A storm system moving toward Northern California's burn areas could dump as much rain this weekend as the area usually receives during the entire month of April, the National Weather Service said Thursday.
The incoming "atmospheric river," as the plumes of water vapor are called, is loaded with tropical moisture that is expected to bring between 3 and 6 inches of rain to coastal mountain ranges, up to 3 inches in the Bay Area and up to 4 inches in Napa and Sonoma counties' wine country, meteorologist Roger Gass said.
The storm arrived Thursday and will peak Friday night into Saturday afternoon and is expected to cause local flooding across much of Northern California until it wrings itself out as it moves south and inland across the Sacramento and San Joaquin valleys, Gass said.
Officials in Santa Rosa, where a deadly wind-driven fire decimated entire neighborhoods in October, told residents to have evacuation plans ready and to monitor local news reports in case there are debris flows.
Elsewhere, the state Department of Water Resources warned residents along the Feather River in Butte County and beyond that the storm could force engineers to open the dam spillway gates at the Lake Oroville reservoir to release water.
The spillway has been out of use for repairs since last year, when water releases triggered by heavy storms partially destroyed the structure.
This weekend's storm is expected to be so big and warm that its rain will likely accelerate snowmelt along the Sierra Nevada, sending runoff down into the reservoir about a day later. The state has increased releases from the reservoir's power plant this week to make room for the anticipated surge of water.
By the time the storm reaches Southern California, Gass said, its strength should be dramatically reduced. Forecasters say the storm could drop up to a third of an inch of rain on Santa Barbara County on Saturday and even less when it reaches Los Angeles.