"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.
This led Mass to look back over satellite photos and regional radar, which at 1 p.m. showed a line of thundershowers moving toward the area from the northeast. By 3:30 p.m., clouds had moved over the fire site. By 4:45 p.m., a pyrocumulus cloud – a towering cloud formed by rising heat from the fire – had punched through the clouds the storm brought. From above, that was a telltale sign the fire had exploded, Mass explains.
Warning signs also were appearing in the output of the high-resolution, rapid-response forecast model from the National Weather Service (NWS), Mass notes. It put the storm over the fire site about the same time that clouds began to cover the area.
The model is designed to provide fresh 18-hour forecasts once an hour. Another forecast model, at the University of Arizona, yielded similar results.