"We have very little wind, we have an increase in relative humidity, and so it's favorable for us to get out and to put out hot spots and work on line construction," Los Angeles County Fire Chief Greg Hisel said. He said calmer winds were helping to keep the fire stationary.
At least 18 dwellings were destroyed over the weekend, and authorities have reported one fatality, an unidentified man found on Saturday evening in a burned-out car parked in the driveway of a home.
Among the properties to go up in flames was the landmark Sable Ranch, a popular location for television and movie shoots.
By Monday morning, evacuation orders had been expanded to about 10,000 homes, encompassing an estimated 20,000 to 30,000 people, fire officials said. Los Angeles County Fire Inspector Joey Marron said some 200 commercial buildings were also in harm's way.
Authorities on Sunday had put the number of evacuees at 1,500.
The blaze was concentrated near the town of Acton, about 50 miles (80 km) north of Los Angeles, as it cast a pall of smoke and soot over a wide area. Much of the Los Angeles basin was dusted with a thin layer of fine white ash from the fire on Saturday and Sunday.
The Soberanes fire, burning north of California's famed Big Sur coastal region, grew by about 5,000 acres (2,020 hectares) in the 24 hours from Sunday morning to Monday morning, the Forest Service said.
Evacuations were ordered for residents of several neighborhoods, and the number of threatened structures was expected to grow above the 1,650 given by emergency officials at around noon on Monday.
The causes of the two fires were under investigation. They are among some 3,750 blazes large and small to have erupted across California since January, a higher-than-normal total, collectively scorching more than 200,000 acres (80,940 hectares), state fire officials said.
The biggest so far was last month's Erskine Fire, which consumed 48,000 acres (19,429 hectares) northeast of Bakersfield, killing two people and destroying about 250 structures.
By comparison, the 2003 Cedar Fire ranks as the biggest on record in the state. It blackened more than 273,000 acres (110,480 hectares), leveled 2,820 buildings and claimed 15 lives.