Lightning strikes are rare in Northern and Central California — so infrequent as to be overlooked by science.
But the subject has been of urgent interest since August 2020, when a massive complex of thunderstorms thrashed its way across the state, dropping not rain but thousands of bolts of “dry lightning”: cloud-to-ground strikes without accompanying rainfall exceeding one-tenth of an inch (2.5 millimeters). The effects were predictable, immediate and immense: wildfires, 650 in total, burning upward of 2 million acres.
The first in-depth look at the region’s dry lightning events was published this month, prompted by that historic event. For a sunbaked land now deep into a drought, the top-line findings are ominous: There may be more of these strikes than realized.
“Our team knew dry lightning happens in California during the summer,” said the paper’s author, Dmitri Kalashnikov of Washington State University at Vancouver. “But we didn’t know that it would be almost half (46 percent) of all lightning strikes in 34 years that were dry.”
Here’s what to know about dry thunderstorms and how they increase wildfire risk
Previous studies have shown that while Southern California sees more human-caused wildfires, lightning-caused fires are more prevalent in the northern section of the state, particularly over mountainous terrain.
There’s currently one active lightning-sparked wildfire in California: the Six Rivers Lightning Complex, about 30 miles east-northeast of Eureka. It had burned more than 27,000 acres as of Tuesday morning and is about 80 percent contained. It began the evening of Aug. 5, when thunderstorms touched off 11 blazes.
However, Kalashnikov said the amount of lightning activity throughout a summer does not increase the probability of a mega-event.
“It just takes a one- or two-day outbreak … to really set off a very costly and destructive wildfire season,” Kalashnikov said. He added that 2020 was a slow lightning season overall — “but that didn’t matter.”
Here’s what to know about dry thunderstorms and how they increase wildfire risk
When the smoke from the fires that year cleared, Kalashnikov discovered that the topic of dry lightning in California was ripe for examination.
“Lightning is just so uncommon … west of the Sierra Nevada and Cascades; a number of studies into lightning in the Western U.S. actually gray out that area because of not enough of a sample size,” Kalashnikov told The Washington Post. “Consequently, there hasn’t actually been sort of a comprehensive climatology of dry lightning in these lowland areas.”
For example, where the Six Rivers Lightning Complex is burning, the average square kilometer receives cloud-to-ground lightning perhaps three times every century. Yet in the monsoon bull’s eye of eastern Arizona, an area of similar size, the average square kilometer receives five strikes a year.
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Six Rivers National Forest, namesake to the fire, measures 957,590 acres and gets an annual average of 0.03 lightning strikes per square kilometer — or about 100 bolts striking within its boundaries every year.
Kalashnikov’s team learned that not only does elevated terrain (above 2,000 meters) get more strikes from dry lightning than lower elevations (below 1,000 meters), but their months of peak activity are also different. That’s based on an assessment of data from the National Lightning Detection Network and of precipitation totals found in the high-resolution gridMET data set.
“The higher elevations, like the Sierra Nevada, they get most of their dry lightning strikes in July and August, sort of during the monsoon season, and then by September and October, their dry lightning mostly goes away,” Kalashnikov said. “Whereas, in contrast in the lower elevations … it’s kind of an ongoing dry lightning season. So, whether you’re in June or July or August or September, you get about the same amount of dry lightning strikes as the other months.”
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Kalashnikov also looked for large-scale atmospheric patterns on days with widespread dry lightning. Applying a clustering technique to the 124 largest outbreaks, he found four patterns — and in all, there was mid-tropospheric high-pressure ridging centered over different portions of western North America. Additionally, three of the four have some kind of troughing features.
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“One of the takeaways is that, yes, the type of large-scale weather pattern that sets up affects the risk or the likelihood of dry lightning in different parts of California,” Kalashnikov said.
Kalashnikov said the next step in his research would be to develop forecasts and climate model projections.
“Somebody could take these patterns we’ve identified, that we know can produce dry lightning in this part of California,” Kalashnikov said, “and they can look at climate models and see if these patterns are increasing in frequency or … decreasing.”
Kalashnikov plans to expand the scope of his study across the western United States, as well as look at the precipitation amounts occurring when lightning starts a fire.
“There’s this commonly accepted threshold for dry lightning, which we use in this paper, of 0.1 inches or less of rainfall — but that varies,” Kalashnikov said. “We know it varies based on the kind of vegetation and how dry the vegetation is where the fire happens.”
That’s something he would like to quantify.
