SAN FRANCISCO — Crews battling deadly Northern California wildfires braced for a weekend of windy, hot weather that could drive the flames into new areas and threaten more homes.

The National Weather Service issued red flag warnings of critical fire weather conditions through Saturday night, saying a series of dry low-pressure systems passing through the region could bring wind gusts of up to 35 mph that could turn small fires or even sparks into racing walls of flame.

“This is a particularly dangerous situation with extremely low humidity and high winds. New fires will grow rapidly out of control, in some cases people may not be able to evacuate safely in time should a fire approach,” the weather service said in its bulletin for the Mendocino area north of San Francisco.

As a precaution, new evacuations were called Friday for an area of Mendocino and Lake counties where week-old twin fires have destroyed 41 homes and threaten about 9,000 more.

The fire has charred an area of the forested, rural area five times the size of San Francisco and is only 30 percent contained. Thousands of people remain evacuated.

The fire remained several miles from the evacuated communities along the eastern shore of Clear Lake but “it looks like there’s dicey weather on the way,” California Department of Forestry and Fire Protection spokeswoman Jane LaBoa said.

However, some days-old evacuations were lifted Friday in an area near Redding, where armies of firefighters and fleets of aircraft have been battling an immense blaze about 100 miles south of the Oregon line. Some areas on the fire’s southeastern flank were reopened to residents.

The Carr Fire was 39 percent contained after killing six people and incinerating 1,067 homes.

Gov. Jerry Brown was scheduled to visit the fire area in Shasta County on Saturday.

The fire burned slowly for days before winds suddenly whipped it up last week and drove it furiously through brush and timber.

The blaze burned so furiously on July 26 that it created a “fire whirl.” The twirling tower of flame reached speeds of 143 mph, which rivaled some of the most destructive Midwest tornados, National Weather Service meteorologist Duane Dykema said. The whirl uprooted trees and tore roofs from homes, Dykema said.

The California Department of Forestry and Fire Protection, which uses acres to describe fire size, said the blaze had blackened nearly 206 square miles.

In the Sierra Nevada, firefighters achieved 41 percent containment of a forest fire that has shut down Yosemite Valley and other adjacent portions of Yosemite National Park at what is normally the height of summer tourism.

The fire had reached into remote areas of the country’s third-oldest national park. Workers who live in Yosemite’s popular Valley region were ordered to leave Friday because of inaccessible roads.

The blaze has killed two firefighters.

A new report says the first firefighter, a California bulldozer operator, nearly slipped off a steep mountain trail three times before his vehicle finally rolled into a ravine and fatally crushed him.

Each earlier slip alone qualified as a “near miss” warning that the century-old mining trail could collapse, according to the Department of Forestry and Fire Protection’s preliminary report.

Braden Varney, 36, was working alone overnight July 14 fighting the wildfire while his assistant went to get a new hydraulic hose. Varney’s radio wasn’t communicating with headquarters, so his assistant relayed messages — until they lost contact.

The report says the death of the 10-year veteran highlights the need for better risk assessment, communication and supervision.