Thousands of people were under orders to evacuate in regions surrounding the San Francisco Bay Area on Wednesday as nearly 40 wildfires blazed across the state amid a blistering heatwave now in its second week.

“Throughout the state of California right now, we are stretched thin for crews” because of the fires, said Will Powers, a state fire spokesman. “Air resources have been stretched thin throughout the whole state.”

California’s governor had declared a statewide emergency late on Tuesday citing an “Extreme Heat Event” that began on Friday, causing record-breaking temperatures and Red Flag warnings – weather events such as heat and lightning storms, that can spark fires.

Police and firefighters went door-to-door early on Wednesday in a frantic scramble to warn residents to evacuate as fire encroached on Vacaville, a city of about 100,000 that lies between San Francisco and Sacramento. Fire officials said at least 50 structures were destroyed and 50 damaged, and four people were injured.

The California Department of Forestry and Fire Protection (Cal Fire) said the blaze was exhibiting “extreme fire behaviour” and challenging firefighters.

To the south in San Mateo and Santa Cruz counties, about 22,000 people were ordered to evacuate because of a fire that was burning in densely wooded parkland but threatened communities, Cal Fire spokesman Jonathan Cox said.

Thousands of homes and businesses were also threatened in the wine-growing counties of Napa and Sonoma in an area devastated by a series of deadly blazes in the past three years. At least seven fires were grouped together as one of two major Lightning Fires in Northern California, a nod to their origins just a day earlier.

In the East San Francisco Bay, a cluster of 20 separate lightning-sparked fires threatened about 1,400 structures in rugged terrain with dense brush. Strong winds and low humidity made the firefight challenging.

The cluster of wine country fires threaten an area that only last year grappled with another massive blaze that forced 200,000 to flee, a task made more complicated this year because of the pandemic.