While much-needed rain falling in parts of Northern California could help firefighters battling the Camp Fire, it could also prompt powerful mudslides and debris flow, adding to the devastation for communities that have already been ravaged by the flames.
Authorities fear the showers -- even as they clear the choking smoke and help extinguish flames -- will trigger debris flows and floods in areas scarred by the Camp Fire, complicating the search for victims' remains and presenting a danger to firefighters. Rain could also pile on misery for people forced from their homes, some of whom are living in tent camps outside.
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"It's pouring rain, which is great for the fire efforts, but making it a little mucky for our search and recovery teams," Sacramento Fire Capt. Dave Lauchner told CNN affiliate KTXL on Thursday.
"It makes the ground really unstable for firefighters," Lauchner explained. "But we just keep our eyes open, make sure we're in safe areas and watch out for each other."
Two weeks after the devastating fire started, 563 people are unaccounted for, Butte County Sheriff and Coroner Kory Honea said. Of the 83 killed in the Camp Fire, at least 58 of them have been tentatively identified as rescue crews sift through debris, searching for more remains.
1 million people are under a flash flood watch
Almost 1 million people in Northern California are under a flash flood watch that remains in effect through Friday morning, CNN meteorologist Haley Brink said.
Paradise recorded 1.5 inches of rain on Wednesday, according to Bill Rasch, a meteorologist for the National Weather Service in Sacramento. Rasch said the rain hit the "sweet spot" and helped firefighters without causing issues with debris flow.
But there's even more rain on the way.
Between Thursday and Saturday, the northern part of the state could see up to 2 inches of rain in valley areas and as much as 4 to 8 inches of rain at higher elevations, according to Idamis Del Valle, another National Weather Service meteorologist.
The upcoming weather system will be the first to have brought "significant precipitation" to the area in some time, Del Valle said, and could help firefighters battling the blaze. But it presents a double-edged sword.
"The area has been dry for a while, so it's great to have beneficial rain," she said. "Although if it comes in high intensive rates then that could lead to debris flows on new burn scars."
"They're basically fast-moving, deadly landslides," Del Valle said of debris flows. They're made up of loose mud, soil and rock, and can occur over the burn scars left by wildfires up to a year old.
Included in the areas at risk for flash flooding are the locations of some of California's other recent fires, Brink said, like the Carr and Mendocino Complex fires, each of which burned hundreds of thousands of acres this summer.
Search for missing to be 'much more difficult'
The first bout of heavy rain is expected to hit Butte County. The heaviest amounts will come Thursday afternoon through Friday afternoon, with between 2.5 and 4 inches expected during that 24-hour period.
When the rain comes, it's going to have major effects on the debris.
Because of the scorched earth, rain that would typically have been absorbed will run off, the National Weather Service said.
In Paradise, most of which was destroyed by the fires, the storms could cause ash flows, or a slurry of ash swept away by rainwater.
"It's going to consolidate the material and make it more dense," said Brian Ferreira, rescue squad officer for California Task Force 4. "And it's going to present much more like soil. So anything we find or hope to find that's still there, it's going to make a difficult task ... that much more difficult."
Honea told KTXL his office would continue to lead the search for the missing as long as conditions weren't too dangerous for search and rescue crews.
Many trees in the area have been damaged in the fires, Honea said, and "there is some concern that as the rain comes in and the ground becomes saturated, there's the possibility that any wind might push those over."
The Camp Fire has charred more than 153,300 acres and was 90% contained Thursday morning. It has destroyed more than 13,600 homes.
Evacuees flee, some brace for flooding
Across Butte County, residents prepared for a possible deluge.
Jennifer Debrunner is staying at the Butte County Fairgrounds in a motor home loaned by a stranger. She told CNN affiliate KCRA that her family covered everything they own with a tarp.
Debrunner said she knows the rain will bring "a lot of mud, a lot of cold" to the area. But this Thanksgiving week, Debrunner said she's grateful her family has a borrowed RV.
Cady Machado has been camping out in a Walmart parking lot with her husband and 9-month-old baby.
Due to the expected rain, Machado told CNN affiliate KTXL, she's sending her child to her sister's in Arizona.
As for herself and her husband?
"There's a nice bridge with my name on it to go underneath where I won't get flooded out with my husband," she told KTXL.
Woolsey Fire now contained
Rain also is expected in Southern California, where the Woolsey Fire has killed three people and torched nearly 100,000 acres. It's now 100% contained.
The Woolsey burn area is expected to get less than an inch of rain. But it's still at risk of mudslides and minor debris flow from Wednesday to Thursday, the National Weather Service said.