Kurt Ruppert of Lake City, Florida, disappeared Thursday after he jumped from out of a helicopter at 6,500 feet while wearing a special jumpsuit known as a "wingsuit," which allows a person to soar and glide before deploying a parachute.
"The only areas left to search are areas (authorities are) not able to search on the ground. They're steep cliffs and ravines, and when the weather clears, we'll hopefully get our helicopter back up there to look," Sgt. Cindy West, a spokeswoman for the King County Sheriff's Office, said Monday.
It is unlikely Ruppert is still alive given the low temperatures on Mount Si where rescuers have focused their search, authorities said.
"Initially we thought he was probably flying 50 to 60 miles per hour," West said, "but after talking to his jump friends, we found more likely he was traveling 80 to 100 miles per hour, which ... in just a matter of a few seconds he's going to be over a large area. And then the other issue is we don't know exactly what direction he went."
In a written statement issued Sunday, West said: "There are cliff areas that could not be searched due to the extreme risk to searchers. The Sheriff's Office said they will search those areas by helicopter when weather permits."
CNN affiliate KOMO reported there was too much fog on Monday for air crews.
Searchers covered 9 square miles looking for Ruppert, West said.