California’s wildfire season started with a roar this year — and we’re only just approaching the critical months of October and November, which in recent years have brought some of the state’s deadliest and most destructive wildfires.
Dry lightning in mid-August ignited the CZU, SCU and LNU Lightning Complex fires, which are now three of the four largest wildfires in California history, and they have yet to be fully contained. More than 7,400 fires have scorched 1.83 million acres so far, according to Cal Fire. In 2018, California’s worst fire year to date, 7,948 fires burned 1.98 million acres.
So, how bad can the rest of the season get? What do wildland fire experts and firefighting agencies look at as they prepare for the rest of the season? And do past data reveal any significant patterns?
Conditions this year are not looking good, according to experts. The meager rainy season left California’s landscape dried out early on in the year, and it has been parched further by heat waves during the dry months. Pair that with the blustery, dry Diablo winds that arrive around this time, and it’s a recipe for disaster, experts say.
‘Significant fire potential’
Predicting the wildfire season depends on a number of factors including: “fuel aridity,” or dryness of vegetation; rainfall and drought conditions; temperature; and more unpredictable weather events, such as wind and lightning.
Strong evidence also indicates that climate change plays a role in wildfires by influencing the weather and the amount of dry vegetation, said Stanford University climate change expert Noah Diffenbaugh. Most of California’s recent major fires have occurred during near record fuel aridity, he said, which is influenced by precipitation, temperature and winds.
“When we look at the historical data and analyze the future of global warming scenarios, we’re confident that because of the very strong effects of global warming on fuel aridity, further global warming will intensify the frequency of extreme wildfire weather in California,” Diffenbaugh said.
In his role as fire weather meteorologist for the U.S. Bureau of Land Management, Stephen Leach identifies upcoming high-risk weather patterns that influence wildfire potential in Northern California and northwest Nevada.
The week after Labor Day looked worrisome, he said. “The upcoming week of hot and dry weather will push fuel indices to extreme levels,” he said. “As we go into the offshore wind season… that danger is there.”
UCLA climate scientist Daniel Swain said fuel conditions don’t seem to be improving as we move into the height of wildfire season across the state.
“Autumn makes me nervous,” he said. “That’s when offshore winds have normally occurred, and it looks more than usual to be dry this autumn… We still could get lucky. You still need ignitions.”
Many of California’s major wildland fires have been human-caused. PG&E’s faulty power lines were responsible for a number of recent wildfires, including the 2019 Kincade Fire in Sonoma County and the 2018 Camp Fire in Butte County. Homeowner equipment, a tire blowout and a rancher hammering a metal stake sparked major fires in 2017 and 2018.
Each month, Leach works with meteorologists from the U.S. Forest Service to put together a monthly and seasonal outlook. September’s outlook is grim, with above normal wildfire risk across most of Northern California. In October, the most northern part of the state goes back to a normal risk. Leach said if there isn’t sufficient rain in October, then November will have the same outlook.
So far, the season’s predictions have been spot on, said Swain. While the recent dry lightning siege across the state was rare, August is usually when lightning-caused fires occur. And the conditions were just right for the surge of blazes that lit up 12 counties across and just outside the Bay Area.
“Northern California had an extremely dry winter, early season heat waves in the spring, and warmer than average temperatures into autumn,” Swain said. “They predicted Northern California would see an early to severe start to fire season beginning in July or August, and Southern California would have a slower start but worse peak.”
A lackluster rainy season
The majority of Northern California saw precipitation below average levels this past rainy season, which began in October of last year. Leach said the rainfall stopped in most areas in May and June, and the snowpack peaked early in April, finishing at two-thirds its normal level and quickly melting off by June.
On top of that, spring rainfall mixed with warm, sunny weather led to an above average growth of grass crop, which Leach said has been happening in recent years due to climate change.
“It’s a critical thing because when we get to September, October and November without rain, the grass is always cured and critically dry,” Leach said. “That’s when you start to see offshore wind events and the highest danger of those big news-making fires below 3,000 feet that impact communities more.”
Meanwhile, big rain events, which mark the end of fire season, are coming later and later, drying out the fuels even further and extending the wildfire risk.
Charting rainfall and acres burned in California through the past 50 years shows that, in general, when there is more rainfall, fewer acres burn. Also unsurprisingly, years with less precipitation tend to result in many more scorched acres.
According to the National Interagency Fire Center (whose figures differ slightly from Cal Fire’s), 2018 was the year with the most acres burned on record, 1.85 million. Rainfall was just over 16 inches that year, far below average. That year also saw the state’s deadliest fire on record, the Camp Fire, in which 85 people lost their lives, and the largest fire on record, the Mendocino Complex, which charred 410,203 acres. Compare those numbers with last year, in which rainfall was above average at nearly 29 inches, and 259,148 acres burned.
