On March 27, a brush fire swept the canyon under the Vermont Street Pedestrian Bridge in University Heights. Despite a five-engine response and helicopter water drops, neighbors watched flames climb another eucalyptus, moving closer to craftsman homes perched on the canyon rim. Luckily it was a mild day, with hillsides still green from light rains. A final water drop doused the tree. Would the story have been different during red flag conditions?

As damaging wildfires rage across California – firefighters are already fatigued, though the typical wildfire season hasn’t even started yet – most of the focus is on forests and fire-prone swaths of backcountry.

Urban San Diego may seem like an unlikely place for an out-of-control wildfire. After all, the city’s many canyons are small — much smaller than the wildlands currently burning in other parts of the state. They are also cut-through with street networks, and close to city fire stations to ensure a quick response.

But San Diego’s unique topography sets the stage for what could actually be a damaging wildfire: flammable vegetation on steep slopes — fire can burn faster upslope — with older neighborhoods sitting at the top of these slopes. This is what’s called the wildland-urban interface, where the city meets nature. The two are tightly woven here.

“Given the right conditions and environment, an urban wildfire is a real threat,” said Eddie Villavicencio, the assistant fire marshal of the San Diego Fire-Rescue Department. “The Fire Department is aware of and proactively addressing the threat.”

A big concern is how quickly a brush fire could reach homes and possibly spread into a neighborhood.

“If you have older construction that is very fire prone, it doesn’t take much to ignite a house,” said Chris Dicus, a professor of wildland fire and fuels management at California Polytechnic State University.

Embers are the key to wildfire spread. They can fly from treetop to rooftop and from house to house, easily crossing streets, especially during extreme fire weather. Dicus, who leads the Wildland-Urban Interface group for the California Fire Science Consortium, has seen fire “hopscotch” around neighborhoods and ignite homes several blocks away.

“Embers can go well past where the flames would be,” he said.

So where does the wildfire threat loom large? The city of San Diego formally adopted wildfire hazard maps, based on recommendations from CAL FIRE, in 2009. The maps show “very high” fire hazard severity zones throughout urban and suburban San Diego, closely following vegetated slopes. For example, these “red” zones run through central areas like Mission Hills, Hillcrest, University Heights, Balboa Park, South Park, City Heights and Normal Heights. The zones often extend beyond the canyons, several blocks into neighborhoods.

Derived from a fire behavior model, the hazard zones offer a warning about where flames or embers could intrude into an urban area. The model considers many factors, such as slope, fuel (anything flammable), weather and fire history. The city adjusted CAL FIRE’s initial maps to include more localized terrain and vegetation, like smaller canyons.

Any brush fire that receives multiple calls becomes a first alarm vegetation fire, which involves at least four fire engines and places a firefighting helicopter on standby. They aim to be on the scene in less than five minutes.

“We’ve been successful at quickly controlling brush fires and limiting their damage and spread to under five acres,” Villavicencio said. “Seconds matter when brush is close to structures.”

That may help explain why urban San Diego hasn’t seen a damaging brush fire since the 1985 Normal Heights Fire. That fire started near Mission Valley slopes and sped into the canyon network to destroy 76 homes and damage 57. A witness described seeing flames “50 feet in the air” and the fire “leaping streets” after it crested the canyon. Ultimately, the fire perimeter extended to Copley Avenue and 35th Street, a roughly two- by seven-block section of the neighborhood.

To protect property in very high fire hazard severity zones, the city now requires that new construction or home remodels meet fireproofing standards under the California building code, such as ignition-resistant roofs and decks, and ember-resistant attic vents. The building standards, however, do not mandate retrofits of existing older homes. And the hazard zones fall within many of San Diego’s classic “streetcar suburbs,” most built by the 1940s.

Canyon residents are also required by law to maintain 100 feet of defensible space, clearing dry vegetation in a way that can slow flames’ easy pathway to the home. Although it’s important, defensible space alone won’t safeguard property — not during the extreme weather and erratic fire behavior seen across California in the last year.

Wind-driven fires are “the fires that really make disasters roll,” said Richard Halsey, director of The California Chaparral Institute, a nonprofit that works to preserve the state’s shrubland ecosystems. Trained as a wildland firefighter, he has long advocated protecting communities from the house out, instead of the wildland in.

“It’s not the canyons, it’s human infrastructure placed in the wrong place, in the wrong way,” he said.

He suggests homeowners take proactive steps to prevent embers from igniting a home. Even simple measures can make a difference, like removing wooden fences, firewood piles, dry leaves, and the most flammable landscaping: Mexican fan palm and acacia. Major retrofits can be expensive but can save an older home, such as flame-resistant roofs and siding and roof and under-eave sprinkler systems.

Urban residents may not know they are living in a wildfire hazard zone. Alexandra Webber and her husband David Shirk, who live on a canyon in University Heights, entered their address into the county’s wildfire hazard mapping tool and found that their area ranks “very high” – the highest hazard level. Even though they’ve worked with inspectors on defensible space, wildfire wasn’t something they actively worried about.

“I thought, given our urban-ness, someone would smell smoke and report it,” Webber said.

Villavicencio laments that preparedness, which always spikes after a devastating event, has waned.

“We are trying to figure out how to get more people engaged,” he said.

A good place to start is the city’s Ready, Set, Go! guide to wildfire preparedness. It covers home hardening (fireproofing to prevent ignition from embers), defensible space (vegetation thinning to keep flames away from the home) and family disaster planning. Canyon homeowners can also consult the Fire-Rescue Department’s brush management guide. Most important? Be ready to go and leave early if a wildfire threatens.

At the neighborhood level, Fire Safe Councils work to bolster preparedness across California. San Diego County has 35 of these community groups, but this map shows almost all are in far-flung suburbs and backcountry communities.

Beverly Barrett established the Kensington Fire Safe Council, the lone group in urban San Diego. Their work has helped to educate neighbors, who now know evacuation routes and participate in an annual brush-clearing program.

“Before, they just didn’t have any awareness,” she said. She points to the finger canyons that branch out from the I-15 freeway and run behind residential streets. “We are just in a bad place here. There aren’t 20 acres you could burn and not get a home.”

The March fire under the Vermont Street Bridge has nearby residents on alert. They’ve written letters demanding that the city clear brush and debris from the area, known as Camelot Canyon.

“It’s just loaded with dead stuff down there,” neighbor Deborah Morrison said.

City Councilman Chris Ward, who represents the neighborhood and has been fielding concerns from constituents, said in a statement: “I hope that this unfortunate incident will act as a catalyst for a more concentrated and coordinated effort to maintain San Diego’s canyons. With so many homes aligning with the dense vegetation of canyons and other open space areas, it is critical that brush management activity is done in an efficient manner that reduces the risk of fire and minimizes the damage to the surrounding environment.”

After a very dry winter and record warmth this summer, fuels – both dead and live vegetation — are extremely dry. The wildfire outlook for the region is ominous, as fuels continue to dry and Santa Ana winds arrive in the fall.

“We are entering the most critical time of our fire season,” said Tom Rolinski, senior meteorologist for the U.S. Forest Service in Riverside. “The potential is very high for significant fires to occur.”