Fire Dynamics, Firefighting, Wildland Firefighting

How Weather Can 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.

The Christian Science Examiner looks at what researchers have learned.

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.

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 Yarnell Hill fire began on 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 show time 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 US, 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 in 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.

Once a fire begins, an incident manager may call on the National Weather Service to send in a team forecasters to provide on-site forecasts.

There, forecasts become more complicated than ordinary weather forecasts, says Heath Hockenberry, who manages the National Weather Service’s fire-weather program, based in Boise, Idaho.

With a tornado forecast, a local forecast office has all the information it needs in-house, he says. With fires, “it’s not only, Will the weather line up to create dangerous conditions? It’s, Will stuff burn? How? What will the nature of the burning be?”

Weather conditions external to the fire range from the passing of frontal systems and the changes they can bring to wind, humidity, and rainfall to powerful downdrafts from passing thunderstorms. These downdrafts hit the ground and generate powerful low-level winds that travel in all directions.

Dry thunderstorms, whose rain evaporates long before it hits the ground, can be particularly troublesome, dropping bolts but no moisture to potentially slow a fire’s spread.

Conditions that would lead to a garden-variety thunderstorm in the eastern US “can cause a lot of headaches and major [fire] outbreaks in the West,” Hockenberry says.

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