Areas near the mountains, in particular, have no trees to stop rainwater due to the blaze, and the scorched earth will not be able to absorb even the moderate amount of rain expected.
The pending rain also brings the problem of mudslides and boulders on local roads as well as ash and debris being carried into streams.
“Cal Fire and weather service officials said the soil just below the surface gets packed by intense wildfire heat and can become impermeable to water, causing an almost instant wash off of debris, ash and loose dirt during extremely heavy rainstorms,” the Sacramento Bee reported.
Those in the vicinity of the Camp Fire in Butte County, the Delta and Hirtz fires near Shasta City and Redding, California, and residents near the Mendocino Complex fire around Clear Lake, have all been warned of the flash floods.
Most people in the area evacuated during the height of the blazes but returned to survey the damage.
Several residents have been staying in caravans or tents in vacant parking lots as they try to recover possessions and rebuild their lives.
Due to the possible flooding though, more indoor shelters have been opened in the region.
The storm is expected to begin midweek and last through part of the weekend, with another storm hitting next week.
Search and rescue crews may also be hampered by the rain.
They are surveying a massive area. “Here we’re looking for very small parts and pieces, and so we have to be very diligent and systematic,” said Joe Moses, a commander with the Monterey County Sheriff’s Office told the local CBS News station.
Sheriff Kory Honea expressed doubt the searches would be completed before the rain began.
There is some positive news from the rain - firefighters and public health officials say it will help to control or stop the blazes and clear up the unhealthy amounts of smoke and debris that have been in the air for the past 10 days.
It will also put a halt to "fire concerns for the winter," meteorologist in the National Weather Service's Sacramento office Robert Baruffaldi told CNN.
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