Nearly two months after it broke out and a month after firefighters contained it, fire investigators said Tuesday they have determined lightning was the probable cause of the nearly 27,000-acre Pole Creek Fire south of Sisters.
The wildfire was discovered on the morning of Sunday, Sept. 9, and ultimately grew to 26,795 acres, putting hundreds of residents south of Sisters on pre-evacuation alert for weeks and sending thick smoke into Sisters and at times other communities, including Bend and Redmond.
The fire investigation team concluded the probable cause of the fire to be the result of lightning from a small isolated thunderstorm in the area of Pole Creek Trailhead late on Saturday, Sept. 8.
"The team’s conclusion comes from numerous eye-witness interviews, lightning occurrence records, the exclusion of all other probable fire ignition possibilities," the announcement said.
To reach that conclusion, the investigation team analyzed evidence of lightning strikes at the origin area.
Officials said the investigation team was challenged by the complete consumption of fuels at the origin, which left no physical evidence.
A large diameter fir tree that burned and broke off at the base was present in the specific origin. The interior of the tree was burned out.
Additionally, numerous trees with lightning scars were present in the general origin area, indicating the propensity of lightning to strike that area.
Eyewitness interviews, photographs, and videos documented an isolated thunderstorm, which built over the Pole Creek Trailhead late in the day on Sept. 8, and further supported lightning as the cause of the fire. Several witnesses independently reported lightning ground-strikes in the area.
The possibility of holdover lightning was considered by the team because lightning strikes were recorded late on August 5, within the vicinity of the Pole Creek Fire's origin.
However, because of the significant time period between the last documented lightning storm and the ignition of the Pole Creek Fire, along with the extreme fuel models and conditions at the origin, the team determined it was unlikely the cause was holdover lightning.
The origin of the Pole Creek Fire is quite remote, a distance from roads and trails, and the dense fuels and ground cover in the area prior to the fire would make access to the area extremely difficult and time consuming, reducing the likelihood that the fire was intentionally set, officials said.
The intensity of the fire activity, safety considerations, and the large number of witness interviews contributed to the length of time the investigative team needed to conduct a thorough investigation.
The fire investigation team consisted of Washington and Oregon Forest Service special agents and law enforcement officers, as well as an Oregon Department of Forestry/Oregon State Police fire investigator.
Next Tuesday, the Deschutes National Forest, Sisters Ranger District, will host a public open house to discuss several items related to the Pole Creek Fire.
The open house will be from 5 to 7 p.m. on Tuesday, Nov. 13, at the Sisters-Camp Sherman Fire District Office, 301 South Elm Street, Sisters.
Forest Service employees will staff tables to answer questions on a variety of issues related to the Pole Creek Fire including:
- long-term and short-term restoration of the fire area,
- impacts to recreation sites and activities and how the impacts may be addressed,
- road closures and area closures and what the public can expect,
- an overall view of the progress of the fire and actions taken during the fire,
- smoke management,
- anticipated prescribed fire and hazardous fuels treatments in the Sisters Ranger District,
- the investigation into the cause of the fire,
- and how to create defensible space around your home.
For more information, the public may contact the Sisters Ranger District at 541-549-7700.