Archive for July 2013
For the past week, the western United States has been faced with a searing heat wave, with temperatures soaring well over 100°F and even over 120°F in spots. This is thanks to a stubborn ridge of high pressure, seen in the figure below as a crest in the 500 hPa (mid-troposphere) wave pattern that is “stuck” over the western portion of the continent. This is in contrast to the eastern half of the U.S. that has been experiencing below-average temperatures.
Along with the oppressive heat, lightning associated with dry thunderstorms have ignited numerous wildfires throughout the region. Extremely dry, windy, and hot conditions have fueled the wildfires and posed a significant challenge to the wildland firefighters who are on the front lines.
This past Sunday, June 30, the Granite Mountain Hotshots, an elite team of 20 wildland firefighters, lost 19 of their members in a severe tragedy in the Yarnell Hill Wildfire. This now ranks as the worst wildland firefighting tragedy since 1933 and is the deadliest firefighting event since 9/11. At the time of this writing, the fire has burned 8,400 acres and is still only 8% contained.
The investigation into what happened is only now beginning, so the details of the firefighters’ last moments are not yet clear. However, what we do know is that the fire shifted direction rapidly and caused the firefighters to deploy their fire shelters, which proved to be no match for the blaze. According to this report from AZCentral:
…winds from the southwest were gusting at 15 to 25 mph at 4:01 p.m. at the monitoring station near the blaze. An hour later, winds were gusting at 30 to 47 mph from the northeast.
According to the Weather Underground archives of MQSTA3 RAWS station records from near the fire, we see that a dramatic wind shift occurred near 5 pm on Sunday, June 30, roughly corresponding to when the firefighters were believed to have deployed their fire shelters.
This dramatic near-180° wind shift from the south-southwest to the north-northeast was accompanied by a wind gust over 40 mph, leaving the firefighters with little time to respond, particularly with their position in a valley between two ridges.
So what was the source of this sudden wind shift?
Despite what may have been initially reported, this was not a serendipitous or random occurrence. A look through archived data from the KFSX radar showed that in the 1.5 hours leading up to the tragedy, numerous thunderstorms were present to the northeast of the fire. Furthermore, many of the storms that dissipated left behind numerous outflow boundaries (fine, light blue lines in the radar image), suggesting the presence of stronger gust fronts that day. An outflow boundary was not detected by the radar near the Yarnell Hill fire most likely because the radar beam that is oriented at 0.5° above the horizon was sampling higher up in the atmosphere by the time it reached that location. However, given the presence of outflow boundaries near most of the showers and thunderstorms around Flagstaff, it would be reasonable to assume that similar features were present ahead of the storms in Yavapai County. The weak radar reflectivities that remained semi-stationary to the southwest of Prescott may have been associated with the fire.
The rapidly decreasing radar reflectivities suggested that the Yavapai County storms were quickly dissipating as they passed Prescott, during the stage of a thunderstorm’s life cycle when downdrafts are likely to be particularly strong. These storms were also clearly evident in GOES-15 visible satellite imagery, as seen here in an animation generated by Dr. Dan Lindsey at NOAA. The red X marks the approximate location of the fire. A more technical discussion of satellite observations is available from the CIMSS Satellite Blog.
This dramatic, time-lapse video was filmed from Congress, AZ, looking toward the northeast at the fire. Toward the end of the video, note the rapid shift of the fire as the gust front passed, which resulted in jumping flames and smoke being blown toward the camera.
So what caused the greater prevalence of these intense downdrafts and associated gust fronts? The 0000 UTC (6 pm MDT) weather balloon sounding from Flagstaff depicted a dry layer of low-level air above the ground, sitting underneath a moist, mid-level layer. The red line shows the vertical temperature profile, and the green line shows the vertical dewpoint temperature profile. Where the lines are closer together indicates more saturated or relatively “humid” air, whereas the lines farther apart indicates drier air.
A similar pattern was evident in the sounding from Tuscon at the same time, with the low-level air being even drier (even greater separation between the red and green lines). This suggested that conditions were favorable for mid-level precipitation falling through the sky to evaporatively cool and accelerate toward the ground and spread out, resulting in particularly strong gust fronts ahead of the thunderstorms that day in Arizona.
Remarkably, the mesoscale evolution and associated wind gusts of these storms were well predicted by numerical models. For example, the NOAA High-Resolution Rapid Refresh model run at 1900 UTC 30 June accurately predicted the wind gusts that would be associated with the thunderstorms approaching the Yarnell Hill Wildfire… four hours in advance! Still images of numerical model output are discussed further in Dr. Cliff Mass’s blog, but the bottom line is that mesoscale numerical models were indeed able to capture gust front dynamics near the wildfire that day.
Although hindsight is always 20/20, it would be wise to consider what could have been done to prevent this tragedy, given that the meteorology was no surprise. Perhaps the results of the investigation should lead to a discussion of how and to what capacity meteorologists can facilitate the decision-making process among fire response teams. When requested by incident commanders, the National Weather Service deploys Incident Meteorologists (IMETs) into the field to help with fire weather forecasting, but one was not present during the fire on June 30 because it was still relatively small and not yet classified as a “Type 1 Incident.” Spot forecasts were provided by the National Weather Service on June 30, but they were not enough to prevent the tragedy. An IMET was not deployed until the following day, yet it would be unrealistic to expect any greater coverage of IMETs for small incidents. So as the formal investigation continues, and given all this advance meteorological evidence of a tragedy in the making, we should ask:
What else can be done to bridge the gap between the science and the decisions?
On a more personal note, although I have never met any of the 19 fallen firefighters, I have friends who were recent wildland firefighters and who have shared with me their passion for being on the fireline. Two months ago, when I taught a FEMA course at the National Emergency Training Center, collocated with the National Fire Academy, I shared the hallways, courtyards, and dining hall with wildland firefighters, whose energy and passion for service were tangible. These individuals are young, energetic teammates who must endure long hours of service in the toughest conditions, but who most likely could not imagine doing anything else. That passion is not unlike the fascination that myself and my fellow meteorologists have when we pursue the most severe storms to improve our understanding of them for the sake of public safety. To the fallen members of the Granite Mountain Hotshots, to their lone survivor, and to all of their family and friends, I offer my most heartfelt condolences. Their sense of service is an inspiration to us all and will live on in our hearts and minds.
For the latest updates on the ongoing investigation of this tragedy, please refer to this website: www.YarnellFallenFireFighters.com.
Update 7/18/13: Initial report about the Yarnell Hill Fire resources deployment, released by the Arizona State Forestry Division on 15 July 2013. Download PDF