Search and rescue teams from Los Angeles, Ventura, Riverside, Orange and San Diego counties are all joining in the search.
Friends say she has been hiking on Mt. Baldy every week to train for an upcoming endurance trek in South America.
She went for a day hike on Saturday and did not return home. Yu's car, a Toyota Prius, was found on Mt. Baldy and Falls roads on Saturday. Her cellular phone was found in the car.
The search for Yu began Sunday after her roommate reported her missing.
"She really knows what to do. She's hiked up Mt. Whitney and all of these major mountains. That's her life," said John Gibson, Yu's roommate.
Yu is part of a Korean hiking group that often climbs Mt. Baldy. Members of that group came out to the mountain to help with the search efforts.
"She is a very smart and experienced person. I think and I hope that she's a survivor," said John Choo, Yu's friend.
One of the volunteers ended up having to be rescued after breaking an ankle, proof of how dangerous the terrain can be.
Authorities say they fear that Yu may have fallen and injured herself.
Tuesday there were more than 50 people scouring the steep, rocky terrain on Mt. Baldy hoping to find any sign that 49-year old Michelle Yu is still alive. Two helicopters were part of the search. Four search dogs took part too.
Also on the mountain were members of the missing hiker's family. Yu's roommate, John Gibson, has been there since Sunday when he first reported her missing.
"I was very optimistic at first, and then, you still have to be optimistic, but it's tough," said Gibson.
Gibson says Yu is an experienced hiker.
Still, it's a long time for someone to be stuck on the mountain in these conditions.
"Saturday night, Sunday night, Monday night, it's three nights out here already. That concerns me," said Gibson.
The other concern is the terrain. The thought is that Yu started her hike north of Baldy Village, heading northwest toward the summit. It's believed she was then going to turn east and hike along a ridge called the "Devil's Backbone." It's an area where the trail is only two feet wide in some spots, with an incredible steep drop down the north face.
"It's very treacherous, it's very steep," said San Bernardino County Sheriff's spokesperson Jodi Miller. "There are some points where it's basically up and down."
That's the area search-and-rescue teams were focused on Tuesday. The Riverside County Sheriff's helicopter shuttled dozens of rescue crews to the summit.
Meantime, all family members and friends can do is wait and hope.
A search helicopter was flying until about midnight Monday and was expected to be out late Tuesday night continuing the search.
The search area covers 20 square miles.