|Above: A home burns along Sunflower Road during the Carr Fire on July 27, 2018, in Redding, California. Image credit: Justin Sullivan/Getty Images.|
“It’s one of the most amazing things I’ve seen in my career in meteorology,” said weather.com’s Jon Erdman. That’s been the refrain of many a weather scientist over the past few days since the Carr Fire near Redding, California, produced an incredibly powerful “fire whirl.” Between about 7:30 and 8:00 pm PDT Thursday night, July 26, a tornado-like vortex developed along the fire front just a few miles north of downtown Redding. According to a preliminary survey led by the National Weather Service and CAL FIRE’s Serious Accident Review Team (SART), the vortex produced surface winds that exceeded an astonishing 143 mph, making it the equivalent of an EF3 tornado on the Enhanced Fujita Scale.
“Preliminary reports include the collapse of high tension power line towers, uprooted trees, and the complete removal of tree bark,” noted the survey team.
The term “fire whirl” usually refers to brief spin-ups that are fairly common along the leading edges of large, intense forest fires. Driven by the buoyancy of the fire-heated air, and the contrast between the fire plume and the adjacent ambient air, these whirls can be dramatic, but they’re usually quite modest in scale: typically 30 to 150 feet tall, 10 to 50 feet wide, and lasting only a few minutes.
The vortex of July 26 was far stronger and longer-lasting, to the point where one wonders if it was something else entirely—perhaps a bona fide tornado generated by a fire. The quest for an answer involves some vocabulary, some atmospheric dynamics, and a dollop of history.
|Figure 1. Summary of the preliminary survey of the Carr Fire vortex. Image credit: NWS/Sacramento|
What is a tornado?
The AMS Glossary, published by the American Meteorological Society, defines “tornado” as “a rotating column of air, in contact with the surface, pendant from a cumuliform cloud, and often visible as a funnel cloud and/or circulating debris/dust at the ground.”
Fire whirls can extend hundreds of feet from the ground, but they aren’t typically connected to a cumulus-type cloud higher up. This makes sense: Because the weather driving the worst wildfires is normally hot and dry, the air would have to rise thousands of feet above the surface in order to condense moisture and form the base of a cumuliform cloud.
The main opportunity for a cloud base and a fire whirl to connect would be in a pyrocumulus, the billowing clouds that sometimes erupt above wildfires at their most intense. Pyrocumulus can sometimes grow large and strong enough to produce pockets of oppositely charged particles, a key stepping stone to lightning—in which case the cloud becomes a pyrocumulonimbus (a fire-generated thunderstorm), or in the lingo of specialists, a pyroCB.
Photos taken on Thursday evening show an intense pyrocumulus cloud atop the Carr Fire, so we know there was at least some potential for a cloud-attached vortex. But exactly what happened during those critical hours and moments on Thursday night?
Birth of a monster
Neil Lareau, an assistant professor at the University of Nevada, Reno, has put together a compelling analysis of the Carr Fire vortex based on Doppler radar data from Beale Air Force Base, CA; Medford, OR; and Eureka, CA. “I'm trying to remain somewhat agnostic and let the analysis of the event lead where it may,” Lareau told me in an email. “My initial take is that it bears a lot of similarity to landspout-type tornadoes, but obviously with some added complexities due to the intense (near-surface) convection due to the fire heat fluxes.”
Landspouts, a type of nonsupercell tornado, are the land-based analogue to waterspouts. Most common across the higher elevations of the Great Plains, landspouts differ from the tornadoes produced by classic supercell thunderstorms in that they don’t require strong vertical wind shear. Instead, they form through a strong updraft that stretches and intensifies weak zones of horizontal wind shear present near the ground.
