With the plume of smoke from the Lake Fire still visible from the Santa Clarita Valley Monday morning, fire officials during Monday’s briefing announced that the blaze had grown to 18,562 acres, with 31% containment.

“The Lake Fire was fast-moving and tenacious, fueled by unforgiving terrain, burning 10,000 acres in just under four hours with minimal wind conditions,” said L.A. County Supervisor Kathryn Barger, whose 5th District includes the Santa Clarita Valley, as she joined fire officials for the morning briefing.

“This fire knows no jurisdictional boundaries, and as we combat back together under a unified command, neither do we,” she said. “Meeting under these circumstances is never ideal, and I want our community to know that they are not alone.”

The blaze, dubbed the Lake Fire after it began in the Lake Hughes area on Wednesday, continued to threaten 4,570 structures Monday, while at least 12 structures and 21 outbuildings had been destroyed, along with three structures damaged. These figures are expected to increase as damage assessments continue to be completed.

Overnight, moderate fire behavior allowed firefighters to improve existing containment lines and construct new fire lines, bringing the containment from 12% to 31%.

When the incident began on Wednesday, Angeles National Forest and L.A. County Fire Department officials formed a unified command, describing the potential challenges that the steep topography and weather could pose for firefighters, along with the fact that fuels in some areas had not burned in 100 years.

“We’re seeing that play out,” ANF Fire Chief Robert Garcia said Monday. “This fire started with some explosive fire behavior and moved into an area that was going to be extremely challenging for us, not only the fuel conditions but the topography.”

A federal incident management team was brought in to assist in long-term planning, managing resources and supporting ground crews.

Steep, rugged and inaccessible terrain continue to challenge firefighters and their safety as they work to build containment lines.

“All fires are challenging. This one’s especially challenging given the hot, dry conditions and the rugged, roadless areas we’re dealing with,” said Rick Young, incident commander of California Incident Management Team 5. “In addition to that, we’re dealing with fuels that haven’t seen fire in a significant period, leading to a very decadent understory and a forested overstory, which lends itself to long-range spotting, further hampering containment efforts.”

By Monday, nearly 1,900 firefighters were assigned to the blaze, including 189 fire engines, 26 hand crews, 25 bulldozers, 23 water tenders and 12 helicopters.

Over the weekend, high temperatures and wind created fire instability, triggering fire spread, along with thunderstorms and plume-dominated fire behavior.

On Saturday, approximately 50 to 100 lightning strikes caused spotting and uphill runs where the strikes occurred, which enlarged several large-scale fires, compromising current and proposed containment lines.

The National Weather Service forecasted high pressure over the region, with temperatures reaching 108 and relative humidity in the 10-19% range, which is expected to bring with it critical fire weather Monday through Thursday.

Evacuations remain in effect for Lake Hughes Road, west of Pine Canyon and north of Dry Gulch Road; east of Ridge Route Road; west of Lake Hughes Road and Fire Station 78; north of Pine Canyon and Lake Hughes Road; and south of Highway 138.

3 Points Road from Highway 138 to Pine Canyon, Old Ridge Route from Highway 138 to Pine Canyon, Pine Canyon from Ridge Route Road to Lake Hughes Road, and Dry Gulch Road from San Francisquito Canyon Road to Lake Hughes Road remain closed.

In addition, both upper and lower Castaic lakes remain closed through the duration of the fire, as firefighters are using the lake and surrounding areas as a base camp.

The smoke from the Lake Fire and Ranch2 Fire burning in the region has continuously caused unhealthy air quality, with a number of smoke advisories being issued by the L.A. County Public Health Department, including one on Monday.

Unhealthy air quality could be damaging to vulnerable groups, and Public Health officials suggest residents in the SCV take precautions, such as limiting outdoor activity and remaining inside as much as possible.

“Children and people who have air-quality-sensitive conditions, such as heart disease, asthma, and other chronic respiratory diseases, should follow these recommendations and stay indoors as much as possible even in areas where smoke, soot, or ash cannot be seen, or there is no smell of smoke,” officials said in a news release.