Lightning storms are common in the Southwest in the summer months due to the annual monsoon pattern. CLIMAS, the Climate Assessment for the Southwest, describes how the Southwest monsoon works:
The monsoon is driven by the sun heating up the land and the Pacific Ocean at different rates, with land surfaces warming more quickly than the ocean. The warm land creates low-pressure zones as hot air rises. Once this pattern establishes across the region, the winds shift to fill in the vacuum.
Effectively, the monsoon helps form low pressure over the land surface which promotes rising air and thunderstorm development.
Although the monsoon pattern sometimes brings beneficial rains to the region, the associated lightning and strong winds can generate hazardous fire weather conditions.
“In the Southwest, lightning has ignited more than 2,300 fires annually since 2001, burning approximately 277,000 acres per year,” adds CLIMAS.
If it was monsoon-driven lightning that triggered the fire, it was wind, dry air, and extreme heat that helped fan the flames.
Winds were gusting between 21 and 28 mph at Prescott, Arizona Sunday afternoon (about 30 miles northeast of Yarnell), when a thunderstorm was reported in the area.
The onset of these winds and a shift in the prevailing wind direction are thought to have helped the fire grow rapidly.
Brian Klimowski, meteorologist in charge of the National Weather Service’s Flagstaff office, said the wildfire area experienced a sudden increase and shift in wind around the time of the tragedy. It’s not known how powerful the winds were, but they were enough to cause the fire to grow in size from 200 acres to about 2,000 in the matter of hours Sunday
Update, 12:30 a.m. Tuesday: Data from a weather station, part of a localized network or “mesonet”, very close to Yarnell show winds doubled in speed (with gusts increasing from 22 mph to 41 mph) and shifted directions (from southwest to north-northeasterly) between 4 and 5 p.m. mountain time. Hat tip: CIMSS Satellite Blog
Yarnell Hill wildfire timelapse. Created by Matt Oss, posted to YouTube
Despite the lightning and the wind, the thunderstorm produced little to no rain to quell the blaze. Just 0.01 inches of rain was recorded at Prescott, due to extremely low relative humidity levels. In the hours before the storm struck, the relative humidity was a bone dry 10-13 percent. Most of the rain falling through the clouds evaporated in this arid atmosphere.
This dry air was also extraordinarily hot. Prior to the thunderstorm, the temperature in Prescott had surged to 102, tying a record high for the date, set in 1950.
The extreme heat resulted from a bulging heat dome – or area of high pressure at high altitudes – which led to all-time record heat in Las Vegas that same day and the hottest temperature ever recorded in the U.S. during June in Death Valley (129 degrees).
When you mix lightning, wind, dry air and heat, the fire weather environment was explosive. And one final factor likely led to the fast growth of the flames: parched ground.
Much of the region around where the fire erupted is classified in moderate to severe drought. The dry ground turned vegetation on the ground into combustible fuel for the wind-driven flames.
Climate scientists have warned an increase in hot, dry conditions due to building greenhouse gas concentrations is likely to increase the risk of dangerous fires in the West.
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A video discussion about the fire