But the pattern doesn’t always hold: 2017 saw plenty of rainfall at 34.5 inches, and yet 1.27 million acres burned. That’s because record rains produced ample vegetation that was promptly shriveled into massive amounts of fuel by the hottest summer in California history, turning the state into a tinderbox. The state saw many of its most destructive wildfires that year, including the Tubbs, Nuns and Atlas fires.
Drought and fuels are critical elements
The U.S. Drought Monitor, produced by the University of Nebraska-Lincoln, U.S. Agriculture Department and National Oceanic and Atmospheric Administration, maintains a drought map categorizing areas by various intensities.
The latest update on Sept. 1 shows “severe” drought conditions for most of Northern California, including a good portion of the Bay Area covering Napa, Sonoma, Marin County, San Francisco and parts of San Mateo, Alameda and Contra Costa counties.
Most of San Mateo and Santa Clara counties are in a “moderate” drought, along with a large area in the center of the state stretching to the eastern border. Siskiyou County and the northern tip of Trinity County in the far north are the only areas of the state currently dealing with “extreme” drought conditions. The map from the same time last year shows nearly the entire state drought-free.
“We’re in a very different place this year with drought conditions through all of Northern California in some aspect,” said Cal Fire spokesman Robert Foxworthy. “A large swath from the Bay Area to the Oregon border is in severe drought conditions, while the rest of the state is in a moderate drought.”
Another important indicator in determining fire potential is the fuel moisture index, used by experts throughout the country.
“The fuel moisture content is the most critical predictor of wildfire risk,” said Craig Clements, director of San Jose State University’s newly formed Wildfire Interdisciplinary Research Center.
Clements said part of the lab’s work is taking monthly samples of a species called chamise — a widespread shrub in California — from three sites in Los Gatos. Right now, the fuel moisture samples are “already at the level they would be in October,” he said, meaning “critically dry.”
Short-term weather events
Short-term weather events are critical to determining fire behavior, but forecasting them is a fickle process. Foxworthy said “it’s hard to predict thunderstorms and lightning” on any given day, and also “what kind of wind conditions to expect.”
“Long-term climatological computer models can tip us off to overall weather trends throughout the upcoming season,” Leach said. “It’s the shorter-term patterns such as heat waves, thunderstorm events, and offshore wind events that are more direct short-term trigger events for wildfire occurrence and not easily seen in forecast models more than a week or so out in the future.”
The multiple heat waves in the spring and summer started the drying out of fuels. The heat wave at the start of this recent spate of wildfires made firefighting difficult.
“Many of the recent large wildfires in California have occurred with a record or near record fuel aridity, which we know is influenced not just by precipitation and winds, but also the temperature,” Diffenbaugh said.
Experts say that overall temperatures have increased because of climate change, which increases the risk of major fires, especially during heat waves.
Adding to the heightened danger is offshore winds blowing from inland toward the ocean. Like the Santa Ana winds in Southern California, the Diablo winds in Northern California have been responsible for aiding the rapid spread of some of the region’s most destructive wildfires, including last year’s Kincade Fire and the deadly 2018 Camp Fire.
Some research suggests that offshore winds might actually decline in the future. But they also will continue to coincide with the lengthening dry season and with abnormal temperature swings as a result of climate change, elevating fire danger.
Clements said the state needs a better handle on wind conditions, even though California has plenty of surface weather stations.
“We don’t have good upper air or wind profiling observations, and the state of California should really invest in that,” he said. “It would help us forecast where the strongest winds will hit.”
Is there any good news?
Leach said the lightning siege that ignited the recent Bay Area megafires is rare, generally happening every 10 to 12 years. He doesn’t expect that to occur again this season.
“We do get some lightning in September and October, but it really dwindles down,” he said. “It’s the one natural fire source. Just about every other fire is human-caused.”
Leach said two factors may actually help quell major outbreaks the rest of this season.
“There’s a huge amount of burned land that won’t be burnable this year, and it’s possible that if fires start, those burn scars will be a fire break,” he said. “In a strange way, there may be good news when the wind events come. Those were really big fires.”
Leach added that the PG&E power safety shutoffs that began last year were helpful to wildfire prevention. If ignitions can be reduced, the wildfire risk will go down.
But still, as past seasons have proved, it doesn’t take much for a fire to spark. So experts and firefighting agencies need to be as prepared as possible with the tools, data and research they have.
“Wildfire danger comes down to fuels and weather,” Clements said. “We have to have a good handle on fuels, and a good handle on weather to be able to predict both fire danger and how a wildfire will spread given those conditions. As we approach fall, the fuels and the weather will drive our most extreme events, so understanding the state of both will help us better mitigate the risk.”
Content written by San Francisco Chronicle's