That’s exactly what happened in the Carr Fire, according to Lareau’s analysis of radar data. A broad zone of cyclonic wind shear extended from the surface up to about 1.5 miles aloft roughly an hour before the fire whirl formed. “It appears to be induced by a combination of fire-modified winds and interactions with southeasterly flow up the Sacramento Valley,” said Lareau.
|Figure 2. Development of the intense plume atop the Carr Fire between 6:00 and 7:26 pm PDT Thursday, July 26, 2018, as derived from radar reflectivities picking up on the thick ash within the plume. Image credit: Courtesy Neil Lareau (University of Nevada, Reno), @nplareau.|
Over the next hour, the deep pyrocumulus cloud formed atop the fire at the same time that the zone of cyclonic wind shear intensified and stretched vertically into the pyroCU. Based on radar returns from the thick ash, the top of the fire plume soared as high as 7.5 miles. A glaciated cap similar to a thunderstorm anvil could be seen atop the pyroCB, pointing to the presence of ice crystals. Because this region is well above the freezing level of the surrounding air, Lareau argued, “the pyroCB community classify this as cumulonimbus rather than cumulus based on the temperature alone. I did not see lightning generation, but it is certainly true that many pyroCBs do not produce lightning or precipitation.”
Daniel Swain (University of California, Los Angeles), author of the California Weather Blog, sent me a Twitter message agreeing with Lareau’s diagnosis. “While there may not have been lightning in the pyrocumulus cloud, it did at one point reach as high as 39,000 feet…with a glaciated anvil top and rotating mesocyclone-like feature. I would classify that as functionally a pyrocumulonimbus cloud, even without the lightning. Sometimes the in-cloud charge separation just doesn't yield actual flashes.”
Roger Wakimoto (University of California, Los Angeles) developed the concept of landspouts during research he carried out in the 1980s with the eminent Ted Fujita. If anything, says Wakimoto, the Carr Fire vortex would be toward the top end of the landspout spectrum, based on the limited data available so far. “The pyrocumulus (parent) cloud was very impressive,” said Wakimoto. He added that all but one landspout tornado he’s researched was no stronger than F3/EF3.
Here is another radar rendering of the #CarrFire plume during the destructive vortex. The plume undergoes rapid vertical development, growing from 6 to 12 km (19->39Kft) in 40 min. Thats a lot of stretching and a possible explanation for vortex intensification. #CAwx #CAfire pic.twitter.com/1CTHAvl6Di— Neil Lareau (@nplareau) July 29, 2018
The Carr Fire vortex may have even tiptoed into behavior more akin to a supercell thunderstorm, according to California tornado expert John Monteverdi (San Francisco State University). The animation above, produced by Lareau, tracks radar reflectivity values of 30 dBz (red) and 25 dBz (orange). it appears that the vortex was nestled on the somewhat concave southwest side of the fire, much as a supercell tornado is cradled by dual flanks of its parent storm. Some of the same processes that convert horizontal into vertical vorticity (rotation) in a supercell could have been at work here, accordingly to Monteverdi.
The experts I consulted agreed that there are more than enough clues to suggest that the Carr Fire vortex was far more than a garden-variety fire whirl. “So was it a tornado? I lean toward yes,” said Lareau.
A class of its own?
The scope of the Carr Fire vortex pushes it into rarefied territory. Fire whirls sometimes spin out ahead of fire fronts before they dissipate, but this vortex remained in sync with the fire front and the pyroCB above for the better part of an hour.
The depth and duration of the vortex, along with its powerful winds, appear to be unprecedented for a fire whirl closely observed by radar. Others that weren’t as well documented include one estimated at EF2 strength that accompanied a catastrophic wildfire near Canberra, Australia, in January 2003, and another reported near Burney, California, during the Eiler Fire of August 2016.
Swain marveled at the raw strength of the Carr Fire vortex. He called it “something distinct—and much more extreme—than a traditional ‘fire whirl.’ Given its intensity (winds around 150 mph), persistence (over an hour), and connection to deep-layered rotation within a pyrocumulus cloud, it would not be much of an exaggeration to call this a ‘fire tornado.’
“Regardless of how we classify it formally, it was an extraordinary and locally devastating event.”
