An early-season storm lashed Northern California with drenching rain on Monday and was moving through the evening toward Southern California, where officials in Los Angeles County issued evacuation warnings in some areas. Thunderstorms could unleash heavy rain that sends torrents of water, mud, sand, rocks, trees and boulders down steep slopes in places recently burned by wildfires, forecasters warned.

The National Weather Service’s office in Oxnard, Calif., said that debris flows were possible in burn scars across San Luis Obispo, Santa Barbara, Ventura and Los Angeles Counties; areas burned by the Palisades and Eaton fires in January and by the Bridge fire last year were particularly at risk.

Evacuation warnings mean that hazardous conditions are possible and that people should keep an eye out for evacuation orders calling for immediate action.

Lisa Phillips, a meteorologist with the Weather Service’s Oxnard office, said that it was unlikely that widespread debris flows would occur in all of the burn scars, but that it was likely that at least one area would get hit with a heavy downpour that creates problems.

“We’re confident that something will happen somewhere, but where that happens, that’s where the confidence is lower,” Ms. Phillips said.

The National Weather Service called the large storm system that’s bringing a chance of rain up and down the state “rare for October.” Rainfall totals across California with this multiday storm could range from a few tenths of an inch to three inches of rain.

Rain started in Southwest Oregon and far Northern California early Monday morning and continued through the day. As of about 5 p.m. local time on Monday, downtown San Francisco had recorded 0.68 inch of rain. Northern California will dry out through the day on Tuesday.

Rich Otto, a meteorologist with the Weather Prediction Center, said the storm was pulling in moisture from the Pacific Ocean and funneling it into California.

“It’s not the highest degree of moisture, but it’s still atypical compared to average for this time of year,” Mr. Otto said. “There’s potential for locally high rainfall rates.”

Snow was falling in the Sierra Nevada on Monday and will most likely continue into Tuesday and early Wednesday.

“This storm is expected to bring the first measurable snow of the season,” said Kate Forrest, a meteorologist with the Weather Service in Sacramento.

A winter weather advisory was issued through Wednesday for the Lake Tahoe Basin, where the highest peaks were predicted to pick up two feet of snow. The heaviest snow in the Sierra will most likely fall Monday night into Tuesday morning.

As of 5 p.m. local time on Monday, the rain had yet to reach Los Angeles County, but Ms. Phillips said the rains to the north were quickly moving south.

“It’s raining in San Luis Obispo County right now,” she said. “We’re expecting it to start pushing further south into the region tonight and into tomorrow morning.”

The chance for rain will continue into Tuesday across Central and Southern California.

“As we go through the day Tuesday, the rain will move eastward into Nevada,” Mr. Otto said.