Now comes the Santa Ana threat

During the four weeks the Station fire has burned, firefighters have battled steep terrain, soaring temperatures and low humidity. Now comes the wind.

On the day fire officials had hoped to achieve full containment of the huge blaze in the Angeles National Forest, Santa Ana winds of 20 to 30 mph were predicted to arrive.

“Wind is a firefighter’s worst enemy,” said Carol Underhill, a public information officer with the U.S. Forest Service who is at the fire command post at the Santa Fe Dam in Irwindale.

Accompanying the winds will be triple-digit heat and low humidity, prompting the National Weather Service to issue red flag alerts for today. The warning, which means that fires can spread rapidly, covered most of L.A. County.

Humidity was also expected to drop into the single digits, perhaps to 5% by this afternoon. “That’s really low,” said Jamie Meier, a meteorologist with the National Weather Service in Oxnard.

In the last few days, humidity near the Station fire has been in the 20%-to-30% range. The dry conditions should last into Thursday, Meier said.

On Monday, officials said the fire was 94% contained. But full containment -- which had already been postponed from Sept. 15 to today -- could be challenged by windy conditions.

On Saturday, helicopters dropped 80,000 gallons of water on hot spots north of Mt. Wilson, according to Underhill, and Sunday they dropped an additional 94,000. “That’s the last area we’re really concerned about,” she said. “If those were to flare up and move, they could move into unburned areas.”

To slow any flames that might be driven into unburned areas, fixed-wing tankers dropped flame retardant along a ridge of brush heading east from Mt. Wilson on Sunday and Monday. “If one of those hot spots were to move, it would move toward that ridge,” Underhill said.

Fire officials were still predicting full containment today for a blaze that has scorched 160,557 acres and destroyed 89 residences. “We feel pretty confident about the steps taken,” Underhill said. “We have an incident fire behavior specialist and a meteorologist. They’re watching it carefully.”

Full containment, whenever it arrives, does not mean every ember is out and nothing else will burn within the area. “There are some unburned islands of vegetation,” Underhill said. “Every once in a while, a tree will go up. They will continue to burn here and there within the perimeter, sometimes for months. . . . Fires aren’t usually out until you get some winter rain on them.”

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carla.hall@latimes.com