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How 'atypical' winds complicate firefighting efforts in Napa and Sonoma Counties

by Austin Cross and A Martínez | Take Two®

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SANTA ROSA, CA - OCTOBER 09: Burned out cars sit next to a building on fire in a fire ravaged neighborhood on October 9, 2017 in Santa Rosa, California. Ten people have died in wildfires that have burned tens of thousands of acres and destroyed over 1,500 homes and businesses in several Northen California counties. (Photo by Justin Sullivan/Getty Images) Justin Sullivan/Getty Images

Multiple fires continue to burn through parts of Northern California with little containment.

At least 17 people have died, and 2,000 structures have burned in one of the worst wildfires in the state. 

Investigators still aren't sure what caused the firestorm, but climate experts have some idea about why the flames may have spread so fast. 

"It was a combination of things that we expect to see in October," says Neil Lareau, assistant professor of meteorology and climate science at San Jose State University. 

He points to two specific factors: dry brush and high offshore winds. 

"When those two things come together, we can get these explosive fire growth patterns that we saw both up here in the Wine Country fires and down south in the Canyon Fire 2," Lareau says. 

Here are four things to know:

The winds are warm.

It's a little bit of a tricky scenario. They originate as cold air over the Intermountain West — kind of east of the Sierra Nevada. That cold air then descends very rapidly from the higher terrain. As it moves offshore and as it descends, it warms up by about 20 to 30 degrees. 

It starts out cold, but by the time it shows up in the coastal typography, it's quite warm and extremely dry. Very low relative humidity — 10 percent as opposed to a typical nighttime value of maybe 75 percent. 

The wind is causing the fire to behave differently.

Under typical wind conditions, fires want to move uphill. They'll head up toward the ridge crests. But when these winds occur, they can drive the fire off of the ridges down the slope and down into town. That's what sets up for these devastating impacts. 

It's like water flowing over a dam. You get this plunging flow that can accelerate the fire as it moves downhill. The strongest winds are occurring right along those slopes, bringing the fire right into the edge of town and across town in some instances. 

The winds are somewhat 'atypical.'

We're looking at 50 to 70 mile an hour wind gusts in Santa Rosa, which is pretty atypical. It's not unheard of with these events, but it's definitely at the higher end of what we'd expect to see. 

That momentum drives the fire itself, but it also spreads embers from where the fire is currently burning well out in front of it, causing spot fires, which are very, very difficult for firefighters to get out in front of. 

More wind could be on the way.

We could see another similar wind pattern setting up in Northern California, so, something to keep an eye on.

Press the blue play button above to learn more about how these winds form. 

Answers have been edited for clarity.

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