Relatório do Acidente ocorrido no Incêndio Florestal de Yarnel Hill

detection systems showed lightning flashes within the parent storms, but lightning strike
locations and trends would not indicate a significant change in storm movement. Field
personnel, using traditional visual indicators, would not easily have noted the approaching
outflow boundary.
Some indicators appeared after 1600. Subtle indicators included the gradual change in wind
direction as the boundary approached (west-northwest) as well as a brief lull just prior to the
outflow passage. Distinct indicators included an abrupt change in wind direction and speeds,
referred to as “ripping” when the outflow boundary hit the northern end of the fire. Other
distinct indicators included “spritzes” of rain, thunder, and some flashes of lightning (both
cloud-to-ground and in-cloud) that coincided with the rapidly increasing fire column and fire
behavior.
FIRE BEHAVIOR
Local firefighters shared that, in their experience during a “normal” year, a strong wind is
required to support active fire spread in the Arizona chaparral, but that this year was not a
normal year. Prior to the Yarnell Hill Fire, local firefighters had been observing higher fire
intensities and rates of spread on other fires. Two contributing factors that can help explain
these observations are:
• Above-average loading of cured grass fuels which increased the horizontal fuel
continuity
• Drought-stressed live fuels
These factors combined to allow for faster-moving fires, with or without winds, and for
increased fire intensities and flame lengths.
The Yarnell Hill Fire exhibited problematic fire behavior almost from the start. Active fire
behavior on the flanks and heel of the fire where there were continuous fuels hampered initial
attack. At one point, the fire burned over a cache of food, water, and other equipment.
Early in the morning of June 30, fire activity was minimal. Mid-morning burning activity picked
up due to a south wind. Variable winds caused the fire flanks to increase and decrease in
activity, best described as pulsating in character. The fire began to spread to the north,
eventually accelerating to one mile per hour. Flames at the head were 15 to 20 feet in length.
By noon, the fire was a long finger that had moved approximately two miles to the north.
Around 1200, the wind developed a partial westerly component and the fire started moving
northeast toward the Incident Command Post in Peeples Valley. By 1500, the wind shifted to
out of the west, and the east flank transitioned into a two-mile wide head of flames. The
northern and southern perimeters continued to spread as flanking fires.
An outflow boundary originating from a line of thunderstorms to the north-northeast swept over
the fire area approximately between 1618 to 1630 MST. The outflow boundary moved from
Yarnell Hill Fire Serious Accident Investigation 76