A helicopter drops water on a wind-driven wildfire in Orange, California, on October 9, 2017.
A helicopter drops water on a wind-driven wildfire in Orange, California, on October 9, 2017. (Mike Blake/Reuters)

Here’s how the “Diablo winds” are sparking historic blazes.

This story originally appeared on Grist and is reproduced here as part of the Climate Desk collaboration.

It’s peak wildfire season in California, and on Monday, more than 31 blazes were underway statewide.

Between Sunday and Monday, more than a dozen wildfires sprung up in and around Napa and Sonoma counties—also known as “wine country,” just north of San Francisco—prompting rushed evacuations of more than 20,000 people. In an attempt to speed the flow of relief and firefighting equipment and make the National Guard available, Governor Jerry Brown declared a state of emergency.

The Northern California firestorm has quickly burned tens of thousands of acres, and is encroaching on neighborhoods in several places. At around 3 a.m. Monday, a Cal Fire official told a local television station that there was “no hope of containment right now.”

In total, the fires have killed one person and destroyed more than 1,500 structures, as of late Monday morning, making them some of the most destructive in state history. (Editors’ note: By Tuesday morning, the L.A. Times placed the death toll at 11 people.) More than 100 people have been treated for burns and smoke inhalation at regional hospitals, according to the San Francisco Chronicle. Nationwide, this year’s fire season has cost more than $2 billion, the most expensive on record.

Smoke from the fires is visible from across the Bay Area, with many residents reporting the smell of smoke and even ashes falling from the sky. The National Weather Service says that, on Sunday night, winds at higher elevations in some parts of Sonoma County exceeded hurricane force, with several areas reporting gusts greater than 50 mph.

Several of the worst wildfires in California history have sprung up during October, near the end of California’s months-long dry season. It’s this time of year when a combination of strong offshore winds and low humidity can quickly fan a seemingly innocent spark into a raging inferno.

These winds are usually formed by a strong inland high pressure center, which pushes air down mountainsides and through canyons, causing it to warm up and dry out—a perfect environment for fast-growing fires. In Northern California, they are generally called “Diablo winds,” after Mt. Diablo in the eastern Bay Area. A 2015 study said that climate change is making these wind events more frequent and more severe in California. According to the Bay Area branch of the National Weather Service, conditions favorable for rapid fire growth will remain until Tuesday morning.

One particularly frightening fire in Northern California, the Tubbs fire near Santa Rosa, jumped the 101 freeway overnight, forcing a hospital to evacuate its patients. Officials report evacuation centers that have been set up have already filled to capacity.

“People are running red lights, there is chaos ensuing,” Santa Rosa resident Ron Dodds told a local television station. “It’s a scary time. It looks like Armageddon.”

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