The death toll from a wildfire raging in northern California’s Cascade foothills climbed to four on Wednesday as firefighters battling that blaze and another wreaking havoc in wine country near San Francisco braced to confront a new bout of high winds.

Diminished winds across northern California assisted fire crews in making some initial headway on Tuesday against both blazes, which have scorched well over 100 000 acres (40 000 hectares) combined since they erupted about 200 miles (320 km) apart on Sunday.

On Wednesday, crews fighting the so-called Zogg Fire in Shasta County and a separate blaze dubbed the Glass Fire in Napa and Sonoma counties scrambled to reinforce their control lines and tamp down hotspots while weather remained in their favor.

A red-flag warning for heightened wildfire risks, including extreme winds, is due to be reinstated Wednesday evening for areas just north of San Francisco Bay, including the world-class wine-growing regions of Napa and Sonoma counties.

Above-normal heat and extremely low humidity has persisted, even after fierce winds that fanned the explosive fires earlier in the week subsided on Tuesday.

Authorities were also on guard for a return of high winds in Shasta County, closer to the Oregon border, California Forestry and Fire Protection (Cal Fire) officials said.

“Now’s the time for our firefighters to buckle down,” Cal Fire Assistant Chief Bill See said during a late-morning update on the Glass Fire. “We’ll be diligently trying to secure the back door on this fire.”

He added that the Napa Valley resort town of Calistoga, known for its hot springs, mud baths and wine tasting rooms but under full evacuation since late Monday, was a particular focal point for fire-protection efforts.

The Glass Fire has already destroyed 80 homes and 32 other structures, including the mansion-like Chateau Boswell winery and a farmhouse containing storage, bottling and fermentation facilities at the Castello di Amorosa winery, built to resemble a 13th-century Tuscan castle. The castle itself was unscathed.

Some 80 000 people have been placed under evacuation orders, including all 5 300 residents of Calistoga.

The Zogg fire, burning near the town of Redding, has destroyed at least 146 structures, with some 15 000 structures listed as threatened and 2 200 residents under evacuation orders or advisories.

The causes of the two fires are under investigation. Both were fueled by overgrown thickets of tinder-dry grass scrub covering the hilly, rugged terrain in each area.

The Glass Fire struck midway through the traditional grape-harvesting season in Napa and Sonoma counties, both world-renowned among California’s wine-producing regions and still reeling from a cluster of large wildfires earlier this summer.

The full effect on the region’s wine business remained to be seen. But industry officials said some vintners would likely scale back production of certain wines due to smoke exposure to grapes still on the vines when the fires struck. Several Napa Valley growers said recently they would forgo a 2020 vintage altogether due to smoke contamination of their crop.

The blazes marked the latest flash points in a destructive spate of wildfires this summer across the US West.

California fires have scorched over 3.9 million acres (1.5 million hectares) since January – far exceeding any single year in state history. They have been stoked by increasingly frequent and prolonged bouts of extreme heat, high winds and dry-lightning sieges that scientists attribute to climate change.

More than 7 200 homes and other structures have burned statewide this year.