By now most of you have probably heard about the 19 fatalities that occurred on the Yarnell Hill Fire, a wildfire about 90 miles northwest of Phoenix, AZ that has also burned at least 50 homes in the small town of Yarnell, AZ. The 19 firefighters killed constituted nearly an entire crew and makes the Yarnell Hill Fire the most deadly wildfire in the US since the Griffith Park Fire killed 25 firefighters in 1933. Many thanks go out to those firefighters who continue to risk their lives to save lives and property.
With each major fatality wildfire, extensive investigations have led to increasing safety though improved equipment and tactics. Some of these investigations, including the investigation of the 1990 Dude Fire in Arizona, have included improvements to the way that fire weather information is forecasted and disseminated. Today every firefighter undergoes training to learn how weather and fire interact. Incident commanders can request a Spot Weather Forecast from the National Weather Service that is specific to the location of their fire. Large wildfires may even employ an on-site Incident Meteorologist (from the National Weather Service) along with several dedicated surface weather stations providing real time weather data across the fire area.
Weather plays a major role in firefighters' ability to control a fire, and the Yarnell Hill Fire was no exception. Two publicly available weather stations exist near the Yarnell Hill Fire, and the time series for one of them, Stanton, is shown below.
|Temperature and relative humidity at Stanton from Mesowest.com|
|Wind speed and direction at Stanton from Mesowest.com|
The above plots show that on Sunday afternoon the fire area was subject to temperatures near 100 F, relative humidities in the teens, and wind from the southwest with gusts into to 20's. This caused the fire to move to the northeast at about 1/2 mile per hour according to some sources. From the limited information currently available, the Granite Mountain Hotshots were working on the south side of the fire on Sunday afternoon and probably expected the fire to keep moving to the northeast. However, as the afternoon progressed widespread thunderstorms popped up across the northern half of Arizona, moving in a southwesterly direction.
Looking at the above image from the Flagstaff, AZ weather radar one can see these thunderstorms along with numerous outflow boundaries.
Outflow boundaries frequently accompany thunderstorms in the Southwest US and can be diagnosed by looking at weather balloon soundings of the atmosphere. The above sounding was taken at 6 pm Sunday in Flagstaff, AZ. The line on the left is dew point and the one on the right is temperature. The further apart these lines are, the dryer the air is at that altitude. You can see that the moisture supporting the thunderstorms is in the mid-levels of the atmosphere, while the lowers levels (below 550 mb) are very dry. In Arizona on Sunday, precipitation falling from the thunderstorms partially or completely evaporated in this dry air and in doing so cooled the air. This cool air, being more dense than the surrounding air, sank rapidly until hitting the ground and fanning out. After hitting the ground the cool air can travel many miles horizontally, producing dramatic changes in wind speed and direction as it passes by a given point. This is the critical aspect of the weather that led to the tragedy at Yarnell Hill.
|Wind vectors at Stanton from Mesowest.com. Vectors point in the direction of wind flow. Down means a northerly wind.|
Looking back at the Stanton station, between 5 and 6 pm (1700 and 1800 on the above plot) the outflow boundary from the thunderstorm to the northeast of the fire hits the station. It probably hit the fire just minutes before. The wind switches to northeasterly and the gust speed doubles to nearly 45 mph, with sustained winds near 25 mph. A slight drop in temperature and rise of relative humidity is seen as the boundary passes, consistent with an outflow, but temperatures stay in the mid-90's with humidities still in the teens. This is a perfect storm. With the strong, northerly wind pushing it, the fire completely switches direction and starts burning furiously toward the Granite Mountain Hotshot crew, who are on the south side of the fire at this time. There was probably very little time to react.
As a former forecaster and former firefighter, I can appreciate the issues faced by both sides when it comes to forecasting, disseminating, and using weather information to fight fires. Conditions like this were certainly foreseeable the morning before the accident. However, predicting where thunderstorms will form and where outflow boundaries will go, if there are any, is extremely difficult outside of a realm of forecasting called nowcasting. Nowcasting involves making forecasts on the order of hours to minutes based mainly on extrapolating forward the currently observed conditions. It is very useful for weather conditions such as those experienced on the Yarnell Hill Fire, but is very labor-intensive and impractical over large areas.
There are no doubt many factors a play in causing a tragedy like this. Even the influence of the weather still remains to be fully understood. Hopefully the Yarnell Hill Fire will provide an opportunity to make firefighters safer as they protect lives and homes from wildfires.