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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.

The loss of 19 experienced firefighters battling an explosive wildfire outside YarnellAriz., 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.

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.

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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.

Indeed, “four times in the last three years we’ve had a fire be named the most destructive in Colorado history,” says Janice Coen, a meteorologist at the National Center for Atmospheric Research in Boulder, Colo., who studies the interplay between weather and wildfires.

Throughout much of the West over the weekend, temperatures soared as a ridge of high pressure sat over much of the western US. Phoenix reached 116 degrees F. on Saturday, two degrees shy of the record set in 1990, according to the National Weather Service forecast office there.

Once a fire begins, an incident manager may call on the National Weather Service to send in a team of 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.

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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.

“They can be tiny. The ones we like to deal with are one or two strikes,” Mr. Hockenberry says, because that usually means firefighters have to respond to a small number of ignition points. “But the really big lightning outbursts, with multiple, multiple strikes – that’s when we start getting overwhelmed.”

With the intense heat rising from a desert landscape, even a little moisture in layers of air above can lead to powerful thunderstorms, even if the rain never reaches the ground.

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.

Terrain also complicates the fire-weather forecast calculus. The terrain forms its own wind as the air rides up mountainside valleys during the day, then travel back downslope at night.

“Those terrain winds will dominate” under hot, dry conditions, Hockenberry explains. Throw in a thunderstorm on a hot afternoon, and the storm’s winds dominate even terrain-generated winds. Upslope winds meeting downdrafts from a dry thunderstorm overhead or a storm nearby can trigger highly erratic fire behavior.

Improvements in forecast capabilities – aided by portable, automated weather stations that firefighters can set up in the vicinity of a fire, fire spotters, and weather balloons launched from the fire’s area – allow forecasters to target their fire-weather forecasts to sections of a large fire as short a mile.

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But while weather is acting on a fire, the fire is creating its own weather, adds Dr. Coen, who has developed computer simulations that capture this interplay with increasing fidelity.

Fire-formed weather “is at the basis for a lot of phenomenon that are problems for firefighters,” Coen says. “It’s also behind some of these situations where even trained, seasoned firefighters can be caught by surprise” by a fire’s sudden turn in their direction.

The most visually obvious influence fire can have on weather in its immediate vicinity can appear as a pyrocumulous cloud. This builds as heat from a fire rises, carrying water vapor with it into a layer of moist, cold air. As the air cools and sinks outside the cloud, it returns to get drawn into the fire, and the process starts over. In effect, the fire inhales air at the surface and exhales it from the cloudtop.

Under the right conditions, these clouds can generate rain that evaporates before it reaches the surface, leaving nothing but cold air to continue the trip down. These downdrafts fall over the fire lines, contributing to gusty winds that can send fire or burning embers into areas with fresh fuel.

On smaller scales, narrow fire whirls and even larger fire tornadoes have appeared during intense fires.

Perhaps one of the most dangerous features, from a firefighter’s standpoint, are the speedy tendrils that can burst from a fire’s perimeter and cover 100 yards in two seconds. These tend to appear in so-called crown fires, in which a fire is burning treetops as it moves upslope.

A crew thinking it is a safe distance from a fire’s front would have precious little time to react if one of these erupted.

“It’s like a flamethrower,” Coen says, one that preheats, drys, then ignites fuel on the ground as well as in the treetops. Observations over the past decade suggest that this effect is basic to fire behavior and, from a training standpoint, represents what Coen has called “an unanticipated safety hazard” indicating that someone doesn’t have to be overtaken by a fire line to be in danger.

With funding from the Federal Emergency Management Agency, Coen says, she is using her computer simulations to explore in detail the behavior of wildfires in which firefighters have been killed on the fire lines when the combination of weather, fuel, and fire produced explosive growth in the blaze.

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Ultimately, she says, the goal is to turn her models into tools firefighters can use for training exercises, for anticipating the best time for igniting fires designed to reduce fuel loads in forests, as well as for on-site forecasting as a fire progresses.

Such modeling becomes especially important along the wildland-urban interface, where wildfires can move through entire communities. Such was the case last month during the Black Forest fire outside Colorado Springs, Colo. There, fire destroyed more than 400 homes in neighborhoods nestled among trees.

That’s a modeling challenge researchers are striving to meet, says the National Weather Service’s Hockenberry.

“But it’s an extremely, extremely complex problem,” he says.

Another crucial area for improvement is in designing the output of these models and other fire-weather forecast products in ways that can be delivered efficiently and in forms that firefighters can quickly and easily interpret in the field.

That’s one for the social scientists, he suggests.