Four million acres have been burned this year by wildfires and leaving behind a trail of death of 30 people and incinerated hundreds of homes in what is already the worst fire season on record in California.

The fires have torched an area larger than Connecticut and fire crews at a blaze in the northern wine country were on high alert as forecasters warned of red flag conditions of extreme fire danger into Saturday morning.

Winds up to 30 mph (48 kph) were forecast to push through the hills in Napa and Sonoma counties as the Glass Fire, exploded in size earlier in the week, continued to threaten more than 28,000 homes and other buildings.

"It's a time of nervousness," said Paul Gullixon, a spokesman for Sonoma County.

Winds were blowing at higher elevations on the western side of the fire and crews expected a long battle to keep flames from jumping containment lines and spot fires from leaping ahead to spark new blazes.

"It's going to be a big firefight for us over the next 36 hours," said Billy See, an assistant chief with the California Department of Forestry and Fire Protection, or Cal Fire.

More crews and equipment were deployed in and around Calistoga, a town of 5,000 people known for hot springs, mud baths and wineries in the hills of Napa County about 70 miles (110 kilometers) north of San Francisco.

The area was also experiencing high temperatures and thick smoke that fouled the air throughout the San Francisco Bay Area.

Around the state, 17,000 firefighters were battling nearly two dozen major blazes. Virtually all the damage has been done since mid-August, when five of the six largest fires in state history erupted. Lightning strikes caused some of the most devastating blazes.

Numerous studies have linked bigger wildfires in America to climate change from the burning of coal, oil and gas. Scientists say climate change has made California much drier, meaning trees and other plants are more flammable.

Cal Fire Deputy Chief Jonathan Cox said wildfires have scorched 3.9 million acres in California since Aug. 15. That figure, which works out to over 6,000 square miles (15,500 square kilometers), is astonishing even in a state that has had its fair share of fires.

"It's likely that over the next day or two we will crest the 4-million-acre mark. The biggest year before this year was 1.54 million," Cal Fire Chief Thom Porter said. "We are dwarfing that previous record and we have a lot of season left to go."

Fire officials said the Glass Fire had first priority. Since erupting last Sunday, the fire has destroyed nearly 600 buildings, including 220 homes and nearly the same number of commercial structures.

Some 80,000 people were under evacuation orders, which were expanded on Thursday.