Scientists say several factors led to this past summer's deadly firenado in Redding, California. 

Atmospheric scientist Neil Lareau at the University of Nevada, Reno has authored a paper in the scientific journal Geophysical Research Letters documenting the rare firenado, finding a number of factors that combined at just the right time and place to catalyze the deadly fire. These observations may help forecasters and scientists identify – and potentially warn – for future destructive fire-generated vortices.

“This paints a clear picture of the sequence of events leading to the vortex development and intensification,” Lareau said. “This sequence suggests the Carr Fire vortex may qualify as pyro-genetic tornado, and not merely a tornado-strength fire-generated vortex.”

In his study, satellite and radar observations document the evolution of the vortex revealing similarities to tornado dynamics. A key factor in the vortex formation was the development of a fire-generated ice-topped cloud (known as a pyro-cumulonimbus) which reached as high as 39,000 ft. The study saus the development of the cloud helped stretch the underlying column of air, concentrating the rotation near the surface and causing the tornado strength winds, estimated at 143 mph, the strength of an EF-3 tornado.

Radar data show that the vortex formed along the fire perimeter and within a pre-existing region of wind shear immediately following rapid vertical development of the convective plume, which grew from four to eight miles high in just 15 minutes. The plume development was fueled by the onset of the pyro-cumulonimbus cloud, a process similar to the development of an ordinary thunderstorm. It is this link to the cloud aloft that distinguishes the Carr fire vortex from more frequently observed fire-thirls, which tend to be smaller and less intense. The only other documented case of a “firenado” is during the Canberra Firestorm of 2003 in New South Wales, Australia

The data in the study came from National Weather Service NEXRAD radars located at Beale Air Force Base, in northern California, Eureka, California and Medford, Oregon. While radar data has been used to study many other wildfires with pyro-cumulonimbus clouds, this is the first instance of NEXRAD radars observing the structure and evolution of a tornadic fire-generated vortex itself. 

Other factors contributing to the firenado include: 

  • Exceptionally low fuel moisture due to unusual warmth throughout July. 
  • Climate impacts of a five-year drought followed by a near record wet season that produced an abundance of vegetation, then another exceptionally dry winter 
  • A near-surface cyclonic wind shear zone that developed in the hour prior to vortex formation 
  • The release of moist instability in the fire-generated cloud aloft  
  • Weather factors including very low humidity, record high temperatures, and terrain-channeled winds due to low atmospheric pressure. 

The Carr Fire started on July 23, and killed eight people and destroyed 1,079 residences. The fire was 100% contained by August 30th after burning 230,000 acres. 

“With the impacts from this fire, a discussion and studies are warranted about the potential to warn for future tornado-strength vortices,” Lareau, an assistant professor in the Physics Department of the College of Science, said. “In this case, the availability of high resolution radar and satellite observations provide advance indications for vortex formation such that watches, or even warnings, may have been possible.”

In the future, Lareau said communities might develop routines to inspect radar data for evidence of rotation in wildfire plumes, and satellite data of fire-cloud formations and storms. 

Funding for this work was provided, in part, by the National Science Foundation and the University of Nevada, Reno Research and Innovation Office. 

The research article, “The Carr Fire Vortex: A Case of Pyrotornadogenesis?” was published in the American Geophysical Union’s scientific journal Geophysical Research Letters.

(University of Nevada, Reno contributed to this report.)