LOS ANGELES (CBSLA.com/AP) — A red flag warning remained in effect Friday for the mountains and adjacent foothills areas of Los Angeles and Ventura counties, according to the National Weather Service.

CBS2’s Josh Rubenstein reported the critical fire conditions are due to the duration of winds and low-relative humidity.

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“A red flag warning means that critical fire weather conditions are either occurring now or will shortly,” the National Weather Service reported within an advisory message. “A combination of strong winds, low-relative humidity and warm temperatures will create extreme fire behavior.”

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The often-fierce Santa Ana winds have been known to topple power lines and trees, and turn a spark into a raging wildfire.

Gusty northeast winds are expected to reach up to 40 miles per hour throughout the day, with gusts possibly reaching up to 60 miles per hour within the passes, canyons and ridgetops, meteorologists said.

Santa Anas are dry, warm and gusty northeast winds that blow from the interior of Southern California toward the coast and offshore, moving in the opposite direction of the normal onshore flow that carries moist air from the Pacific into the region. They typically occur during the fall months and continue through winter and into early spring.

The gusty winds are created by high pressure over the Great Basin — the vast desert interior of the West overlapping several states. The sinking air loses its moisture and flows in a clockwise direction toward Southern California, where it must get past towering mountain ranges that separate the desert from the metropolitan region lining the coast.

Like a slow-moving river that suddenly narrows and turns into rapids, the air speeds up as it squeezes through mountain passes and canyons, becoming drier and warmer as it descends.

Humidity levels often plunge to single-digit percentages during a Santa Ana wind. The extreme lack of humidity in the air causes vegetation — living and dead — to significantly dry out and become susceptible to fire.

“We have approximately 170 extra firefighters working today strategically placed throughout the county,” said Inspector Randall Wright of the Los Angeles County Fire Department. “We have extra fire engines, patrols and crews out there ready to go if a fire does break out.”

The tremendous wind speeds can stoke any spark — from a fallen power line, for example — into a rapidly spreading conflagration. Santa Anas are linked to some of the worst wildfires Southern California has experienced.

A commonly accepted explanation is that the name is linked to Santa Ana Canyon in Orange County. Other theories persist, along with other nicknames such as “devil winds.”

Meanwhile, strong winds are expected to be weaker tonight and Saturday, but very warm and dry conditions will continue.

The advisory remains in effect until 6 p.m. this evening.

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