ST HELENA (REUTERS) - A wind-driven wildfire erupted on Sunday (Sept 27) in the heart of northern California's Napa Valley wine country and spread across nearly 2,000 acres (809ha), forcing the evacuation of several hundreds homes and a hospital, authorities said.

Fire crews were out in force, scrambling to fend off flames threatening neighbourhoods and vineyards in the north-west corner of the famed wine-growing valley, about 120km north of San Francisco.

The blaze, dubbed the Glass Fire, broke out before dawn near Calistoga and raced towards the adjacent towns of Deer Park and St. Helena, with flames advancing to within a mile of the Adventist Health St Helena hospital.

All 55 patients who were at the hospital at the time were safely evacuated by ambulance and helicopter over the course of five hours, beginning around 7am in the morning, hospital spokeswoman Linda Williams told Reuters.

"We had ambulances lined up from all over the Bay area," she said, adding that while the facility was surrounded by smoke, the skies over the hospital itself remained clear enough for helicopters to land and take off with patients who needed to be evacuated by air.

It was the second wildfire-related evacuation of the 151-bed hospital since August, coming on the heels of a massive cluster of lightning-sparked blazes that swept several counties north of the San Francisco Bay region.

Authorities ordered about 600 homes evacuated on Sunday, with residents of 1,400 more warned to be ready to flee at a moment's notice, said Tyree Zander, a spokesman for the state's forestry and fire protection department (CalFire).

The notices affected at least 5,000 people, he added.

By evening, flames stoked by winds gusting up to 80kmh had scorched about 728ha of grassy rolling hillsides and oak woodlands, with little or no containment, Zander said.

The fire's cause was under investigation. There were no immediate reports of injuries, but a Reuters photographer in St Helena saw a number of structures that had been burned.

The blaze erupted midway through the traditional grape-harvesting period in the Napa Valley, world renowned as one of California's premiere wine-producing regions.

The area's 475 wineries account for just 4 per cent of the state's total annual grape harvest but half of the retail value of all California wines sold, according to the Napa Valley Vintners trade group.

Of Napa's 16 wine-growing districts, or sub-appellations, the Howell Mountain area may have faced the greatest threat, said Lisa Covey, a spokeswoman for Hall Family Wines, which kept open during the day all its three tasting rooms in the county.

Napa and other wine-growing regions have been hit by wildfires in and around the Bay area for several years.

Susan Krausz, co-owner of Arkenstone Estate Vineyards in Angwin, said it would take days or weeks to assess the impact of the latest blaze on valley vintners.

"Most people have harvested," she said, adding, "Any time's a bad time for a fire."

Tom Kaljian, 78, a realtor who owns a house about halfway between Calistoga and St. Helena, defied evacuation orders to spend the day with his wife hosing down their home and dry brush along a fence line separating their property from the Silverado Trail, one of the area's main north-south roadways.

"We were told to get out of here, but I was trying to protect our little abode, so we stayed," he told Reuters by phone, adding that firefighters stopped by later to tell him his house was no longer in danger.

"I stopped watering at that point, and came in and took a nap," he said.

The latest blaze came as the Pacific Gas and Electric Company announced it was temporarily shutting off power to transmission lines in portions of 16 counties across northern and central California as a precaution against heightened wildfire risks posed by hot, windy, dry conditions.

The public safety power shutoffs were expected to affect some 65,000 homes and businesses in the region, said PG&E, the state's largest electric utility.

A red flag warning for extreme wildfire risks was to remain posted for Napa Valley through Monday morning, Zander said.

CalFire said a fire weather watch will start on Monday across much of Southern California due to a forecast return of hot, gusty Santa Ana winds and low humidity from San Diego to Los Angeles and Ventura counties.

California wildfires have scorched more than 1.49 million hectares in the first nine months of 2020, far exceeding any single year in state history, killing 26 people and destroying more than 7,000 structures.