(AP) — California will see widespread rain and heavy Sierra Nevada snowfall through midweek, potentially bringing travel problems and raising the risk of damaging runoff from wildfire burn scars, forecasters said Tuesday.
The wet pattern from a deep atmospheric fetch of Pacific moisture marks a significant change in the weather following conditions that contributed to disastrous and deadly wildfires up and down California, where hundreds of thousands of acres have burned this year.
"This is good news to help minimize that fire activity," Cal Fire spokesman Scott McLean said. "But remember that if you are in an area that has seen recent fires this year or latter part of last year it could mean trouble as that soil is much more prone to mudslides and debris flow."
UC Berkeley forestry specialist William Stewart says the threat of a debris flow will last for three or four years.
These flows contain a destructive mix of mud, trees, and boulders that can cause widespread damage, as witnessed last December in Montecito.
"Especially if we have a really big winter and later in the winter when the soil is just full of water, it just has a lot less strength to stay on the hillside compared to before it was burned," Stewart said.
The risk is higher for residents near burn scars in Southern California because of the population distribution.
Another concern is storm runoff containing heavy metals and toxic chemicals coming from the charred remnants of homes.
Stewart says the toxic sludge looks like simple mud, but it's harmful to wildlife living downstream from these burn scars.
He says most will flush out if this is a normal water year, but some will remain in the ecosystem for years.
The National Weather Service said there was a risk of heavy rainfall in northwest California through Tuesday night, then spreading farther south down virtually all of the coastal ranges and some interior sections of the state through Wednesday and Thursday.
Snow accumulations in the Sierra could range from 2-4 feet, the NWS said.
In the Sierra, chain controls were put into effect on Interstate 80 between Kingvale and the Donner Lake interchange, the California Department of Transportation said.
On the coast near Big Sur, Caltrans planned to close a section of Highway 1 between Mud Creek and Paul's Slide for 48 hours starting Wednesday morning because of potential instability.
The scenic route perched between towering mountainsides and the ocean has been dogged by slides since late 2016. But the one that hit Mud Creek near Ragged Point in May 2017 was monumental. Millions of tons of earth moved, displacing 75 acres of land and extensive work was required to rebuild the highway over the slide.
Caltrans also warned that chains will be required for travel through the mountains of San Bernardino and Riverside counties east of Los Angeles when the storm arrives there Wednesday evening. Flash flood watches were to go into effect Thursday morning in those area as well as parts of Orange County.
Forecasters also warned of very high surf along the coast.
Capital Public Radio's Randol White contributed to this report.