Some of the lightning was triggered by “dry thunderstorms,” which produce pinpoint bolts of electricity but very little rainfall. Dry air at the low levels means much of the rain evaporates before hitting the ground, allowing some channels of electricity to potentially ignite new wildfires without any water to extinguish the flames.
Here’s what to know about dry thunderstorms and how they increase wildfire risk
Additional dry thunderstorms are possible in the days ahead, blossoming on the nose of monsoonal moisture working throughout the desert southwest. The same humid air mass is wafting north and bringing torrential downpours to the ordinarily arid landscapes of New Mexico, Arizona, the Great Basin of Nevada and Southern California.
“This is one of the top 5-6 events since 2015,” Chris Vagasky, a lightning solutions manager and meteorologist with Vaisala Group, said in an email. “Most of the ‘big’ California days are around 20-30k lightning events.”
According to Vagasky, the National Lightning Detection Network (NLDN) registered 66,897 events across California between late Tuesday night and early Wednesday morning. About a third — 21,768 of them — were cloud-to-ground strokes.
Vagasky noted this latest round surpassed the “lightning siege” of August 2020, which ignited a rash of wildfires across Northern California, in terms of both total lightning events standpoint and number of cloud-to-ground flashes. It also did so over a shorter window of time, meaning temporal flash density was greater — in other words, much higher lightning rates with “sparkier” storms.
Where lightning hits the most in the US in 2021
However, climate scientist Daniel Swain tweeted that the societal impacts will probably not be as severe as the August 2020 episode, which took place after a record-breaking heat wave. He said these thunderstorms were significantly wetter, and “even a modest amount of rain co-occurring with lightning can greatly reduce (though not eliminate) the likelihood of a lightning-caused wildfire ignition.” The storms also occurred much earlier in the fire season, when vegetation isn’t as dry as later in the summer.
Vagasky said most California lightning sieges occur between now and the fall. “The primary culprit is the influx of monsoon moisture,” he wrote, “but if you add an upper level low, like we’ve had the last couple days, you can get extra moisture from the Pacific, in addition to the added lift that storm system provides.”
Upper-level lows are pockets of cold air, low pressure and spin at high altitudes that can destabilize the lower atmosphere. That makes it easier for parcels of air near the surface to ascend and produce towering storm clouds.
Some of the storms brought moderate rainfall to a place that desperately needs the water. Chilao South in Los Angeles County received 1.3 inches, the Cogswell Dam got 0.99 inches, and Palmdale picked up three tenths of an inch. That was mainly in the mountains, however.
Other places experienced no virtually relief: Downtown Los Angeles saw only 0.01 inches, as did Burbank. Long Beach received 0.11 inches.
More dry thunderstorms are likely over central parts of the Sierra Nevada and adjacent Nevada on Thursday. Storms will move more slowly farther to the west, enhancing the potential for saturating a column of atmosphere enough to get water down to the ground.
Some of the storms also produce “hot lightning,” also known as “continuing current” lightning. Most bolts of lightning you see are actually made up of several or even dozens of individual pulses of electric current that follow the same path — hence the flickering appearance of lightning. But sometimes a particular pulse may persist unusually long — for 40 milliseconds or longer — which given its greater duration, has a better chance of heating up its contact point on the ground and producing a wildfire.
“While continuing current is probably the most important factor in whether a fire is started, multiple strokes of lightning to dry brush or wood (correlated with high nonmetal damage potential) can also start fires,” Vagasky said.