Credit: California National Guard

All eyes are on Northern California this week as devastating wildfires tear through wine country. At least 31 people have died and thousands more have been left homeless, mostly in Napa and Sonoma counties.

This infrared satellite image from Planet shows fire damage Tuesday near Napa’s Silverado Resort. Blackened areas represent scorched ground. Healthy vegetation, such as the resort’s golf course, is in red. We’ve added imagery from Google for comparison.

Even with thick smoke covering the area Wednesday, the hardest-hit fire zones were clearly visible.

But the conditions for the unfolding catastrophe were set long ago, both in wine country and throughout California.

Large portions of both Napa and Sonoma counties have been rated by the state Department of Forestry and Fire Protection as having a high chance of destructive fire based on the vegetation types and condition of the area. Nearly all the fires there are burning through areas with the most severe classification.

Most Northern American forests are supposed to burn at regular intervals – every few years or so. Northern California is no different, but many of the current fires are consuming trees left untouched by flame for decades.

The Tubbs and Nuns fires both began in areas that last burned in 1964. The Atlas Fire sparked in a region that hasn’t seen flames since 1981. CAL FIRE has no recorded fire history for the Partrick Fire to the south.

The flames did not stop at treelines. They moved into communities, coursing through streets, claiming house after house, building after building. These communities are part of what is known as the wildland-urban interface, or WUI.

The WUI is defined as areas where homes and development border on and intermix with natural, uncultivated vegetation such as forests and grasslands. About 30 percent of homes in California fall into one of these zones.

Fighting fires in WUI areas is both dangerous and expensive. A 2015 U.S. Forest Service audit revealed that it costs an average of $1,695 per acre to fight a fire in a WUI zone. The price for fighting a fire in the forest is $779 per acre and $61 per acre in areas dominated by grass and shrubs.

In WUI areas, there is an added threat beyond the potential loss of life and property. Humans cause as much as 90 percent of the wildfires in the United States. Developing in wildland areas increases the chance of fires starting as more people move into the countryside.

And the WUI zones themselves are growing. A Reveal analysis found that as many as a third of the homes built nationwide since 2000 have been in a WUI zone.

Officials are estimating that as many as 3,500 buildings have been lost in the Northern California fires so far. In many cases, the exact causes of the fires are unknown and under investigation.

Meanwhile, hundreds of fire engines and crews are streaming into the state from other parts of the country. They will relieve firefighters who have been working throughout the week and deal with any new blazes that might occur over the weekend.

Eric Sagara can be reached at Follow him on Twitter: @esagara.

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Eric Sagara is a senior data reporter for Reveal. He joined Reveal following a news applications fellowship at ProPublica, where he worked on projects about pharmaceutical payments to doctors, deadly force in police agencies and the trail of guns in the United States. Prior to that, he was a reporter on The Newark Star-Ledger's data team. Sagara is originally from Arizona, where he reported on business, education, crime, wildfires and government. He is based in Reveal's Emeryville, California, office.