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Wildfire wildcard: How weather can help trigger an explosive fire

This week's loss of 19 elite firefighters battling Arizona's Yarnell Hill wildfire raises questions about how scorching temperatures and low humidity may contribute to explosive fires and unanticipated safety hazards. Here's what researchers have learned.

By Staff writer / July 2, 2013

Firefighters from Prescott, Ariz., and other area departments embrace during a memorial service Monday in Prescott for the 19 Granite Mountain Hotshot firefighters killed Sunday when an out-of-control wildfire overtook the elite group.

Julie Jacobson/AP

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The loss of 19 experienced firefighters battling an explosive wildfire outside Yarnell, Ariz., highlights the difficulty – and danger – involved in halting the growth of fires burning through rugged, drought-stricken, fuel-rich terrain under scorching temperatures and low humidity that only serve to encourage a fire's growth.

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Now, scientists are working to turn a decade's worth of research into the interplay between fire, terrain, fuel, and weather into tools that fire managers might be able to use to try to reduce the risk firefighters face of being caught off-guard by sudden shifts in fire behavior.

Indeed, while terrain and fuel abundance play crucial roles in fire behavior, weather – including weather conditions that fires themselves foster – often is the wildcard in combating fires. It's a card that, without warning, can send thin tongues of flame lancing ahead of the main fire at speeds of up to 100 miles an hour, covering 100 yards in two seconds, only to vanish.

The deaths of 19 members of the elite Granite Mountain Hotshots on Sunday represents the largest loss of life while fighting a wildfire in 80 years, according to statistics kept by the National Fire Protection Association. Only two other blazes, one in 1910 and 1933, claimed more firefighters' lives.

The Yarnell Hill fire began June 28, triggered by lightning. By Sunday, it had burned only a few hundred acres. By 7 a.m. local time Monday, the fire had expanded to nearly 8,400 acres, according to the Arizona State Forestry Division.

Firefighters battling the Yarnell Hill fire are keeping an eye out for a seasonal break from the dryness – the Southwest's annual monsoons. "Until we get a significant showing of the monsoons, it's showtime and it's dangerous, really dangerous," Roy Hall, incident commander for the Yarnell Hill fire, told The Associated Press. The monsoons typically reach the region in early July.

At the broadest level, global warming has loaded the dice for climate and weather patterns that affect wildfires in the western United States, climate researchers have noted. Since the mid-1980s, large wildfires in the western US have been occurring more often, the length of the wildfire season has grown, and the fires are lasting longer, consistent with projections for global warming.

The changes have been most pronounced in the northern Rockies. But warming also has triggered large fires in areas where, at least in recent history, such fires have been scarce, according to a draft of the Third National Climate Assessment report, released for public comment in January. These regions include Alaska and the desert Southwest.

In New Mexico, the past 12 months have been the driest on record, according to the latest drought report issued by the National Drought Mitigation Center at the University of Nebraska at Lincoln. The past 24 and 36 months have been the second driest on record. Meanwhile, Arizona and Colorado also are reeling under prolonged drought.

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