In August, the higher-risk zones are mainly in the inland valleys and foothills in the northern part of the state. By October, the danger zone extends up and down the coast and into the mountains.
Experts sometimes refer to two separate fire seasons in California: summer wildfires fed by heat and fall wildfires driven by winds.
This summer had been off to a slow start. Then a heat wave at the end of July, which had inland valley temperatures soaring into the upper 90s and 100s from Riverside to Redding, seemed to flip a switch. Firefighters battled blazes near Los Angeles, Santa Barbara, Monterey and Big Sur. The largest fire of the year so far burned in the state’s remote northeastern corner.
“The summer fire season looks to be very active but compressed to August and early September,” said Bryan Henry, a meteorologist and acting manager of the National Fire Weather Program who oversees the regional wildfire reports.
Because of the wet winter and spring and late snow melt, higher elevations had remained moist, suppressing summer forest fires in the Sierra. The rain was “a little too good to us,” Henry said — it produced an especially tall and continuous grass crop that can carry fire easily. “Right now we’re at a critical juncture in the low and middle elevations,” he said. These are the same areas that saw extreme wildfires in 2017 and 2018, including wine country (Tubbs Fire), Redding (Carr Fire) and the Sierra foothills, where the Camp Fire erupted, California’s deadliest and most destructive blaze on record.
Brent Wachter, a predictive services meteorologist for the Northern California region, said conditions are better than at this time last year. “That doesn’t mean we won’t have large fires and fires that make the news,” he said. “We still expect above-normal activity, but it’s not going to be the extreme year we had last year.”
Summer 2019 had a much different start than the previous one, and that can be seen in maps of the Evaporative Demand Drought Index (EDDI) for May through July of the past two years.
EDDI measures the “thirst” of the atmosphere and has shown promise in predicting fire danger. In 2018, the very hot summer dried out vegetation nearly a month ahead of schedule. This summer, with more moderate temperatures and higher daytime and nighttime humidity, plant moisture levels remained near expected values. However, the drying rate seems to be accelerating. “Fuels are starting to really dry out due to the heat,” Wachter said. “We’re seeing shrubs contribute more to fire growth than we were two weeks ago.”
Even brief heat waves can intensify fire danger because they occur on top of a background warming trend that has its own drying effect.
“A heat wave today is going to have a much more potent influence on flammability than one 150 years ago when temperatures were 3.5 degrees cooler,” said Park Williams, a climate scientist at Columbia University.
In a new study released in July, Williams and his co-authors found a heat signature in the dramatic increase (405 percent) in California’s burned area since 1972, one that appears as a rise in big summer forest fires in the Sierra and the forested northern coast regions — like 2018’s Ferguson Fire in Yosemite and the record Mendocino Complex Fire. These changes were driven by increased daytime temperature, creating a thirstier atmosphere that can more quickly draw moisture from plants.
Although the relationship between a warming climate and fall wildfires was less clear, especially in coastal Southern California, they did find that a significant statewide upward trend in fall temperature (October-November) since the mid-1940s has boosted the chance of large autumn fires. Williams said the trend can’t be attributed entirely to climate change because of a mid-century cool period, but it does show that a small increase in background temperature can force a big increase in fire potential. That’s important, he said, “because we expect in the coming decades for the human-caused warming fingerprint to become increasingly apparent in fall, as it already is in summer.”
Big, damaging fall fires are sporadic because they depend on strong offshore wind events meeting a very dry landscape. But that’s exactly what happened in California the last two years. In 2017, after a wet winter, summer and fall heat waves punctuated a long drying-out period (May through December). In 2018, extreme summer heat primed the state for big autumn fires.
A common thread in both years was the late arrival of the rainy season. Climate models project a future decrease in autumn rain in California, but the Williams study found no clear statewide trend toward delayed winter precipitation in the observed record. There is, however, a hint that Southern California autumns have moved in that direction.
Wachter, the northern California meteorologist, said it’s too early to say what weather patterns will take hold this fall, though he does expect August through November to be warmer and drier than normal. “If I were to place bets, the fire season in California would go on a little longer than normal, perhaps similar to last year.”
Park Williams thinks wetter soils could possibly work to take the edge off heat waves this year, making an extreme fire season less likely. He added, “If this summer or fall happen to be record-breaking hot, all bets are off.”