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Authorities urged residents in the foothill communities of the San Gabriel Mountains to prepare to evacuate as a massive fire burning in the Angeles National Forest threatened to spread quickly with the arrival of the Santa Ana winds Tuesday.

The blaze, dubbed the Bobcat Fire, is burning closest to the cities of Duarte and Monrovia, officials said. It could also endanger Azusa, Bradbury, Arcadia and Sierra Madre.

“Get your families set, include your pets, medications, important records,” said David Richardson, deputy chief with the L.A. County Fire Department.

“Don’t wait to get things ready… If that evacuation order comes, you need to leave,” he said. “Prepare now.”

An official evacuation warning was issued around 7:30 p.m. for residents of Duarte, Bradbury, Monrovia, Arcadia, Sierra Madre, Pasadena and Altadena.

Residents can sign up for emergency alerts from the county at www.lacounty.gov/emergency/alert-la.

The Bobcat Fire charred an additional 1,800 acres during the day Tuesday, to cover a total of 10,344 acres. That’s up from the previous evening’s estimate of 4,870 acres.

The blaze erupted during record-breaking heat around noon Sunday near the Cogswell Dam above Azusa.

As of Tuesday night, it was 0% contained. It remains unclear how the flames started.

Forecasters predict Santa Ana winds to arrive Tuesday afternoon, with the strongest gusts expected in the night into Wednesday morning. Wind speeds could reach 50 to 60 mph in the mountains, the National Weather Service said.

The humidity could also drop to the single digits as winds develop, according to officials.

“Fuels after this historic heat wave will be at critical levels as we enter into the Santa Ana wind event,” NWS said in an advisory.

The winds could push the smoke toward the southwest and into Sierra Madre, Monrovia, Duarte, Azusa and Glendora, the South Coast Air Quality Management District warned.

The smoke has caused unhealthy air quality levels in the San Gabriel Valley as well as in Central L.A. and the southeast portion of the county, according to the L.A. County Department of Public Health.

A red flag warning was in effect from noon Tuesday through 8 p.m. Wednesday.

By around 2:30 p.m. in Monrovia, crews had started cleaning up vegetation and clearing access points.

Federal officials had already shut down the Angeles National Forest, along with seven other California forests amid the dangerous fire weather across the state.

Authorities also closed Mount Wilson Road as flames threatened Mount Wilson’s seismic station. They also issued a temporary flight restriction over the area, according to the federal website InciWeb.