NOAA’s Storm Prediction Center (SPC) will be taking an extra-close look at the Carr Fire vortex to determine whether it ought to count as a tornado. “I am not aware of any other fire whirls in the SPC tornado database,” said Patrick Marsh, warning coordination meteorologist at SPC. “I am not sure how we (SPC) will handle this one, as it does not meet the traditional definition of a tornado in the severe storms community, where the cumuliform cloud is primarily driven by latent heat release within deep (moist) convection.” He added: “I am sure we will have ongoing discussions in the near term as to how to best handle/categorize/record this highly unusual event!”
It’s impossible to know what kind of whirls developed on the most terrible wildfires in U.S. history—those that developed during westward expansion in the 1800s, when forests were slashed and burned without heed to fire risk. According to fire expert and author Stephen Pyne (Arizona State University), “There are accounts from the massive U.S. fires of the 19th century in which people used terms like fire tornadoes, fire balloons (probably bubbles of gases that kindled above the main flaming zone), and fire storms (the first use in print that I’m aware of comes from a report on the 1881 Michigan fires from a sergeant with the Army Signal Service—the predecessor to the weather service). Mostly, observers were overwhelmed and trying to flee.”
|Figure 3. This impressive fire whirl developed in California’s Polo Fire on March 7, 1961. “It was interesting on several counts,” said fire expert Stephen Pyne (Arizona State University). Not only was the fire almost completely out of phase with California’s traditional fire season, he noted, but the debris being tossed suggests a more intense circulation than is seen in most fire whirls. The vortex destroyed one house and an orange grove. Image credit: Courtesy Stephen Pyne.|
California’s checkered history of tornadoes and fire whirls
There have been only two confirmed F3 tornadoes in state history, according to Monteverdi. They occurred in Southern California within a few miles of each other and less than five years apart—on August 16, 1973, and February 9, 1978—as noted by weather writer Dennis Mersereau.
However, one of Monteverdi’s students carried out a reanalysis suggesting that damage from a pair of F2-rated twisters in Santa Clara County on January 11, 1951, was consistent with the EF4 level. Thus, Monteverdi said he’s not yet fully convinced that the Carr Fire is the strongest tornado-like vortex in state history.
It’s possible, though not yet confirmed, that at least some of the deaths in the Redding area on July 26 were directly associated with the vortex. If so, this would only be the second tornado or tornado-like feature to result in a California fatality. The other one emerged from a bizarre circumstance almost a century ago—a fire whirl that had nothing to do with wildfire.
On the morning of April 7, 1926, a surge of warm, moist air spread into the San Luis Obispo area. An article in Monthly Weather Review by J.E. Hissong, from the city’s U.S. Weather Bureau office, details what happened next: “At 7:35 am, a very intense discharge of lightning occurred accompanied by a crash of thunder; this was followed instantly by a terrific explosion that shook the town of San Luis Obispo severely….The lightning bolt had struck the large oil reservoirs at the tank farm of the Union Oil Co.., located about 2 ½ miles directly south of the center of town.”
|Figure 4. Two photos of fire whirls that developed during the oil fire of April 7-11, 1926, near San Luis Obispo, California. Image credit: J.E. Hissong, “Whirlwinds at Oil-Tank Fire, San Luis Obispo, Calif.,” Monthly Weather Review (April 1926), courtesy American Meteorological Society.|
Close to 6 million barrels of oil burned over the next five days, leading to a spectacle unlike anything in modern U.S. history. The fire produced what Hissong described as “hundreds of violent whirlwinds, many of tornadic character and force, probably the strangest meteorological phenomenon ever noted in connection with a fire.” According to Hissong, one of these fire whirls moved about 3000 feet out from the fire, picked up a cottage, and dropped it in a field about 150 feet away, killing a father and son who were in the house. “An examination of debris scattered around the Seeber and Banks places indicates clearly that the direction of rotation in these funnels was counterclockwise,” noted the article.
In his epic book Significant Tornadoes 1680-1991, researcher Tom Grazulis concluded, “This was California’s only killer ‘tornado,’ and not a true tornado at that.” It’s just one of the many pieces of California weather history that sit in a new context after the catastrophe in Redding last week.