LOS ANGELES — The weather conditions were unremarkable for coastal Orange County, and even a bit pleasant: Mild temperatures, relatively moist air and a seasonal onshore breeze.
But when firefighters struggled to contain a 50-by-50-foot brush fire on a sere hillside in Laguna Niguel on Wednesday afternoon, officials grew concerned. Within a few hours, multiple homes were ablaze and spewing hot embers as the Coastal fire chewed methodically through an upscale development overlooking the Pacific Ocean. By the time the fire’s spread slowed, at least 20 homes had been destroyed, many of them overlooking the canyon where the fire began.
The sudden and severe destruction has left many to wonder just how such a fire could erupt amid mundane conditions. Yet experts say that preliminary reports suggest the devastation was due to an unlucky combination of factors. Moderate winds, steep terrain and drought-ravaged vegetation worked together to drive flames into a community where homes had been constructed before fire-hardening building codes took effect.
“That’s not atypical, for a fire to happen when those pieces start to align — not aligned in our favor but aligned against us,” said Capt. Sean Doran, spokesperson for the Orange County Fire Authority. “As they start to stack, it becomes a more self-propagating force.”
After the fire started on the hillside due west of the South Orange County Wastewater Authority treatment plant, county firefighters arrived within minutes to find the blaze had spread to at least an acre, Doran said.
Firefighters trudged up the hillside with equipment and formed two flanks in hopes of extinguishing the blaze, he said. However, an ocean breeze drove the flames downhill and to the east. Winds peaked with gusts of up to 25 mph around 4 p.m., according to the National Weather Service in San Diego.
Fire brands then ignited hills to the east of the treatment plant, torching dry grasses and shrubs. The wind-driven flames continued west, and radiative heat and flames naturally reached higher and higher, swiftly racing up to homes perched on the hilltop, Doran said.
“Once the fire hit the base of the hill below the homes, it was like an arrow that just shot to the top,” said OCFA Chief Brian Fennessy.
On hillsides, fire creates a chimney effect as a column of hot gases, smoke, ash and debris rises, potentially overriding the ambient wind direction and creating a draft that pulls in oxygen. Features such as gullies, ravines or chutes concentrate this hot, rising air, called a convective column. The hot gases preheat fuels ahead of the fire as the blaze roars up a slope. Hot embers can be carried by this rising air and the wind, touching off smaller fires ahead of or behind the main blaze.
“A matchstick — or any fire — is doing the same thing,” said Issac Sanchez, a battalion chief for communications at the California Department of Forestry and Fire Protection. Hot air rises, and because nature abhors a vacuum, air from the surrounding environment rushes in to fill the void, he said.