Firefighters have been struggling to combat the blaze, hampered by strong, erratic winds and rounds of thunderstorms. On Wednesday, the fire was 9 percent contained.
But fire officials have been getting a little bit of help, thanks in part to the fire itself. Twice in two days, the Hog Fire has helped produce its own thunderstorms, which then dumped rain on the flames below. Hail even pelted the wildfire Tuesday, due to a hybrid storm that was both occurring on its own and exacerbated by the wildfire plume.
A week of wild weather
On Monday afternoon, the Hog Fire began producing its own precipitation shortly after 4 p.m. local time. The fire’s smoky updraft plume towered into a hulking “pyrocumulonimbus cloud,” or a fire-induced thunderhead.
The surrounding environment did favor thunderstorms — so scattered thunderstorms might have formed in this region regardless. But Monday’s thunderstorm near Susanville was specifically triggered by the wildfire, thanks to the heat from the blaze inducing a strong upward motion.
Doppler radar allows meteorologists to peer into thunderstorms, gleaning valuable information about the size, shape and motion of the water droplets, ice crystals, or, on occasion, smoke and ash particulates contained therein.
By using a radar product called the “correlation coefficient,” we are able to determine how uniform the materials inside a cloud are. All rain or all snow will yield high correlation and values near 1; mixed precipitation will see slightly lesser values.
For an assortment of tornadic debris, or smoke, soot or ash, values will be lower. Those are plotted in blue (as shown in the bottom panel of the image below).
Neil Lareau, a professor of atmospheric sciences at the University of Nevada at Reno who studies PyroCb clouds, tweeted a radar animation (above) showing the Hog Fire’s smoke plume producing its own rain shower — with lightning — Monday.
Notice (in the bottom panel) the cloud is depicted as blue, denoting low correlation values corresponding to smoke or ash, with a pinprick of red eventually emerging and expanding in the middle. That’s indicative of rain beginning to fall, with higher correlation values showing up as more uniform rain droplets fall to earth.
In a zoomed-out version, you can see nearby ordinary storms resolved in red, while the plume associated with the Hog Fire stands out in blue.
A hybrid thunderstorm drops hail Tuesday
On Tuesday, the Hog Fire did it again, this time with a bit more help from the atmosphere. Conditions favored strong to locally severe storms, and several feisty storm cells developed in Lassen County.
Near Susanville, which was included in a severe thunderstorm warning, a pair of updraft plumes soared into the sky.
Except one was a smoke plume, and one was an ordinary thunderstorm.
The two became entangled, and the resulting thunderstorm dropped hail and stirred up gusty winds in the community.
Remarkable photos emerged of dime-size hail peppering a charred ground, logs continuing to burn as the landscape singed by the fire smolders.
“It is not rare for a fire to create its own [thundercloud],” said Alex Hoon, a meteorologist with the National Weather Service in Reno. He is frequently dispatched into the field as an incident meteorologist to help with fire weather forecasts.
“It does happen from time to time, it is something that we actually have to forecast,” said Hoon. “Part of what we have to teach to the firefighters, we teach them that fires can create their own weather.”
Hoon noted that, while fires release an exorbitant amount of heat, environmental moisture is usually the limiting factor precluding the development of a PyroCb cloud.
While Monday’s thunderstorm was a bona fide smoke plume spitting out lightning, Hoon said that Tuesday’s was a bit more of a hybrid setup.
“We have severe weather going on anyway,” recounted Hoon. “[A thunderstorm] could have combined with the updraft from the fire.”
That’s supported by radar imagery. With a product known as differential reflectivity, we are able to probe the ratio of an object’s height to its width. That’s because “dual polarization” radars emit two pulses of energy — one horizontally and the other vertically.
Values in the 1-to-3 range, plotted in blue and green, mark rain — which flattens when it falls. But with smoke, the differential reflectivity should be highly irregular and varied.
That is evident on radar that shows the largely white, gray and colorful aberrations of the smoke cloud contrasted against ordinary rain showers. Initially, the northwestern updraft is primarily smoke, but over time it is entrained into the main convective, weather-driven updraft.
Thunderstorms in the vicinity of fires can be both good and bad. On one hand, rain is beneficial in tempering a fire. But conversely, erratic winds and lightning can both worsen ongoing fires and ignite new ones. In addition, shifting and gusty winds can put firefighters in danger.
“When you get a thunderstorm over the fire, it will moderate fire activity somewhat,” said Hoon. “The rain will help to moisten up some of the finer fuels … the grasses and the small sticks. They get affected by the change of relative humidity within a matter of hours.”
Larger trees, however, respond to humidity changes over hundreds or thousands of hours, explained Hoon — so, it will take more than a couple of quick downpours to make a meaningful dent on the area’s fire risk.
Hoon says firefighters are trying to seize the opportunity to make progress battling the blaze.
Fire weather can get even more wild than generating thunderstorms or mixing smoke plumes with preexisting storms. In 2018, for example, the Carr Fire near Redding, Calif., produced a bona fide fire tornado. Its deadly 140 mph winds left a damage swath more than a third of a mile wide.
On May 11, 2018, a wildfire in the Texas Panhandle produced a severe and rotating, thunderstorm that brought hail west of Elk City, Okla.