Fire three times the size of San Francisco keeps raging in Napa, Yolo counties

Hot Shot crews from Mendocino use backfires to help contain the County Fire along Highway 129 near Lake Berryessa in Yolo County, California, Tuesday, July 3, 2018. (Randall Benton/The Sacramento Bee via AP) Hot Shot crews from Mendocino use backfires to help contain the County Fire along Highway 129 near Lake Berryessa in Yolo County, California, Tuesday, July 3, 2018. (Randall Benton/The Sacramento Bee via AP) Photo: Randall Benton / Associated Press Photo: Randall Benton / Associated Press Image 1 of / 24 Caption Close Fire three times the size of San Francisco keeps raging in Napa, Yolo counties 1 / 24 Back to Gallery

A ferocious inferno burning in Yolo and Napa counties was nearly three times the size of San Francisco early Thursday as firefighters made some headway gaining control despite the difficult terrain.

The County Fire, which erupted Saturday afternoon, grew to roughly 88,000 acres burned while threatening more than 1,000 homes by Thursday night, according to Cal Fire. By comparison, San Francisco is about 30,000 acres.

Officials said the fire is roughly 33 percent contained.

Nine structures had been destroyed as of Thursday night, officials said.

Guinda, a rural enclave along Highway 16 north of Lake Berryessa in Yolo County, is where the fire started, according to the California Department of Forestry and Fire Protection.

A red flag warning was issued for the region Thursday due to gusty winds, low humidity and high temperatures, officials said.

New evacuation orders were also issued for the area, including west of Highway 16 to the Yolo and Lake county lines, and north of County Road 40 just south of Yolo County. Highway 16 was closed between Highway 20 and Rumsey Hall.

Mandatory evacuation orders remained in place for residences along Highway 128, between Monticello Dam and Pleasant Valley Road, west of Highway 16 to Berryessa Knoxville Road, south of Old County Road 40 and north of County Road 53.

Wednesday brought more favorable weather conditions, but the blaze continued to aggressively burn through brush, Cal Fire said.

Steep and inaccessible terrain prevented firefighters from establishing a containment line on the northern end of the fire, officials said.

A dry winter combined with the ongoing effects of the five-year drought that started in 2012 resulted in more dead trees and dry brush and grass, contributing to what has been an especially early and volatile wildfire season.

Officials said hotter and drier weather conditions may impact fire growth heading into the weekend.

Cal Fire said that 3,475 firefighters are working to establish containment lines with the help of 309 fire engines, 73 fire crews, 21 helicopters and 73 dozers.

Sarah Ravani is a San Francisco Chronicle staff writer. Email: sravani@sfchronicle.com Twitter: @SarRavani