The fires began at about 10 p.m. Sunday and were fanned by wind gusts moving faster than 50 miles an hour, Ms. Upton said. The cause remained under investigation on Monday afternoon.
The worst fires in Northern California tend to hit in October, when dry conditions prime them to spread fast and far as heavy winds, known as north winds or diablo winds, buffet the region.
Ms. Upton said that conditions were critically dry, given the lack of moisture in the air and the buildup of grass, brush and trees.
“Combined, that’s a recipe for disaster,” she said.
Smoke billowed into the Bay Area, but the Marin County Fire Department reported that there were no separate fires in the area.
Reports suggested that residents in the region were caught unaware, many of them fleeing the area in cars and on foot as firefighters rushed to contain the outbreak. A number of roadways, including highways, were blocked by a fire.
Neighborhoods in Santa Rosa, the county seat of Sonoma, were evacuated, according to the city manager, who said the Kaiser Permanente and Sutter hospitals were being cleared out.
The fires raged through the hills that are home to some of the country’s most prized vineyards. The main north-south highway that connects San Francisco to the northernmost parts of California was closed Monday as fire engulfed both sides of the freeway. Santa Rosa is a hub for tours into wine country. At least two large hotels that cater to the wine tourism trade were destroyed by the fires.
North of Santa Rosa’s downtown, residents of the Overlook, a hilltop apartment complex, used fire extinguishers to put out flames engulfing cypress trees planted along a building’s edge. Minutes later, the flames returned. At least three engines and ladder trucks arrived but could not stop flames on one of the buildings from spreading to the roof.
“It looks like they’re giving up on that one,” said Derek Smith, a Santa Rosa resident watching the blaze whose house was several blocks away.
When Mr. Smith awoke at 2:30 a.m. Monday to prepare to leave for work as an installer of laboratory equipment, he noticed very high winds and smelled smoke.
“It’s weird — I didn’t get any warnings or messages,” he said. “I left the house and then went back to get my mother’s jewelry.”
“The fire is everywhere now,” he said.
Traffic lights at multiple intersections in Santa Rosa were not functioning. Columns of black smoke could be seen in the evergreen forests on the northern outskirts of the city. A pall of white smoke across the city blotted out the sun.
Lisa Kaldunski, an operator at Sutter Santa Rosa Regional Hospital, said around 6:30 a.m. local time that the facility was being evacuated and that patients were being taken to other hospitals.
Marc Brown, a spokesman for Kaiser Permanente, said about 130 patients had been evacuated from the Santa Rosa medical center because of the fires.
Belia Ramos, the chairwoman of the Napa County board of supervisors, said the county was dealing with three main fires. One has threatened more than 10,000 acres in northern Napa County, another has endangered 8,000 to 12,000 acres, and a third has affected about 2,000 acres, she said.
Ms. Ramos said the fires were moving quickly and unpredictably. She said she did not know how many people had been evacuated early Monday, but added that the areas being evacuated were large and densely populated.
“Certainly we know that the numbers are high,” she said. “As day breaks and we get a better handle on this situation, we’ll be able to update those numbers.”
California was hit by fires throughout the summer. Late last month, several blazes led to the evacuation of about 1,000 people in Southern California.
“I’ve been with the department for 31 years and some years are notorious and they’re burned in your memory,” Ms. Upton said. “I’m afraid that 2017 is going to be added to that list now.”