As fast-moving wildfires spark evacuations and devastate communities, California residents shared these first-hand videos. USA TODAY
A brutal combination of ferocious winds and near-record low humidity fueled the deadly wildfires that are scorching northern California’s wine country and leaving a breathtaking trail of destruction.
Fierce northeast “Diablo” winds that circulated around a ridge of high pressure over the Great Basin blew through the region late Sunday, according to Brian Mejia, a National Weather Service meteorologist in Monterey, Calif.
The winds allowed the runaway fires to jump fire lines and decimate entire neighborhoods, seemingly coming out of nowhere and causing residents to run for their lives in the middle of the night — the worst possible time for such an emergency to occur.
The toll on Tuesday was staggering and could get worse from the more than a dozen blazes, officials warned: 15 people killed, more than 100 injured, more than 2,000 businesses and homes destroyed, and about 3,200 people in shelters.
Through much of the summer, winds blow into California from the ocean. But winds can switch in late September or early October to northeasterly from the bone-dry deserts of Nevada or Utah.
This is a “classic wildland fire pattern in California, after five months of dry weather plus high-pressure over the Great Basin, creating warm, dry winds,” said meteorologist Jan Null of Golden Gate Weather Services.
In addition, as the wind howls down from the mountains toward coastal areas, they are also compressed and warm up. Then, as the winds squeezes through the canyons and valleys they speed up, further fanning the flames.
Mejia said sustained winds were at least 40 mph in some spots; a gust registered as high as 79 mph in northern Sonoma County.
Earlier on Sunday, the weather service had issued a “red flag warning” for the area, meaning that weather conditions were ripe for the spread of wildfires.
Extremely low humidity, in the single digits, which is unusually low for this area, was also a contributing factor, Mejia said. The humidity helps dry out vegetation, making it a better fuel for fires.
Another issue: A record wet winter of 2016-17 allowed plenty of trees and brush to grow this spring, which acted as fuel for the fires. That fuel, dried out by the summer heat and lack of rain, helped intensify the blazes.
The weather was similar to the conditions that led to the most destructive fires in California history: the October 1991 firestorm that struck the Oakland and Berkeley hills and killed 25 people and destroyed about 2,900 structures.
October is always a difficult time in California for wildfires, but this year, the wildfire eruptions seem extreme even to the most seasoned Californian.
The fires that roared across northern California were likely not started by lightning, according to the weather service, which did not detect any strikes late Sunday or early Monday. This leaves the probable cause for the blazes as man-made, whether accidental or deliberate.
This isn’t surprising: About 84% of wildfires in the U.S. are started by people, according to a comprehensive study undertaken earlier this year that was published in the Proceedings of the National Academy of Sciences.
Officially, the cause of the fires remained under investigation, according to Barry Biermann, deputy incident commander for the California Department of Forestry and Fire Protection (Cal Fire) But he wouldn’t say the rash of fires seemed suspicious.
“The wind were extremely erratic,” he said. “During those conditions of high winds, it doesn’t take much to start a fire.”