Timber and brush parched from a years-long dry spell and thick grass that grew after drought-busting winter downpours are making for early and unpredictable wildfire behavior that California officials haven't seen for years, if at all.
Dense layers of new grass are providing a "fine fuel'' for flames that then gain speed and intensity by moving through "standing dead fuel'' made up of vegetation and trees that shriveled during the state's six-year drought, said Kathleen Schori with the California Department of Forestry and Fire Protection.
"It's difficult to remember a year quite like this one,'' she said Tuesday. "There's such a mix of fuels that these large damaging fires are starting at least a month earlier than usual.'' The result, she said, could be a longer and more destructive fire season than California has experienced in a while.
Crews were making progress against dozens of wildfires across California, Colorado, Arizona and New Mexico.
Authorities surveying the damage from a blaze in Northern California said Tuesday that at least 36 homes and 37 other buildings had been destroyed near the town of Oroville, about 150 miles (241 kilometers) northeast of San Francisco.
Residents had started to return home after fleeing a wildfire in the grassy foothills of the Sierra Nevada, about 60 miles (97 kilometers) north of Sacramento, but at least 4,000 were still evacuated. The blaze burned nearly 9 square miles (23 square kilometers) and injured four firefighters. It was partially contained.
Schori said this year's conditions were similar to California's 1979 wildfire season, which came on the heels of a two-year dry spell and saw blazes blackening a total of 386 square miles (999 square kilometers) of grass, brush and timber and caused more than $30 million in damage. However that year's major fires didn't kick off until well into August, she said, as did the destructive 1992 blazes that followed a drought that started five years earlier.
Major downpours last winter pulled the state out of years of drought but also brought a layer of grass that early-summer fires are greedily feeding on.
"That creates faster moving fires, hotter fires, it carries fire much more readily,'' said Santa Barbara County fire Capt. Dave Zaniboni, whose department was battling two large wildfires.
Older, dried out trees and vegetation are especially dangerous for wildland blazes, but enough new and drying grass can provide links between such tinderboxes.
With the dense grass as the "carrier,'' the firefight becomes much more challenging because "you have to make sure the water is getting all the way down to the smoldering areas below,'' Schori said. "It takes a lot more effort to extinguish grass fires.''
In Southern California's Santa Barbara County, at least 3,500 people remained out of their homes due to a pair of fires. The larger of the two charred more than 45 square miles (116 square kilometers) of dry brush and has burned 20 structures since it broke out. It was 45 percent contained.
To the south a 17-square-mile (44-square-kilometer) wildfire that destroyed 20 structures is 25 percent contained. Crews were getting a break from rising humidity and light winds.
In Colorado, crews were winding down the fight against a wildfire that temporarily forced the evacuation of hundreds of people near the resort town of Breckenridge. Firefighters built containment lines around at least 85 percent of the blaze.