Flagstaff Fire: Blaze at 228 acres, 2,400 pre-evacuation orders for south Boulder. "We're about one ridge over from the city of Boulder," Boulder County sheriff's spokesman Rick Brough said of the fire at a news briefing this afternoon. The fire started near the 1500 block of Bison Drive in the Walker Ranch area around 1:15 p.m. and is believed to have been sparked by lightning, Brough said.