The fire has burned 27,245 acres and was 49 percent contained as of 11 p.m. Saturday. With higher humidity and lower temperatures, firefighters were able to make significant gains in the area above Trails End, officials said.
The blaze was burning in very steep and rugged terrain in the southern portion of the San Jacinto Wilderness along the Desert Divide and in the Apple Canyon and Bonita Vista areas.
The area hardest hit is Bonita Vista Road. Some residents there have lost everything, including their homes and vehicles.
Dicey weather conditions are expected to cause the fire to burn aggressively and generate a significant smoke column. Officials said these kinds of smoke columns are often capable of creating unstable weather conditions including thunderstorms, lightning, and erratic winds.
"We're still expecting a 20 to 40 percent chance of thunderstorms coming over the area," said Chris Gaulding of the U.S. Forest Service. "We are seeing some low-level cloud build up coming across the mountains right now."
Gaulding says lightning and wind could hamper efforts as rain can also bringing flooding and create slick and dangerous working conditions. Firefighter safety would become a major concern.
Thousands of people were under evacuation orders in the Idyllwild area and nearby communities Saturday. More than 4,100 homes are threatened by the fire, and an estimated 6,000 people have already left the area. Yellow tags mark which homes and businesses were evacuated.
More than 3,000 fire personnel are battling the blaze, which broke out Monday in Mountain Center near the 243 and 74 highways. Ground troops are getting help from 20 water-dropping helicopters and air tankers.
Firefighters are reportedly working to secure the southern flank of the fire with a combination of hand line construction, running hoselays, and water drops.
"The air power is still going out there. They're hitting hot spots in numerous locations in the interior of the fire, and they're also dropping water in support of the firefighters on the ground building a line," said Cal Fire spokesman Edwin Simpson.
Twenty-three structures have been destroyed, including seven homes. Fire officials said three mobile homes and three other residences were destroyed in Bonita Vista. In Pine Springs, one commercial building, a workshop, garage and cabin were destroyed. Eleven outbuildings and several vehicles also were destroyed.
The cause of the blaze remains under investigation but authorities say the wildfire was "human-caused." The cost of the Mountain Fire is so far estimated at $18.4 million.
Mandatory evacuation orders are in effect for residents in Idyllwild and Fern Valley. A voluntary evacuation order was issued for Pine Cove, where 700 homes are threatened.
Evacuations were lifted for the communities of Trails End and Camp Joe Scherman. Apple Canyon, Bonita Vista, Fobes Canyon, and Pine Springs Ranch areas were also open to residents.
Camp Ronald McDonald has been voluntarily evacuated due to health concerns related to air quality.
Meantime, the Riverside County Office of Emergency Services on Friday put out an advisory for residents in the Pine Springs Ranch and Zen Center telling them to boil their water due to contamination from the Mountain Fire. Officials say the problem should be resolved in the next two to three days.
Evacuation centers for residents and animals have been set up:
-Hemet High School, located at 41701 East Stetson Avenue in Hemet
-Hamilton High School, located at 57430 Mitchell Road in Anza
-Beaumont High School, located at 39139 Cherry Valley Boulevard in Beaumont
-San Jacinto Animal Shelter, located at 581 South Grand Avenue in San Jacinto
-Beaumont Civic Center (small animals), located at 550 E. 6th Street in Beaumont
Evacuated residents are encouraged to check in with the Red Cross in Anza at Hamilton High School so the Red Cross can help them with any needs.
A hard closure remains in place on Highway 243 from the intersection of State Highway 74 to the Idyllwild Nature Center, south of Pine Cove. Highway 74 has since opened to traffic.