Relatório do Acidente ocorrido no Incêndio Florestal de Yarnel Hill

Weather: The Southwest region of the United States experiences a weather phenomenon
known as the Summer Monsoon. The monsoon period represents a switch in wind patterns
from a drier westerly flow to a moist southerly or easterly flow. This typically occurs from mid-
June through mid-July and generally lasts into September. During the initial stages of the
monsoon, drier thunderstorms are dominant but generally give way to larger footprints of
wetter storms. Gusty outflow winds dominate the drier thunderstorm period. Storms typically
form over the higher terrain such as the Mogollon Rim and then try to move off the higher
terrain as the day progresses. Terrain features pertinent to thunderstorm formation include the
Bradshaw Mountains and Mogollon Rim. There is typically a one to two-week period when the
moisture shift is dynamic and provides highly variable weather.
Temperatures are typically hot just prior to and during this transition. Relative humidity values
will start out low and then fluctuate widely as storms and cloud cover become more numerous.
Winds are highly variable during this period with highest wind speeds tied to thunderstorms.
These winds, known as downdrafts, micro-bursts, outflows, and gust fronts, generally deliver
erratic wind shifts and short bursts of strong speeds.
The dynamic monsoonal transition started the last week of June. On June 28, widely scattered
thunderstorms formed over the Mogollon Rim. The cells tracked southwestward bringing cloudto-ground
lightning to the Yarnell area (Figure 5). The region was in its initial stages of a
significant heat wave prior to the thunderstorm. The Phoenix airport, about 110 miles southeast
of Yarnell, reported a high of 110 o F, 4 o above normal. The fire area reported high temperatures
between 101 o to 107 o F. Humidity was low, ranging between 8 to 10 percent.
During the thunderstorm
on June 28, temperatures
fell and humidity values
rose. The Peeples Valley
weather-observing site
seven miles north of
Yarnell reported
measurable rain (0.04").
Storms generally were drier
and produced gusty
outflow winds. The RAWS
reported gusts between 38
to 43 mph during the
thunderstorm passage.
Figure 5. Cloud-to-ground lightning observed between 0900 to 1800
MST, June 28, 2013. Graphic courtesy of BLM.
Temperatures increased on
June 29. The Phoenix
airport reported a high temperature of 116 o F, 10 o above normal and 1 o away from tying the
record for that day. Storm coverage was lighter with less intensity than the 28. Storms did not
Yarnell Hill Fire Serious Accident Investigation 68