Santa Ana Winds Heighten Fire Risk in Southern California

Santa Ana winds combined with low humidity levels and unseasonably warm temperatures are bringing a heightened risk of wildfires to parts of Southern California just west and north of Downtown Los Angeles this week.

The National Weather Service office issued a red flag warning, the highest level of fire alert, through 6 p.m. Wednesday for two separate pockets: One is spread between Los Angeles and Ventura Counties from the western San Gabriel Mountains into the Santa Susana Mountains. The other falls within the valleys of southeastern Ventura County and includes the cities of Thousand Oaks and Simi Valley.

The wildfire risk this season was significantly reduced in most of Southern California by early fall rains, but this region did not receive enough rain to fully end the threat. “The heavier brush is still dry, and that’s conducive to fire spread,” said Kristen Stewart, a fire weather meteorologist with the U.S. Forest Service.

Santa Ana winds, also known as offshore winds, blow dry air from inland areas toward the coast, pulling moisture from trees and brush and making them more flammable. Imagine a large blow dryer over interior desert valleys that is pointed toward the coast when it’s turned on. Santa Ana winds are most common in the colder months, and this is their first appearance of the season, which runs fall through spring.

When catastrophic fires ignited in Los Angeles County in January, the flames were spread by Santa Ana winds blowing as fast as 100 miles per hour.

On Tuesday and Wednesday, the winds will not be nearly as strong. Sustained speeds are forecast to reach 15 to 30 m.p.h., with isolated gusts up to 35 to 45 m.p.h. The strongest winds are expected through the early afternoon on Tuesday, and again late Tuesday night into Wednesday morning.

Ms. Stewart said the fire risk was significantly higher in January because the exceptionally strong winds hit at a time when the landscape was especially dry — the region did not see any rain in the fall and through the early part of winter last year.

“Our fire indices were very, very high in January, but this time they’re below average because of the recent rain,” Ms. Stewart said. “But, of course, it doesn’t mean there’s no risk since it’s still going to be dry and breezy out there, and hot.”

One factor signaling fire risk this week is the humidity levels, which are predicted to fall to 8 to 15 percent on Tuesday and down to 5 to 10 percent on Wednesday. Low humidity means the atmosphere is generally more dry, and more conducive to fires.

What’s more, temperatures across Southern California are expected to be unseasonably high on Tuesday, about 10 to 15 degrees above normal in many locations, with Downtown Los Angeles poised to record a high of 92, and San Diego one of 90.

“Temperatures are going to be pretty close to some records in some areas,” said Bryan Lewis, a meteorologist with the Weather Service office in Oxnard, Calif.