Cal Fire officials say the woman, who has not been publicly identified, was last seen inside a Forest Falls home that was buried in mud on Monday.
In all, about 30 homes were damaged or destroyed in the mud and debris flows.
"I've cleaned it up three times already this year," Oak Glen resident William Buster said of the mud. "Nothing this bad -- I thought it was bad but this is the granddaddy of them all."
"We're clearing debris, we're opening culverts, doing whatever we can to minimize the damage," Yucaipa Fire Department Chief Grant Malinoski said at a news conference. "But the takeaway from this is: The only thing that will heal that hillside is time."
This week's mud and debris flows stemmed from the burn scar left by the 23,000-acre El Dorado Fire that occurred in 2020. Since then, officials say they've prepared for the possibility of debris flows by consulting hydrologists and an emergency watershed response team.