Destruction in California: What caused the LA fires to spread so rapidly?

Editor's Note: For updates on the Los Angeles wildfires in California, please read USA TODAY'S live updates for Saturday, Jan. 11.

A growing number of wildfires spread rapidly across Los Angeles, fueled by powerful Santa Ana winds, low humidity and dry vegetation due to a lack of rain.

At least five fires were active in Los Angeles County including the Palisades Fire, which grew from 10 acres to more than 17,000 acres in just three days and the Eaton Fire, which has swelled to more than 10,600 acres east in the foothills of the San Gabriel Mountains, according to Cal Fire.

The causes of the largest fires are still under investigation, according to Cal Fire. The environment was primed for ignition. The National Weather Service office in Los Angeles warned that low humidity and widespread, damaging winds up to 100 mph in some areas would fuel massive blazes "with extreme fire behavior."

"We knew that the vegetation was very dry already and then on top of this, with strong winds like Santa Ana winds that are very dry and gusty, the fire risk was really very high," said Luca Carmignani, an assistant professor at San Diego State University and former fire advisor for the Wildland Urban Interface in Southern California. "So it's not surprising that once the fire started it spread that quickly."

Santa Ana winds fuel fires

One of the nation's most notorious wind events has helped fuel the destructive wildfires.

The Santa Ana winds, which occur most often in the fall and winter, push dry air from over the inland deserts of California and the Southwest toward the coast, the National Weather Service said. As high-pressure systems move east to west over the Santa Ana Mountain range, wind is forced down where it’s compressed and warms up.

An area of high pressure over the Great Basin, the high plateau east of the Sierra Nevada, combined with a storm in northwestern Mexico to create the conditions for strong winds over Southern California starting on Tuesday, said AccuWeather meteorologist Gwen Fieweger. The Santa Monica and San Gabriel Mountains also have canyons and valley, which may act as funnels for the Santa Ana winds and further accelerate the spread of wildfires in these mountainous areas.

Janice Coen, a project scientist at the National Center for Atmospheric Research in Boulder, Colorado, said these winds also dry out vegetation on the mountain ridges as they pass and can carry embers over long distances – allowing fires to spread rapidly.

Lack of rain, low humidity create dry conditions

The rapid spread of the fires were likely also aided by the extremely dry season that preceded them.

Over 83% of Los Angeles County was in a drought, according to the most recent U.S. Drought Monitor. Los Angeles has not received a quarter of an inch of rain since April, Accuweather reported.

Several wet winters allowed vegetation to flourish, which the current drought turned into an abundance of dry fuel, according to Heather Zehr, a senior meteorologist with AccuWeather. Extremely low humidity also helps dry out vegetation, making it a better fuel for fires.

"It is that dramatic change from very wet to very dry that can set up these more extreme situations, and that is likely affected by climate change," said Zehr.

Contributing: Karissa Waddick and Doyle Rice, USA TODAY

This article originally appeared on USA TODAY: What caused the LA fires 2025 to spread so rapidly?