The Science Behind The ‘Fire-Induced Tornado’ Warning In California

Meteorology is a relatively young science so we are always seeing interesting things within my discipline. This weekend is a good example. The National Weather Service in Reno, Nevada issued a tornado warning for southeastern Lassen County, which is in Northern California. While tornado warnings are already rare in California, what made this one particularly fascinating is that it may have been associated with or caused by the Loyalton wildfire. Let’s explore further.

The San Francisco Chronicle reported that The Loyalton Fire began Friday and by late Saturday evening exceeded 20,000 acres. Around 2:35 pm PDT, the National Weather Service issued a tornado warning. Specific wording of interest to meteorologists or anyone fascinated by the weather is this line in the warning, “At 228 PM PDT, a pyrocumulonimbus from the Loyalton Wildfire is capable of producing a fire induced tornado....” There are images all over social media of the Loyalton Fire. Meteorologist Paul Delegate has an outstanding one posted on his Facebook page. Ok, let's take a step back and decode some of the meteorological jargon in that tornado warning.

The picture above shows a pyrocumulonimbus (cumulonimbus flammagenitus) clouds associated with the Loyalton fire from a CalFire remote camera. According to NASA’s website, “Pyrocumulonimbus clouds are similar to cumulus clouds, but the heat that forces the air to rise (which leads to cooling and condensation of water vapor) comes from a fire instead of the sun-warmed ground.” These wildfire-generated thunderstorms will typically have very cold cloud top temperatures (-40°F or cooler) according to NASA.

While fire tornadoes are not new, KCRA News and other media outlets are reporting that it is the first fire tornado warning issued in the state of California and perhaps ever. WeatherFlow, Inc. meteorologist Shea Gibson tweeted the weather radar image below showing the likely pyrocumulonimbus system and the tornado warning box. How does a fire tornado happen?

For more information on the process of “pyrotornadogenesis,” I highly recommend this 2018 study in the American Geophysical Union journal Geophysical Research Letters. It documented a fire-generated vortex associated with the Carr fire in 2018. For now, I will try to convey the basics. Pyrotornadoes form from pyrocumulonimbus clouds by taking pre-existing spin (vorticity) at the surface and stretching it upward through a combination of strong upward motion (updraft) and intense latent heat release (energy associated with phase changes of water vapor to cloud water). According to the aforementioned study, this is different from other fire-generated vortices (commonly identified as fire whirls, firenadoes fire devil, fire swirls or fire twisters) which may focus spin only through fire-related processes.

While technically, these fire tornadoes differ from the classic Great Plains tornadoes that you may envision, these fire-induced vortices come in different manifestations and can be dangerous. They are undoubtedly fascinating.