RANCHO CUCAMONGA, Calif. — The wildfires ravaging Southern California are fueled by drought- and insect-ravaged vegetation and stoked by hot, dry winds racing over terrain untouched by flame or ax for decades.

The combination primed the region for a disaster that began to unfold a week ago when fires — some linked to arson — began spreading through dense chaparral and forest, officials said.

"We're set up for what we have all been afraid of for a while," said Dan Felix, a fire behavior analyst for the U.S. Forest Service.

By Sunday, the flames had destroyed hundreds of homes and forced tens of thousands of people to flee.

Fierce desert winds gusted as high as 70 mph.

"This is all wind-driven," Martin Esparza, a Forest Service spokesman, said of the wildfires.

The dreaded Santa Ana wind gets started in the Great Basin, the vast expanse of desert that covers much of Nevada, Utah and southern Idaho.

High pressure over the Great Basin forces cool, dry desert air toward the southwest and through the mountains of Southern California, including the Cajon Pass area, 60 miles east of Los Angeles, that was at the center of the largest of the fires burning Sunday.

As the wind travels downhill toward the coast, it is funneled through narrow canyons, compressing, heating and accelerating the air, said Robert Balfour, a National Weather Service meteorologist.

The hot, dry wind quickly parches vegetation already stressed by years of drought.

"The fuels are so dry, they too are dictating the growth of the fire," Balfour said.

Much of the brush has grown unchecked for decades.

"Some of these places do not have any recorded history of fire," Esparza said.

The Santa Ana wind typically blows between September and February. A decade ago, the wind exacerbated fires that charred thousands of acres, killed three and destroyed 1,000 buildings in Malibu, Altadena and Laguna Beach.

The Santa Anas tend to push flames downhill, toward subdivisions built up against Southern California's mountain ranges.

When the wind subsides, flames climb uphill, mounting the chaparral-covered slopes to push into denser forest.

Officials fear that prospect, since the region's forests contain millions of dead and dying trees devastated over the last year by tiny beetles.

Since the spring, the Forest Service and private land owners have struggled to remove dead trees and brush within the 820,000-acre San Bernardino National Forest. Vast swaths of the forest, home to 80,000 people, have not been logged for more than a century.

"If the fire starts to crown, racing from one tree to the next, it will be an extreme situation," said Stanton Florea, a Forest Service spokesman.

Briefing papers circulated among fire experts this summer said fuel levels near Lake Arrowhead in the forest approached those that fed the Peshtigo firestorm in Wisconsin on Oct. 8, 1871, which killed more than 1,500 people.

Decades of fire suppression mean a region that could support a healthy forest of about 40 trees an acre is crowded with as many as 568 trees per acre, according to a fire safety briefing prepared by the San Bernardino National Forest and the interagency Southern California Coordination Center in Riverside. The Peshtigo fire region had about 640 trees per acre.