Fire, lighting across Sisters Country

The wildfire that started on Sunday morning, September 9, near the Pole Creek Trailhead southwest of Sisters tripled in size from Friday afternoon, September 14, to late Saturday evening. By press time the area of burn was estimated to have grown to approximately 17,500 acres.

Fire officials blame "active weather" Friday afternoon into early Saturday for the fire's rapid spread.

Pole Creek Fire Incident Meteorologist Jon Bonk explained that lower relative humidity and gusty winds associated with a passing heat low Friday afternoon and evening allowed the fire to spread.

Of particular interest was the lightning activity in Sisters Country Friday evening. Many wondered if the fire actually caused the thunderstorms to form. While large, intense wildfires can generate thunderstorms, Bonk says the activity we experienced on Friday evening would have occurred even without the fire.

Bonk explained that a heat low at the surface, and a passing weak upper-level trough resulted in an unstable atmosphere on Friday evening. This allowed columns of rising air near the surface to keep ascending into the upper levels of the atmosphere until the resulting cooling temperatures caused water vapor to condense into clouds. The additional heat released by this process allows the ascending air columns to rise even higher, triggering the thunderstorms.

Steve Smith, who lives in the Squaw Creek Canyon Estates development located approximately six miles northeast of Sisters, witnessed the light show from his hot tub. "It was about 9:30 p.m. when I saw the first lightning flashes almost directly overhead. Then I saw 10 or more flashes to the northwest, about a quarter-mile away," says Smith. "There were several distinct lightning bolts that came from the cloud, while many other flashes seemed to light the cloud from within," he adds. Smith says the show lasted for about 45 minutes as the activity slowly drifted off to the northeast.

From her home in Camp Sherman, Joan Weiler saw numerous flashes to the southeast from about 8 to 9 p.m.

"The flashes seemed to come from the same place in the sky; they didn't move, and I didn't hear any thunder," said Weiler.

Available weather data from stations in Central Oregon show that these storms were generally moving from southwest-to-

northeast. At 11:56 p.m. on Friday evening, Redmond reported a thunderstorm and 0.03 of an inch of rain. Eighteen minutes later the thunderstorm had reached Madras (precipitation at this station was not available).

It is believed that Friday evening's lightning strikes may be responsible for a new 200- to 300-acre fire on the Warm Springs Reservation.

Bonk said that cumulus clouds were beginning to form on Friday afternoon as they moved from south to north, generally paralleling the Cascade mountains. This was an indication that the atmosphere was beginning to destabilize prior to the thunderstorms that were to occur later that evening.

Meteorologist Jon Bonk is on loan to the Pole Creek Fire Incident team from the National Weather Service office in Portland.