Communities nestled next to wilderness are used to girding for fire season, which typically occurs in the summer. Compared with decades past, however, the traditional fire season now lasts two months longer and first responders sometimes find themselves beating back flames in the winter.
Rising temperatures all over the West, for one, have created dangerous, dry conditions.
Over the past 35 years, Arizona has seen dramatic warming, with the state's 10-year average temperature jumping from 59.1 degrees Fahrenheit in 1977 to 61.4 degrees last year — an increase of 2.3 degrees. By comparison, the entire continental U.S.' 10-year average temperature jumped only 1.6 degrees during the same period. Experts say every little spike in temperature makes a big difference.
"Even a degree or so warmer, day in day out, evaporates water faster and that desiccates the system more," said fire ecologist Steve Running of the University of Montana.
In Arizona, where a drought has persisted for nearly two decades, the manzanita, evergreen, mount mahogany and oak in the Yarnell area were so crispy Sunday that a nearby state fire-monitoring station recorded a near-maximum level of potential fuel in area vegetation.
In many places, decades of aggressively snuffing out wildfires also have led to a buildup of fuel ready to ignite. On top of that, more people are living in fire-prone areas near forests, grasslands and shrub lands, which complicates firefighting logistics.
Over the past years, firefighters on the front lines have complained about how flames "go berserk in ways they never used to see," Running said.
Though the Yarnell Hill Fire, at 13 square miles, was not considered huge compared with previous fires in Arizona, its ferociousness caught many off guard. Investigators said it appeared the Granite Mountain Hotshots were overrun by flames fanned by erratic winds.
At one point, the fire raced four miles in just 20 minutes, fed by the dry brush and 41 mph winds that suddenly switched direction, said Yavapai County Sheriff's Capt. Jeff Newnum.