Dry Vegetation Fuels L.A. Fires as Wind Speeds Drop

The focus this week has been on how extreme winds have fueled the most destructive fires in Los Angeles’s history. But that’s not the only concern.

On Friday, even as slowing wind speeds increased hopes that firefighters would contain the blazes, dry vegetation and steep terrain pushed the Palisades fire, the biggest, east, putting a new swath of Los Angeles under mandatory evacuation orders.

The blaze was burning along the tops of the ridges of Mandeville Canyon, said Kenichi Haskett, a division chief with the Los Angeles County Fire Department, on Friday night. The fire tore through a steep area full of dry vegetation and threatened the neighborhood of Encino in the north.

The spread was being driven by the landscape rather than wind, Mr. Haskett said. “We’re not getting strong winds the way we got on Tuesday and Wednesday.” The Palisades fire has now burned more than 21,000 acres in five days.

The rains that usually fall in autumn and early winter did not come, leaving most of Southern California bone dry and leaving vegetation primed to burn. Most locations south of Ventura County have recorded about a quarter-inch of rain or less in the past eight months, while the Los Angeles area has received only sprinklings of rain since April.

That means the Santa Ana winds, the strong, dry gusts that have driven the wildfires, have had a particularly dramatic effect. Even as they have subsided, the parched vegetation has continued to fuel the Palisades fire, experts said. Stronger winds are expected to return to Los Angeles and Ventura counties Saturday afternoon, reaching the highest speeds overnight into Sunday morning and heightening the risk of rapid wildfire spread.

Wind speeds over the fire were light — under 15 miles per hour — on Friday night, said Dave Gomberg, a meteorologist with the National Weather Service. In comparison, Wednesday saw wind gusts of over 90 m.p.h. “I think a big component is the fuels are exceptionally dry,” Mr. Gomberg said of Friday’s expansion.

The Palisades fire was “following the terrain and the fuels,” said Craig Clements, director of the Wildfire Interdisciplinary Research Center at San Jose State University. Fires thrive in hilly terrain and move faster uphill than downhill, he said, adding, “The steeper the terrain, the faster the fire can go.”

The fire chewing its way through Mandeville Canyon is a “plume-dominated fire,” that is being fueled by its own wind, said Redondo Beach Fire Chief Patrick Butler, a former assistant chief for the Los Angeles Fire Department who has led the response to many Southern California fires. Such blazes often shoot upward and then collapse, scattering embers for miles in concentric patterns, he said.

On Friday evening, ash was falling in the Brentwood neighborhood to the south of the canyon.

Wildfires are notoriously hard to fight in Mandeville Canyon, which has poor radio communication and an extremely narrow road, Mr. Butler said: “There’s basically one way in and one way out.”