(CNN) - First, the good news: Heavy rain expected this week won't just drench Northern California's Camp Fire. It could also snuff out the risk of new wildfires for the rest of the year.
Now the bad news: Flash flooding, mudslides and rivers of debris could hit cities already devastated by the wildfire.
A flash flood watch is in effect Wednesday through Friday for the Camp Fire area, where 3 to 6 inches of rain are expected.
The rain will end "fire concerns for the winter," said Robert Baruffaldi, a meteorologist from the National Weather Service's Sacramento office.
But it could also make conditions treacherous for firefighters battling the Camp Fire, which has charred more than 152,250 acres and was only 75% contained as of Tuesday.
The inferno has already killed 81 people and destroyed 12,900 homes. And with hundreds of residents still unaccounted for, authorities fear the death toll could rise. Statewide, 84 people have died in the Camp Fire and the Woolsey Fire burning west of Los Angeles.
Rain could impede the search for victims
Authorities are scrambling to find 870 people who are unaccounted for in Butte County. That number keeps fluctuating as more residents are found.
But with heavy rain on the way, searchers feel a greater sense of urgency in finding the remains of Camp Fire victims.
"What we're looking at here is potentially remains, or cremains -- it's kind of a cremated remains state," said Brian Ferreira, rescue squad officer for California Task Force 4.
When the rain comes, "It's going to consolidate the material and make it more dense. 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."
Ferreira said it's possible some remains could be washed away. If that happens, "it's out of our hands at that point. We're going to go as hard as we can, as long as we can, until we can't go anymore. That's what's going to happen."
Butte County is keeping a list of the people who are unaccounted for because of the Camp Fire. Sheriff and Coroner Kory Honea urged those who see their own names or the names of loved ones found safe to contact the sheriff's office.
Officials lifted evacuation orders and warnings for several more areas Tuesday, but cautioned residents "to take steps to ensure they have food, water and fuel for their vehicles."
Evacuees could endure flooding
Across Butte County, residents were bracing 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.
The Red Cross and other aid groups have opened additional shelters for people and pets in the area, KCRA reported.
"We want to make sure those people who are staying in tents know that these spaces are available for them so they can get out of the elements," said Shawn Boyd with the California Governor's Office of Emergency Services.
Ash flows expected in Paradise
The first bout of heavy rain is expected to hit Butte County on Wednesday, bringing about an inch to Paradise -- a town virtually wiped out by the Camp Fire.
The National Weather Service's Baruffaldi said the rain could cause ash flows, or a slurry of ash swept away by rainwater.
A second rain system arriving Thursday night will likely bring 1.5 inches of rain to Paradise and up to 4 inches in places of higher elevation, Baruffaldi said. That means mudflows are a real threat.
Southern California is in danger, too
Rain also is expected in Southern California, where the Woolsey Fire is burning west of Los Angeles. As of Tuesday afternoon, the blaze had killed three people and torched more than 96,000 acres.
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 in Los Angeles said.
Mudslides are more likely in the Woolsey Fire area because fires in the southern part of the state tend to destroy vegetation roots, Baruffaldi said.
Officials in Ventura and Los Angeles counties were urging residents to prepare for the rain. Fire departments in both counties are making an emergency supply of sandbags available to the public at fire stations.
Wildfires lead to widespread health concerns
Even Californians who never saw the flames could be affected by the wildfires' health hazards.
Smoke from the Camp Fire hangs heavy in parts of the state, forcing schools to close and shutting down some public transportation.
The air quality is so bad that San Francisco, Stockton and Sacramento became the world's three "most polluted cities" on Friday, according to Berkeley Earth, a nonprofit that aggregates data from air-quality monitoring sites, worse than hot spots in China and India.
Experts say residents in those areas of California should minimize the amount of time they spend outside.
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