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Fires due to humid, stormy weather now more common in California

by Hannah Hagemann, Santa Cruz Sentinel |

A little bit of moisture goes a long way in increasing the likelihood of thunderstorms, and subsequent wildfires, according to new research out of the Department of Energy’s Pacific Northwest National Laboratory.

Atmospheric scientists with the Pacific Northwest National Laboratory analyzed environmental and meteorological data, looking at temperatures, wind patterns as well as precipitation and humidity levels that preceded wildfires in California.

“We found a large number of fires related to conditions that are generally wet, where you have onshore winds with a lot of moisture blowing from the ocean to land,” study author Ruby Leung said.

“These conditions can create thunderstorms and dry lightning, but these storms aren’t big enough to produce a lot of rain,” Leung explained.

Just enough humidity combined with onshore winds can trigger lightning, and in an area with years of built-up fuels wildfire can ignite, as it did in Santa Cruz and San Mateo counties Aug. 16.

The researchers found that the area burned by wildfires associated with onshore winds, humid weather and thunderstorms – or hot-wet wildfires – increased by 7.5 percent per year across California from 1984 through 2017. That year, hot-wet fires burned approximately 300,000 acres Leung said.

“Consistent with the increase in wildfires during hot-wet days, we also see an increase in the number of lightning strikes over California by 2.8 percent per year,” Leung said. “A lot of wildfires are triggered by lightning, but they don’t necessarily burn a huge area.”

Over the 30-year study period, these humid and wet wildfires ignited just 12 percent of the time. That’s compared to 60 percent of fires that occurred on hot and dry days, when humidity is low, the sky is clear, and temperatures are high, Leung said.

But Leung said, the same type of associated humid and wet conditions that precede fires such as the CZU Complex, has been increasing in frequency, more quickly, compared to their arid counterparts.

“Although hot-wet wildfires only account for 12 percent of all wildfires in California, this type of wildfire has been increasing at a faster pace than the more common hot-dry wildfires,” Leung said.

Leung and her coauthors also found that in the last 30 years, soil moisture levels decreased across the state, and temperatures rose.

In the short time, Leung said she hopes this research could tailor wildland fire forecasts to be more precise.

Next, she aims to examine how different environmental and meteorological conditions comingle in sparking fires. The scientist said no single factor causes a wildfire to ignite, but rather a combination.

“If we can establish the relationship between wildfires and these kind of meteorological conditions, we can also better connect changes in wildfires over time with changes in the environment because of global warming,” Leung said.