Tens of millions of Californians are under heat warnings and advisories Thursday as a record-breaking, relentless heat wave continued to roast areas from San Francisco to San Diego. The heat, associated with an unseasonably intense and stubborn area of high pressure parked over the region, is sending temperatures soaring up to 30 degrees above average for this time of year in some areas.

The heat elevated wildfire risks to “critical” in some areas, including parts of the North Bay, where the devastating Glass Fire continues to burn near hard-hit Calistoga. Red-flag warnings were also in effect for the South Bay, central Mendocino County, western Monterey County and most of the Los Padres National Forest, Cal Fire stated.

Although Glass Fire destroyed vineyards and homes and forced thousands to evacuate with little warning in Napa and Sonoma counties, the heat and shifting winds are expected to make the fire more difficult to contain. The extreme temperatures and erratic winds may be ideal for generating tall plumes of smoke and ash above the Glass Fire and other preexisting blazes, allowing them to spread farther. Any new fires could also spread rapidly, given conditions through Friday.

In the Los Angeles metro area, numerous record highs were set Wednesday, with more likely to fall again Thursday. For example, Long Beach Airport reached 105 degrees Wednesday, breaking its old record of 100, set in 1992. The heat Thursday is expected to reach coastal areas, which had been spared by a breeze off the relatively cool waters of the Pacific Ocean.

Higher elevations in the Los Angeles area failed to see temperatures drop below the upper 80s to low 90s early Thursday morning.

Similarly, in Northern California many locations had temperatures in the upper 80s to low 90s as of 5:53 a.m. local time, according to the National Weather Service.

Particular fire weather concerns in the Bay Area center on the Glass and Dolan Fires. Satellite imagery overnight showed the Glass Fire was active in the hills above Calistoga, and a red-flag warning is in effect from 1 p.m. Pacific on Thursday through Friday at 6 p.m. for the blaze. The Glass Fire has grown to nearly 57,000 acres and was 5 percent contained Thursday morning.

The Weather Service is predicting that northwesterly winds of 25 to 30 mph are likely to reach the Glass Fire region this evening, with continued hot and breezy conditions Friday. These conditions could push the fire to the south and east while allowing for extreme fire behavior.

The fire has forced about 80,000 to evacuate from Napa and Sonoma Counties and has taken a heavy toll on the wine industry, with Sherwin Family Vineyards, the Meadowood resort, Newton Vineyard, Burgess Cellars, Behrens and Château Boswell all damaged or destroyed, according to news reports. The Calistoga Ranch and Black Rock Inn were also damaged, Eater reported.

California is in the middle of its worst wildfire season on record, with more than 3.9 million acres burned and five of the top 20 largest wildfires in state history taking place this year.

Scientists say California’s 2020 fire season, and the severe blazes that have extended north to Oregon and Washington, have clear links to human-caused climate change. There has been an increase in acres burned in large fires across the West because of global warming, and projections call for the region to become even hotter and drier, making it more susceptible to fire sieges like the one taking place. A study published in August shows California’s frequency of fall days with extreme fire weather conditions has more than doubled since the 1980s.

Heat waves also show a significant human fingerprint, with greenhouse gas emissions boosting the odds and severity of these events. There have even been some case studies published showing that individual extreme heat events would not have occurred without human-caused global warming. This is California’s third significant heat wave since mid-August.