On a new Bay Area wildfire-risk map that gauges the threat of wind-driven embers, Lars Guntvedt’s neighborhood in the hills above Los Gatos sits in the highest-danger zone.

Like many of his of Aldercroft Heights neighbors, and plenty of residents around the Bay Area, the 58-year-old school administrator felt confident he had reduced the danger to himself, his wife and their wooden home by diligently clearing the surrounding vegetation. The ember-risk map, Guntveld said, made him consider “what additional hardening I can do with my house.”

Based on NASA satellite imagery, the map shows where vegetation is most likely to produce dangerous, fire-spreading embers during high winds and low humidity, threatening homes far from main blazes.

Recent firestorm catastrophes, including the deadly 2017 razing of the Coffey Park neighborhood in Santa Rosa after bits of burning debris from the Tubbs Fire floated over six lanes of Highway 101, and the devastating Los Angeles fires earlier this year have highlighted the role of embers in dramatically speeding and spreading wildfires.

Embers can send spot fires far ahead of a main blaze, igniting yard plants and transporting fire to houses.“The fire can be down the hill or across the valley and you’re going to get embers falling on your property,” said Craig Clements, director of San Jose State University’s Wildfire Interdisciplinary Research Center. “One or two embers get in the attic and that’s what sets off the fire and the house starts burning down.”

The map, a collaboration between NASA’s Ames Research Center and the fire-protection nonprofit Santa Clara County FireSafe Council, shows areas where the types and densities of vegetation would be expected to generate embers, also known as firebrands, that could ride winds to new locations.

“When it comes down, it has a very high chance, if it’s a dense ember, very hot in the middle, to more or less explode, and spew flames out of it, which can ignite other vegetation or even a structure,” said Christopher Potter, an scientist at NASA Ames who leads the map project.

The redder the area on the map, the higher the chance it could launch a barrage of dense firebrands, Potter said.

The map comes as climate change increases the frequency and severity of wildfires in California; four out of five of California’s most-destructive wildfires and nine of the 10 largest have occurred since 2017. Fire experts say the map provides a valuable new layer of information for Bay Area fire officials.

NASA has not published a publicly available map or links, but the data are available to local agencies.

“NASA maintains and updates the map and data, and the Santa Clara County FireSafe Council works with towns and municipalities that are interested in using the data for their wildfire programs,” said Seth Schalet, chief executive officer of the Santa Clara County FireSafe Council.

Among the most ember-vulnerable communities in the South Bay region, fire experts said, are Saratoga, Los Gatos, Los Altos Hills, Morgan Hill, Gilroy, and in San Jose, Almaden and Coyote Valley, along with Willow Glen, which Potter described as “a perfect example of a place where a fire could rip through there, burn house after house, if it came out of the hills.”

“I worry about Los Altos and Mountain View burning,” Potter said, “because of all the vegetation that’s up there in Los Altos Hills.”

A map for Marin County shows the highest risks mostly concentrated in the areas west of the Highway 101 corridor.

“Marin Wildfire is very much invested in a science-based approach in all of our mitigation efforts,” said Mark Brown, executive officer of the Marin Wildfire Prevention Authority. “We analyze many different modeling tools and like to overlay their results with each other. This helps us to see where the models agree and therefore increases our confidence in the accuracy of the models.”

“Different models, looking at different aspects of the wildfire environment also helps us to choose the correct mitigation efforts for individual areas as a one size fits all solutions really don’t exist,” Brown said. “This particular modeling is something we find useful in describing the importance of home hardening and fire smart landscaping, particularly emphasizing the need for ‘Zone 0’ compliance.”

“Zone 0” refers to a state law, yet to be implemented, requiring a 5-foot ember-resistant buffer around residences in high-risk areas.

Bay Area downtowns, where concrete dominates, are generally considered safe from wildfire, but many of their outskirts and suburbs are not.

High-risk areas on the ember map are often identified in other maps, including Cal Fire’s. But the new firebrand-focused map shows areas considered engines of wildfire spread that menace surrounding neighborhoods and landscapes.

“During the conditions that generate a lot of embers you’re going to get heavy ember casts out to 1 or 2 miles,” said Michael Wara, a researcher at Stanford University’s Woods Institute for the Environment.

Jared Lewis, manager of environmental planning and natural resources at San Jose Water, the utility that owns and manages much of the Los Gatos Creek watershed, said the ember map’s “higher level of detail and resolution” than the Cal Fire maps is already helping the utility identify new areas of risk and prioritize where to thin and clear vegetation.

To produce the map, satellite imagery is analyzed to show the landscape by categories of vegetation — forest, shrub and grasses — and how thickly they grow.

That imagery is combined with data showing which vegetation species tend to produce dense embers that can travel long distances without burning out, plus additional mapping by NASA and the U.S. Department of Agriculture of where plant species grow around the Bay Area, Potter said.

Many plants common in the Bay Area can produce dangerous embers, including coyote brush, chamise, broom, eucalyptus, manzanita, pampas grass, acacia, bay, many pines and Douglas fir, Potter said.

The map is intended to guide work to cut fire risk through controlled burns, fire breaks, and thinning and trimming of firebrand-prone trees and other vegetation, particularly in locations close to residential and commercial areas “where these embers don’t have to travel far to cause a conflagration,” Schalet said.

The map also will provide a compelling tool to persuade local authorities and private landowners to take on that work or contract with the council to have it done, Schalet said.

Wara noted that once large numbers of houses catch fire, “they’re generating plenty of embers all on their own.” He favors extensive prescribed burns, including “right around houses.”

The FireSafe Council is working to add layers of information on top of the new map, including which areas have the worst routes in and out, and which communities — like Aldercroft Heights — have many older homes less resistant to fire, Schalet said. He would like to see ember-risk maps statewide.

NASA Ames is looking at wind patterns during past fires to add another layer, to determine, for example, that “not only is this a really bad ember production place but it also routinely gets winds at 60 mph during the dry periods of the year,” Potter said.

The Independent Journal contributed to this report.