Behind the weather that led to the deadly Yarnell Hill Fire

A wildfire burns homes in Yarnell, Ariz. on Sunday, June 30, 2013. An Arizona fire chief says the wildfire that killed 19 members of his crew near the town was moving fast and fueled by hot, dry conditions. The fire started with a lightning strike on Friday and spread to 2,000 acres on Sunday amid triple-digit temperatures. (AP Photo/The Arizona Republic, David Kadlubowski)

A wildfire burns homes in Yarnell, Ariz. on Sunday, June 30, 2013. An Arizona fire chief says the wildfire that killed 19 members of his crew near the town was moving fast and fueled by hot, dry conditions. The fire started with a lightning strike on Friday and spread to 2,000 acres on Sunday amid triple-digit temperatures. (AP Photo/The Arizona Republic, David Kadlubowski)

Nineteen firefighters are dead after a fire erupted in Yarnell, Arizona Sunday. The Arizona Republic says this is the worst firefighting disaster in the state’s history and worst in the U.S. since 25 people died fighting a fire in Los Angeles in 1933.

Lightning sparked the fire and strong winds, dry air, record heat and severe drought all share in the blame for its fury.

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:

Surface weather map simulation from the GFS model showing monsoon low over western Arizona which helped thunderstorms develop over the central part of the state. (WeatherBell.com)

Surface weather map simulation from the GFS model showing monsoon low over western Arizona which helped thunderstorms develop over the central part of the state. (WeatherBell.com)

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.

Reports the Associated Press:

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).

Weather map simulation showing very high pressures at high altitudes over the Southwest which led to the record hot conditions (WeatherBell.com)

Weather map simulation showing very high pressures at high altitudes over the Southwest which led to the record hot conditions (WeatherBell.com)

Arizona drought assessment as of last Wednesday.  (National Drought Monitor)

Arizona drought assessment as of last Wednesday. (National Drought Monitor)

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.

See related posts:

Arizona Wallow fire largest in state history; climate change projections suggest far worse in pipeline (from 2011)

Southwest drought, climate warming and fuel: an explosive combination for record wildfires (from 2012)

A video discussion about the fire

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