The Bay Area has a fire season, and it has grown more dangerous over the past few decades. The risk builds through the dry summer and peaks in the fall, when strong offshore winds can arrive after months without rain. Some of the region's worst fires have struck in October, when those winds turn a small spark into a fast-moving disaster. The same hills and valleys that give the Bay Area so many microclimates also make it vulnerable. Steep slopes, dry grass and brush, and homes built right up against the wildlands all raise the stakes when fire weather arrives.
The Fuel and Dryness Cycle
Fire danger in the Bay Area builds slowly through the summer. The region has a Mediterranean climate, so almost no rain falls from May through September. Grass and brush that grew green with the winter rains begin drying out in spring. By late September, they have gone five months or more without a soaking.
Firefighters call the quick-burning grass, small plants and twigs "fine fuels." These ignite easily and carry fire quickly, and they reach their driest point in September and October. Larger fuels, such as dead branches and the trunks of shrubs, dry out more slowly over the summer. By fall, both kinds are often ready to burn.
Much of the Bay Area's native vegetation evolved alongside fire. Chaparral, the dense shrubland that covers many inland hills, burns every few decades, and some of its plants need fire or smoke to sprout their seeds. Oak woodlands can usually survive low and moderate fires. However, a century of putting out fires as quickly as possible has let dead wood and brush pile up in many places. Combined with hotter, drier summers, that buildup means fires today can burn more intensely than the fires that shaped these landscapes.
Diablo Winds: The Fire Weather Trigger
Diablo winds are the weather pattern that turns dry fall hills into an emergency. They form when high pressure builds over the Great Basin, the high desert east of the Sierra Nevada, while pressure falls along the California coast. Air then flows from the interior toward the ocean, reversing the usual sea breeze.
As that air pours down the slopes of the coastal hills, it is squeezed by rising pressure and warms as it sinks, so it arrives dry and gusty. In the East Bay hills and the mountains of the North Bay, gusts can reach 40 to 70 mph and temperatures can run well above normal. At the same time, relative humidity can drop into the single digits.
Under those conditions, a downed power line, a spark from a car or a carelessly tossed cigarette can start a fire that grows within minutes. The wind blows embers far ahead of the main fire, starting new spot fires and letting flames jump roads and firebreaks. A wind-driven fire in dry grass and brush can cover several miles in an hour, faster than many people can evacuate.
Most Dangerous Areas and Recent History
The highest risk falls on neighborhoods built where homes meet wildland, in the hills around the edges of the region. The East Bay hills, including parts of Oakland, Berkeley, Moraga, Orinda and Danville, are among the best-known examples. Steep canyons, dense brush and eucalyptus groves sit right next to homes, and the area catches the full force of Diablo winds.
That danger became clear on October 20, 1991, when the Oakland-Berkeley Hills Fire swept through the hills. Driven by strong, dry offshore winds, the fire killed 25 people. It destroyed 2,843 single-family homes and 437 apartment units in a single day, making it the most destructive wildfire in California at the time.
The North Bay has seen even larger losses in recent years. On October 8 and 9, 2017, Diablo winds drove a series of fires across Napa and Sonoma counties. The Tubbs Fire alone destroyed more than 5,600 structures, including whole neighborhoods of Santa Rosa, the North Bay's largest city. In October 2019, the Kincade Fire started during another offshore wind event and led to the largest evacuation in Sonoma County history.
Not every big fire follows that pattern, though. The LNU Lightning Complex of August 2020 was started by dry lightning during a heat wave, not by wind, and burned across several North Bay counties. It showed that the late summer heat can bring major fires even before the offshore wind season begins.
Fires far outside the Bay Area can affect it too. In November 2018, the Camp Fire destroyed the town of Paradise in Butte County, more than 150 miles away. Its smoke drifted south and gave the Bay Area some of the worst air quality on record for nearly two weeks. During any fire season, it is worth checking the air quality forecast along with the fire weather forecast, because smoke can travel hundreds of miles.
Deep Dive: How Fast Do Grass and Brush Dry Out?
Fire scientists track how wet vegetation is with a number called fuel moisture. It is the weight of water in a plant or twig compared with its dry weight. Dead fuels, such as cured grass and fallen branches, follow the weather within hours or days. Living shrubs dry out more slowly, over the whole summer.
How Quickly Do Dead Fuels Respond to the Air?
Dead plant material constantly trades moisture with the air around it. Given enough time, it settles at a moisture level set by the temperature and humidity, called its equilibrium moisture content. Thin fuels get there quickly, while thick logs take much longer.
The National Fire Danger Rating System sorts dead fuels into four classes by that response time. One-hour fuels are less than a quarter inch thick, such as grass and pine needles. Ten-hour fuels are a quarter inch to 1 inch, 100-hour fuels are 1 to 3 inches and 1,000-hour fuels are thicker still. Each name gives the time a fuel takes to cover about 63 percent of the distance toward its new equilibrium.
That is why grass fires can turn dangerous so quickly. When a Diablo wind drops the humidity into the teens, cured grass dries to its new low moisture within a few hours. Large logs barely change over the same night, but the fine fuels that carry a fire across a hillside are ready to burn by morning.
When Do Living Shrubs Reach Critical Levels?
Living plants hold far more water, often more than their own dry weight in spring. Their moisture falls slowly through summer as the soil dries out. Fire agencies sample shrubs such as chamise, the most common chaparral plant in California, every few weeks to track that decline.
Philip Dennison of the University of Utah and Max Moritz of UC Berkeley studied that record for Los Angeles County. They published the results in 2009 in the International Journal of Wildland Fire. They found strong evidence for a threshold near 79 percent live fuel moisture in chamise, below which large fires became far more likely. The timing of that drop depended mostly on spring rain, especially in March.
The study looked at Southern California, but the same pattern shapes the Bay Area's calendar. A wet spring keeps the shrubs green into summer, while a dry one brings them to the danger zone weeks earlier. By September and October, chaparral across the region is usually at its driest. That is the same season when Diablo winds start to blow.
Fire Weather Season and Preparedness
The National Weather Service issues a Red Flag Warning when strong wind, low humidity and dry vegetation come together to make fast fire spread likely. A Fire Weather Watch comes a day or two earlier, when those conditions look possible. On the most dangerous days, utilities may cut power to some neighborhoods to keep their lines from sparking fires. These Public Safety Power Shutoffs have become a regular part of fall in hill communities of the East Bay and North Bay.
If you live in or visit a fire-prone area, get ready before the season peaks. Pack a go bag with documents, medicine, chargers and a change of clothes by September. Learn more than one way out of your neighborhood. Sign up for your county's emergency alerts as well, such as AlertSCC in Santa Clara County or SoCoAlert in Sonoma County.
When an evacuation order comes, leave right away instead of waiting to see what the fire does. In the 2017 and 2019 fires, many people escaped because they left quickly, while some who waited ran out of time. Clearing brush and dead plants from around your home also gives firefighters a better chance to save it. Like earthquakes, fire is a risk that comes with living in the Bay Area, and simple planning makes a real difference.
