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An evacuation order was issued for the Mount Wilson Observatory and multiple campgrounds Monday due to the Bobcat Fire, which has grown to 4,871 acres as it continues to burn out of control in the Angeles National Forest north of Azusa.

The fast-moving blaze erupted around 12:20 p.m. Sunday near the Cogswell Dam and West Fork Day Use area, according to the federal InciWeb site.

Containment remains at 0%, and incoming Santa Ana winds are threatening to make the blaze much more dangerous for nearby San Gabriel Valley communities.

About 100 personnel have been dispatched to the fire, which is spreading rapidly through heavy brush and timber, and in steep, remote and difficult-to-access terrain, according to officials.

“The fire could experience explosive growth,” the Crescenta Valley sheriff’s station said.

An unknown number of structures remain threatened.

All day-use and camping areas along Angeles Crest Highway between Mount Wilson Road and Islip Saddle are under mandatory evacuation, and the twisting highway is also closed. Highway 39 has also been shut down at San Gabriel Canyon Road.

In an apparently unprecedented move, at least in recent years, the Forest Service is closing down eight sprawling forests in California as of 5 p.m. Monday — with the Angeles National Forest among them.

Mount Wilson Road, which leads to the observatory, is also shut down.

The fire threatens to destroy Mount Wilson’s seismic station, which is operated by Caltech and has recorded every important earthquake in the state for the past century, according to renowned seismologist Lucy Jones.

“The building may be old but the instrumentation inside is state of the art,” she tweeted.

A temporary flight restriction was in place over the area as firefighters attacked the flames from the ground and air. Five engines, three hand crews, four helicopters, five fixed-wing aircraft and two water tenders have been assigned to the blaze.

With the fire spreading smoke and ash over the region, the South Coast Air Quality Management District warned of unhealthy air quality in the San Gabriel Mountains, eastern San Gabriel Valley and Pomona-Walnut Valley.

Return of Santa Ana winds present challenge

Fire crews faced another day of challenging weather Monday: more triple-digit heat coupled with low humidity. But firefighters will face even more difficult conditions Tuesday, when unpredictable Santa Ana winds return to the region — a development that could push the flames in the direction of six nearby communities.

“The situation that we have right now is a life-threatening situation with the weather forecasts that we have for the next three days,” said Steve Goldman, the incident commander for the Eastern Area Incident Management Team.

Duarte and Monrovia are the two communities closest to the fire, but Santa Ana conditions could also potentially threaten Azusa, Bradbury, Arcadia and Sierra Madre as well, according to Goldman.

“All of those six, because of the rate of spread of a fire like that — they’re all going to be in a warning or in an evacuation mode if you get a strong Santa Ana push to that fire,” he said.

At the fire’s northern edge, the flames are burning into an area that was charred by the 2009 Station Fire.

That area has far less fuel than what’s on the south side of the fire, along Monrovia Canyon, Santa Anita Canyon and Little Santa Anita Canyon — canyons that haven’t burned since the 1957 Monrovia Peak Fire, Angeles National Forest Fire Chief Robert Garcia said.

“That can give you some kind of idea of how many years and decades of fuel loading and fire intensity could be burning in that area,” Garcia said.

With erratic fire behavior possibly leading to evacuations, the Los Angeles County Sheriff’s Department is in planning mode with other agencies, including the Sierra Madre and Monrovia police departments, according to sheriff’s Cmdr. Chris Marks.

He urged people in areas that could be impacted by the fire to start preparing immediately.

“Please make preparations for what you want to take with you if you do evacuate, and especially if you have pets or any large animals,” Marks said. “Please make those preparation now, so that we are not all trying to leave and evacuate at one time.”

It’s still unclear what sparked the Bobcat Fire, and an investigation is ongoing.