though it’s still too dangerous to go above 10,000 feet.

“Our challenge continues to be super-unstable snow conditions on the upper mountain,” said Mark Morford of Portland Mountain Rescue, one of the organizations helping with the search. “The guys we’ve had up high at all in the last two days have observed numerous slides. With the warming weather and all the precipitation this week ,that creates a very dangerous situation.”

If the aerial search pinpoints Adams’ location, the helicopter can hoist Adams up or drop searchers in the area to provide aid.

“If we know we’ve got a live subject and we know exactly where he is, we’ll figure out some way to get there, whatever the conditions,” Morford said.

Adams was reported missing Saturday night, hours after he was due to return from a climb on the west side of Oregon’s tallest mountain.

He had been making frequent trips to Mount Hood in preparation for a climbing trip to Nepal. He is believed to have a cellphone, but searchers have been unable to pinpoint the signal. His mountain locator beacon was found at home, apparently with gear he was planning to take to Nepal, said Sgt. Robert Wurpes, a sheriff’s spokesman.

Adams has scaled numerous peaks since the 1970s. His experience and general preparedness give family and search crews hope that he is alive in a snow cave, awaiting help. In March, an injured college student, Mary Owen, survived six days on Mount Hood before her rescue.

“By all accounts from fellow climbers, he’s very meticulous, determined,” Wurpes said. “If a guy can survive for a week up there, then he is that guy.”

Mount Hood, 50 miles east of Portland, has seen dozens of accidents and fatalities over the years. Thousands climb the 11,239-foot peak each year, mostly in the spring.