Why California Wildfires Are Particularly Destructive in Fall

Fall has historically been a destructive, deadly time for California wildfires, due in part to the state's notorious Santa Ana winds.

2021 has been yet another devastating year of wildfires in California, fueled by exceptional drought.

According to Cal Fire, 7,595 wildfires have burned more than 3,600 square miles – an area larger than the state of Delaware – in California so far in 2021 through Sept. 19. That's about 600 more fires than the latest five-year average, and double the average area burned year-to-date.

Among these was the state's largest single-source wildfire on record, the Dixie Fire, which charred over 1,500 square miles and more than 1,300 structures in northeast California, including the town of Greenville.

The Caldor Fire, while not nearly as expansive, scorched about 1,000 structures and prompted an evacuation of South Lake Tahoe in late August.

In California, as in much of the West, the number of wildfires typically peaks in summer.

From 2008 through 2019, July averaged the most California wildfires, according to Cal Fire.

Summer is the heart of the state's dry season, when searing heat, low humidity, dried-out vegetation, lightning strikes and human causes combine to ignite fires.

These summer fires, while still dangerous, tend to burn more slowly and usually in remote areas.

Larger, Destructive Fires More Often in Fall

A recent study from the University of California-Irvine found about 20% of California's wildfires from 2000 through 2019 were responsible for 97% of the total area charred.

Over 90% of casualties and property losses in the state were due to larger wildfires over 500 acres, according to Tirtha Banerjee, one of the study's co-authors.

When it comes to these most destructive wildfires, the fall months lead the pack in the Golden State.

According to Cal Fire, the four deadliest and most destructive wildfires on record in the state happened in either October or November.

November 2018's Camp Fire, which incinerated the town of Paradise, topped both lists with 18,804 structures destroyed and 85 killed. It was the nation's deadliest wildfire since 1918.

If we examine acres burned per wildfire over the same 12-year period, only July tops the fall months for these larger wildfires.

Tinderbox Ingredients

Unlike most locations in the Lower 48 States, California has pronounced wet and dry seasons, and the last weeks of the dry season happen in the fall.

"California’s Mediterranean climate makes it unique as our warmest months coincide with our driest," Jan Null, certified consulting meteorologist and adjunct professor in the Department of Meteorology and Climate Science at San Jose State University, told weather.com.

Both Los Angeles and San Francisco typically pick up less than 10% of their average annual precipitation in the six-month "dry season" from May through October.

"By the end of the summer and into early fall, the state’s vegetation has dried out," said Null.

And that's a particular concern this fall, with the state in the grips of an exceptional drought.

(MORE: California Just Had Its Driest 'Rain Year' Since 1895)

But the main driver of California's destructive fall wildfires is wind.

California's infamous offshore winds, known locally as Santa Ana (Southern California) or Diablo (Northern California) winds, typically kick into gear in October and November and can happen occasionally through April, according to a 2019 study.

"We begin seeing high pressure developing over the Great Basin (in the fall) and this creates dry warm offshore winds," Null told weather.com.

"Since the Great Basin is nominally 4,000 feet in elevation, the air is compressed as it descends to sea level, warming and drying it. When this flow is forced over mountains and through canyons, it accelerates."

If the jet stream is also located just to the east of the state over the Great Basin, a downward push of strong winds can occur, intensifying the offshore wind event.

These winds sometimes gusting over 70 mph can quickly whip up either an existing wildfire or just-developed small brush fire into an inferno, blowing embers downstream and starting many more spot fires, often in heavily populated areas.

That's precisely what happened in each of the state's four most destructive wildfires, including the November 2018 Camp Fire that burned through Paradise, the October 2017 Tubbs Fire in Santa Rosa, the October 1991 Oakland Hills firestorm, and the October 2003 Cedar Fire in San Diego County.

According to a 2015 study, fires fanned by Santa Ana winds spread three times faster, burn much closer to urban areas and inflicted 80% of cumulative economic losses from all California fires from 1990 through 2009.

Troubling Trends

California's wildfire season is becoming a nearly year-round concern.

According to Climate Central, the average wildfire season in the West is 105 days longer, has three times as many large fires and burns six times as many acres than in 1970.

A 2020 study found that, due to the combination of hotter temperatures and less precipitation, California now has twice the number of days with extreme fire weather conditions in the fall than in the 1980s.

Another study released earlier this year found California's rainy season is arriving later in the fall compared to six decades ago, increasing the potential overlap of dried-out vegetation and Santa Ana wind events.

Then there are land-use trends as more population moves into forested land or the outer edges of suburbs.

"Before 2000, there were almost no human-caused wildfires along California's Pacific coastline, but now nearly every coastal county is experiencing increased risk," said Shu Li, a Ph.D. candidate and lead author of the aforementioned University of California-Irvine study.

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