Why California's Wildfires Are Worse in Fall
Published: October 10, 2017
The recent firestorm that swept through Napa, Sonoma and Mendocino Counties, and a separate destructive wildfire in the Anaheim hills are only the latest examples of why fall is a peak time of concern for California wildfires.
(MORE: N. California Firestorm | Anaheim Hills Fire | Photos)
Entire neighborhoods were leveled by the 1991 Oakland Hills firestorm, California's most destructive single wildfire on record.(Cal EMA)
Hurricane season may be in its last active month, and, for most in the U.S., it's still too early for big winter storms, much less the season's first snow, but October in California has a destructive reputation.
Prior to the October 2017 firestorm, five of the top six most destructive wildfires of record in California occurred in October, according to Cal Fire.
Fifteen of the top 20 most destructive fires on that list have occurred in the fall months of September through November.
There are two main meteorological reasons why California's firestorms typically happen in the fall.
The Tail End of the Dry Season
Unlike most locations in the Lower 48 States, California has pronounced wet and dry seasons. It just so happens the last weeks of the dry season happen in fall.
"California’s Mediterranean climate makes it unique as our warmest months coincide with our driest," said Jan Null, certified consulting meteorologist and adjunct professor in the Department of Meteorology and Climate Science, San Jose State University.
Average rainfall in Downtown Los Angeles in the wet season (November - April) compared to the dry season (May - October). (Data: NWS)
Both Los Angeles and San Francisco typically pick up only 9 percent of their average annual precipitation in the six-month "dry season" from May through October.
This amounts to a six-month average rain total of only 1.3 inches in L.A. and 2.25 inches in San Francisco.
"By the end of the summer and into early fall the state’s vegetation has dried out," Null said.
But what about the all-time wettest "wet season" in parts of the state in 2016-2017? Incredibly, that may have made the firestorm worse.
"Recall we had nearly five years of extreme drought followed by record rains last winter that produced a bumper crop of grasses and fine fuels on top of drought and disease-stressed heavier fuels," wrote the Monterey, California, National Weather Service office in an Oct. 9, 2017 forecast discussion following the Napa/Sonoma firestorm.
"Fuel analysis just ahead of the winds showed they were at all time record dry levels preceding the front."
Which leads us to the main driver of California's fall firestorms.
High Winds Return to Fan the Flames
Summer's wildfires in California tend to burn more slowly, starting in more remote areas, often due to lightning.
In the fall, Calfornia's infamous offshore winds, known locally as Santa Ana (southern California) or Diablo (northern California) winds, typically kick into gear, according to a 2017 climatology study.
A typical Santa Ana (or Diablo) wind set up in California.
"In September and October, we begin seeing high pressure developing over the Great Basin and this creates dry warm offshore winds," said Null.
"Since the Great basin is nominally 4000 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 intense offshore winds can occur, at times, from fall through spring.
What makes them particularly dangerous in October is that they occur when, as mentioned earlier, soil moisture is at its driest after the dry season.
These Santa Ana or Diablo winds can quickly whip either an existing wildfire or just-developed small brush fire into an inferno, blowing embers downstream, starting many more spot fires, often in more heavily-populated areas.
"October is sort of the perfect storm month for California firestorms," said Null.
Jonathan Erdman is a senior meteorologist at weather.com and has been an incurable weather geek since a tornado narrowly missed his childhood home in Wisconsin at age 7. Follow him on Facebook and Twitter.
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