Why unusually late rains could delay start of wildfire season

Experts say an above-average and unusually lengthy rainy season could bode well for California’s wildfire fortunes as the state heads into the summer months.

After a historic storm drenched Northern California over the course of less than 24 hours at the beginning of the month, dumping roughly an inch of rain in downtown San Francisco and more than a foot of snow at some of the state’s highest elevations, the rest of May has stayed almost completely dry.

Still, even that amount of precipitation alone — the most rain during the month of May in more than 100 years in some counties — helped delay the start of the wildfire season, said Scott Stephens, a professor of fire science at UC Berkeley.

“We generally get some rain in May, but it’s generally light — a quarter of an inch to a tenth of an inch, something like that,” he told The Examiner. “But this last storm — that was a lot. So we’re going to benefit from that in Northern California.”

Fears over wildfires — and the pollution they spew into the skies — have surged in recent years. Ten of the largest California wildfires in history have occurred in the last 20 years — five of them in 2020 alone. A report this month from the nonprofit Climate Central found that wildfire seasons are intensifying and lengthening throughout the western U.S.

There have been nearly 1,000 wildfires throughout California this year, totaling almost 4,000 acres burned. The acreage total is more than last year — which ended up being a mild year for wildfires — but less than the average of the past five years, according to the California Department of Forestry and Fire Protection.

The storm earlier this month helped lessen the effects of a significant warm front in the middle of May, when temperatures rose to their highest levels of the year. Temperatures approached 80 degrees then but have remained relatively moderate since.

“I think in many parts of the state, we are still pretty moist,” Stephens said. “That’s a big plus, because that just means that vegetation will remain wetter for a period.”

He said he hopes the state receives about another quarter-inch of rain in the coming weeks, which would “retard the beginning of wildfire season” even longer. He said his rule of thumb is that if a region receives a half-inch of rain, that will prevent fires from igniting for about the following two weeks.

Cal Fire chief Robert Carvallo said the state’s forest-fire agency is taking advantage of the mild conditions so far, reducing fire-conducive vegetation and debris, as well as educating at-risk communities about ways they can fortify their homes against wildfires, including mowing weeds.

“We take advantage of when the weather is on our side,” Carvallo said. “We don’t call it a fire season anymore — we call it a fire year. So what’s the 2024 fire year going to bring? With the different weather patterns, it’s hard to determine. But [is] Cal Fire preparing for the fire year? Yes, we are.”

Stephens said the state is “headed into our summer and in a decent position from a fuel-moisture standpoint” — with the exception of grassland fuel, which is roughly 50% higher than normal across the state. That’s expected during high-volume rain years, he said, but it is “something that will get our attention in fire season.”

Toxin-carrying wildfire smoke has become an increasingly concerning hazard throughout the state, including in the Bay Area. UC Davis released a study this week which found that in parts of 2020 and 2021, 70% of California was covered by wildfire smoke .

Those concerns are magnified in San Francisco, the least air-conditioned city in the U.S. Most homes in The City don’t have ventilation systems to filter out the toxins.

Even though 2023 was a mild wildfire season, it still resulted in 15 Spare the Air Alerts in the Bay Area and the worst San Francisco air quality in three years , most of which was due to smoke from a half-dozen wildfires raging along the California-Oregon border during the fall.

Stephens said that even during El Niño years , the northwest generally gets drier — acting opposite of how the phenomenon impacts Southern California. But that hasn’t been the case in 2024.

“Even though El Niño was relatively strong, we still had pretty good precipitation levels in Oregon and Washington,” Stephens said, which indicates less likely significant wildfire risk and potentially less smoke wafting from the north into the Bay Area.

Stephens said it remains to be seen how long this moderate weather period lasts and how fast the summer will heat up.

“I think no one’s been able to predict that very well,” he said. “But those types of events certainly help to dry things out quickly and really make fire weather worse. So that’s a part I don’t really have a good feel on.”