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Flames jumped a contingency line Monday, advancing on Little Santa Anita Canyon after the Bobcat Fire ripped through nearly 38,300 acres in the Angeles National Forest and prompted mandatory evacuation orders in the foothill communities.

Firefighting aircraft quickly began making continuous retardant drops to stop the fire’s spread.

Residents in Sierra Madre were told to prepare to evacuate as the fire grew, burning just 1.6 miles away from the historic Mt. Wilson Observatory, which contains state-of-the-art equipment that recorded all California’s significant earthquakes for 100 years.

“The fire will probably be upon us today,” Mount Wilson Institute Chairman Sam Hale said in a statement.

In sharing a dramatic image of flames and smoke approaching Monday night, the observatory wrote: “The Bobcat is knocking on our door.”

Backfires were being set in an attempt to slow the blaze’s progress before it could reach the institution. Shortly after 11 p.m., fire officials said in a tweet that their protection operations to were “going as planned” and the flames had yet to reach the observatory.

The Bobcat Fire, which has been burning for more than a week, grew to 38,299 acres Monday evening with 3% containment — down from 6% containment in the morning, according to the U.S. Forest Service’s incident website.

Firefighters planned to spend the night using strategic firing operations and fixed wing aircraft to protect the communities south of the fire, the U.S. Forest Service said.

Officials are concerned about the fire’s southward spread toward the foothill communities.

Mandatory evacuations were ordered Sunday for some residents of Arcadia and Sierra Madre.

The evacuation order included all residents north of Elkins Avenue and east of Santa Anita Avenue. San Gabriel Canyon, including Camp Williams, also remains under an evacuation order, according to the Forest Service.

Evacuation warnings are in place for residents north of Foothill Boulevard and East of Santa Anita Avenue. The communities of Monrovia, Bradbury, Sierra Madre, Altadena, Duarte and Pasadena are also under an evacuation warning.

Residents have been asked to be ready to leave immediately if ordered to do so.

By 4:20 p.m., the flames had jumped the contingency line and started burning on a ridge line above the eastern side of Little Santa Anita Canyon. Sierra Madre police told residents in the area to prepare for an order to remove any vehicles and personal property from the street, or they’ll be towed around 6 p.m., according to a news release.

A Red Cross evacuation center has been reopened at Santa Anita Park, located at 285 W. Huntington Drive in Arcadia. Residents were asked to enter through Gate 5. Those who need lodging will be sent to hotels after arriving at the evacuation point, officials said.

The fire is mainly sitting in the west side of Santa Anita Canyon, below Mount Wilson, and the flames progressed up through chantry flats, Operations Chief Chad Cook said in a virtual update Monday.

Flames were burning “very rapidly” in the canyons, Cook said.

He assured residents that fire officials have a surge capacity of resources in and around the communities, strategically placed, to immediately respond if the fires spots into vegetation around homes. 

Fire activity was heavy on the west side of the blaze near Winter Creek and the Santa Anita Wash as it made its way to the Mt. Wilson Observatory.

The north end of the fire has reached Bear Creek and is expected to spread along the crest. So far, crews have successfully put out all spot fires that have crossed Highway 2. Overnight, the fire is expected to continue push up towards the highway.

In areas scarred by the in Ranch2 and Fish fires, firefighters saw little to no activity Monday.

Officials said visibility has been “extremely difficult” for all the pilots battling the fire, but the southwest push of the winds Monday helped aircraft make good progress to protect the foothill communities.

More than 1,000 firefighters are working to improve containment of the blaze.

The Bobcat Fire began on Sept. 6 near the Cogswell Dam and West Ford Day Use area. The cause of the fire remains under investigation.

Along with the El Dorado Fire, which has also burned for more than a week in the San Bernardino Mountains, the Bobcat Fire has brought moderate to unhealthy air quality conditions to the region.