"The winds were coming from the southeast, blowing to the west, away from Yarnell and populated areas. Then the wind started to blow in. The wind kicked up to 40- to 50-mile-per-hour gusts, and it blew east, south, west – every which way," Prescott, Ariz., city councilman Len Scamardo told the AP. "What limited information we have was there was a gust of wind from the north that blew the fire back and trapped them."
The question for meteorologist Cliff Mass, at the University of Washington in Seattle, is whether anyone had pointed out in advance the strong likelihood that this would happen. The downdraft, known as a gust front, came from an approaching storm and served as a blacksmith's bellows, quickly reversing the wind direction that had prevailed through much of the afternoon and suddenly turning the fire back onto the firefighters.
"We have satellite images every 15 minutes and radar every six minutes," says Dr. Mass, a meteorology professor at the university who performs weather forecasts for firefighters in the Pacific Northwest. "This was not a difficult forecast."
Balloon-based measurements of the atmosphere taken at Flagstaff, Ariz., at 1 p.m. local time Sunday showed that in terms of the potential for powerful downdrafts should thunderstorms build, the atmosphere was loaded for bear.
A remote weather station about five miles from where the fire started and set up to monitor conditions in potentially fire-prone areas tells the story from the local perspective, Mass says.
From midday through 5 p.m., winds were blowing generally out of the south at about 30 miles an hour, driving the fire away from Yarnell. At 5 p.m., the wind abruptly shifted, blowing out of the north at about 40 miles an hour with gusts up to 65 miles an hour. At the same time, measurements of sunlight had been falling in an uneven, stair-step pattern since about 2 p.m., indicating that clouds were spreading over the region.