David Kadlubowski / The Arizona Republic via AP
Homes burn as the Yarnell Hill Fire approaches Glenn Ilah on Sunday, June 30, 2013 near Yarnell, Ariz.
YARNELL, Ariz. — The wildfire that killed 19 firefighters came at the start of Arizona's monsoon season — a weather phenomenon that brings lightning strikes, gusty winds, dust storms and sometimes rain to the state during the summer months.
The winds that helped fuel the fire were highly unpredictable and contributed to the rapidly shifting nature of the blaze.
Arizona's monsoon season began on June 15, but didn't produce many storms until this past weekend. Unlike the flash-flood-producing storms late in the summer, the early monsoon storms typically have plenty of lightning and wind, but often have little rain.
The rain-less storms combined with the dried out conditions of a prolonged drought in Arizona — the state has been in rain deficit for more than a decade — greatly increase the potential for devastating fires.
"If we're talking about Arizona, this is definitely the most dangerous time as far as wildfires go," Ken Waters, warning coordination meteorologist for the National Weather Service in Phoenix, said on Monday. "Basically, June into about mid-July, that's definitely a problem. It's a time when fire starts by lightning are more likely."
The monsoon begins when upper-level winds shift to bring moisture from the Gulf of Mexico into states like Arizona, New Mexico and Colorado.
When the moisture collides with the high heat of the Southwest, it creates thunderstorms across the region, producing produce multiple lightning strikes, gusty winds and dust storms, along with and isolated downpours.
The firefighters, part of the elite Granite Mountain Hotshots based in nearby Prescott, were apparently overwhelmed by the erratic nature of the lightning-sparked fire near Yarnell on Sunday in the nation's biggest loss of firefighters in a wildfire in 80 years.
Fire officials aren't sure how the firefighters became trapped, but outflow boundaries from thunderstorms and a channeling effect caused by the topography of the mountainous area can cause varying wind speeds and directions during the monsoon season.
Yarnell sits along a stretch of a highway that climbs about 2,500 feet in a span of four miles from the desert below. The town of about 700 is at an elevation of just over 4,700 and in a rugged area once known for its mining.
The up-and-down topography creates a funneling effect for the wind over the passes and through the valleys, creating unpredictable speeds and directions.
"There are so many variables here with the terrain, and the terrain tends to modify the winds where you get this channeling affect where the winds can change direction and intensify as they get compressed," Waters said.
"Basically, you're trying to squeeze the winds into a smaller area when you get a pass or something and you get what we call channeling. And if you throw in the outflow boundaries from these thunderstorms, that greatly increases the erratic nature of these winds."