October through December, the three months where wind is up and moisture is down, have the strongest correlation with high risk fire days in the Santa Monica Mountains, according to a recent report from LA County Assistant Fire Chief Drew Smith.
The turning of the seasons from summer into fall may be associated with crunching leaves and sweater weather elsewhere in the country, but Californians can usually also expect the smell of smoke in the air as the risk increases for significant wildfire spread over the autumn months.
Last week, Malibu residents were invited to attend a virtual community fire season briefing, where Smith — LA County Fire Department’s fire behavior analyst — shared the department’s outlook for this year’s wildfire season.
The season was projected to begin with normal to above normal monsoonal showers through the middle of September — Smith said the forecast was made even before the recent rain events in the mountains — followed by rainfall “well below normal” from late September through December. Temperatures were predicted to be “well above normal” through December, although Smith specified that “well above normal” could mean as little as three to seven degrees above average temperatures, not dramatic heatwaves. Finally, there were expected to be a “near normal” amount of Santa Ana wind events from October through December.
It is the combination of dry vegetation and Santa Ana winds that drive the most devastating wildfires. While Santa Anas, the dry, hot winds that blow in from the east and drive fires toward the coast, peak in January and February, the start of the rainy winter season means fuel moisture is usually high enough to compensate for the additional fire danger, making October through December the most dangerous period for wildfire spread.
The two most recent highly destructive Southern California wildfires, the Woolsey and Thomas fires, both occurred in that period (Woolsey in November 2018 and Thomas in December 2017).
“The largest winds are usually in January and February; however, we have the least amount of large fire growth due to the fact that we’ve had precip[itation] that has stimulated annual grass, which we consider to be called greenup,” Smith said. “So, when we have green grass, we don’t have the same fire spread probabilities or components as we would in October or November because we don’t have a greenup factor.” According to Smith, vegetation turns greener from February through April, driving down the chance of a wildfire that could grow up to 10,000 acres or larger.
LACoFD teams measure the moisture levels of live fuel in the Santa Monica Mountains and other natural areas around LA County every two weeks by gathering samples, weighing them and baking them in ovens to “cook all the water weight that’s in it” out, before weighing them again. Statistically, live fuel moisture that dips below 75% is considered the high risk threshold. When live fuel moisture falls below 60%, vegetation has reached critical danger. Live fuel moisture fell below 60% in the Santa Monicas in mid-August.
On average, from 1981 through 2022, fuel moisture dips into the high risk zone from mid-September through December, but does not fall below the critical threshold. However, last year, live fuel moisture was “critical” from mid-August through early November.
It takes more than one brief rainstorm to improve fuel moisture conditions.
“If we don’t get two inches of rain spread out through three days or greater, it really just knocks the dust off the fuels, or the dead fuels will absorb what precip[itation] they get, but over the course of a lag time, it’ll dry out, and it’s really Mother Nature’s sponge,” Smith said. “So, really, what we look at is, when we get two inches of rain or greater, how is it distributed? Two inches in an hour isn’t the same as two inches over the course of two-and-a-half days.”
Smith also detailed the robust LACoFD preparations, including the state-of-the-art 69 Bravo helispot, a blufftop command center with four enormous water tanks containing 10,000 gallons of water poised to refill helicopters and air tankers to make drops on fires in the Santa Monica Mountains. See the view from 69 Bravo at 69bravo.com.
emily@smdp.com
