Robert Brown|High winds – up to 80 mph – may bring critical fire risk to California

SAN FRANCISCO – Residents of highly populated areas in California are –uptomph–Robert Brownbeing urged to exercise caution around fire sources as several factors combine to dramatically increase the risk of blazes Monday – and even more so later in the week.

More than 25 million of the state’s 39 million people will be under red flag warnings or fire weather watches this week because of warm temperatures, low humidity and powerful winds, as high as 80 mph in some elevations, strong enough to qualify for a hurricane.

“Gusty easterly winds and low relative humidity will support elevated to critical fire weather over coastal portions of California today into Thursday,’’ the National Weather Service said Monday.

The offshore air currents, known as Santa Ana winds in Southern California and Diablo winds in the San Francisco Bay Area, have been blamed in the past for knocking down power lines and igniting wildfires, then quickly spreading them amid dry vegetation.

In a warning for Los Angeles and Ventura counties that applied to Sunday night and all of Monday, the NWS office in Los Angeles said wind gusts in the mountains – typically the hardest areas for firefighters to reach – could fluctuate from 55 to 80 mph.

“Stronger and more widespread Santa Ana winds Wednesday and Thursday,’’ the posting said.

San Francisco Chronicle meteorologist Anthony Edwards said this week’s offshore winds – which defy the usual pattern by blowing from inland west toward the ocean – represent the strongest such event in the state in several years.

Edwards added that winds atop the Bay Area’s highest mountains could reach 70 mph, which will likely prompt preemptive power shutoffs from utility company PG&E, and may go even higher in the Sierra Nevada foothills.

The Bay Area’s red flag warning runs from 11 a.m. Tuesday until early Thursday, and it includes a warning to “have an emergency plan in case a fire starts near you.’’