A fire weather warning is an alert from the National Weather Service that the weather could make wildfires spread fast and burn out of control. In the Bay Area, these alerts come from the Weather Service office that covers San Francisco and Monterey. Forecasters there watch for a dangerous mix of strong wind, low relative humidity and very dry vegetation. When those line up, a single spark can grow into a fast-moving fire within hours. The main warning for this danger is the Red Flag Warning, and it comes with a related watch that is worth understanding too.
What Are the Fire Weather Alert Tiers?
The Weather Service uses two main fire weather alerts. The first is the Fire Weather Watch, which means dangerous fire weather is possible within the next one to three days. A watch is a planning tool that tells fire agencies, park managers, utilities and the public to get ready in case the threat develops.
The second alert is the Red Flag Warning. It means critical fire weather is happening now or will start soon. Some of the Bay Area's worst fires burned during Red Flag conditions, including the 2017 Tubbs Fire in Santa Rosa and the 2019 Kincade Fire in Sonoma County. A warning means forecasters are confident the dangerous weather will arrive.
For the most extreme events, forecasters often add stronger wording to the warning and to their written forecast discussions. They may describe the setup as a high-end or rare event, compare it to past fires or urge people to prepare to leave quickly. That extra language does not create a new type of alert, but it signals that the coming wind event could be among the worst in recent memory.
What Thresholds Trigger a Fire Weather Warning?
There is no single national formula for a Red Flag Warning. Each Weather Service office sets its own criteria, based on the local fuels, terrain and wind patterns. In the Bay Area, forecasters look at wind and humidity together, along with how dry the grass and brush have become. Very strong wind with somewhat low humidity can be just as dangerous as moderate wind with extremely dry air. A hot, dry day without much wind usually does not qualify.
The most dangerous setups usually come during Diablo wind events. These dry offshore winds blow from the northeast and sink down the slopes of the East Bay and North Bay hills. As the air sinks, it warms and dries out. Ridgetops can see gusts of 40 to 60 mph or more, and humidity can fall into the teens or single digits.
Humidity is one of the key numbers. On a normal summer day, damp ocean air keeps the coast fairly humid, while inland valleys such as Concord and Napa are drier. During a Diablo wind event, the whole region can turn bone dry, even places that are usually damp and foggy. Dead grass, leaves and twigs lose their last moisture quickly in that air. A spark from a downed power line, a car exhaust or a cigarette can then light grass that carries fire into brush and trees.
Wind matters because it drives how fast a fire spreads. In light wind, a grass fire may move slowly enough for crews to catch it. Strong gusts push the flames forward and carry burning embers far ahead of the main fire, starting new spot fires. That is how the 1991 Oakland-Berkeley Hills Fire, driven by dry offshore wind, destroyed 2,843 single-family homes and 437 apartment units in a single day.
How Is a Fire Weather Watch Different From a Red Flag Warning?
The difference between the two comes down to timing and confidence. A Fire Weather Watch comes first, often a day or more ahead, when forecasters see a weather pattern that could meet Red Flag criteria. It gives fire agencies time to move crews and equipment into place and to prepare warnings for the public.
In fire-prone areas such as the East Bay hills or the slopes above Napa Valley, a watch is the signal to get ready. That means reviewing evacuation routes, charging phones and making sure go bags are packed.
A Red Flag Warning replaces the watch once forecasters are confident the dangerous conditions will arrive. At that point, the plans start turning into action. PG&E may shut off power in high-risk areas to keep its lines from sparking fires, a step called a Public Safety Power Shutoff. Fire agencies add staff and move extra engines into the hills. Since PG&E began these shutoffs in 2018, they have become a regular part of fall in hill communities, at times affecting hundreds of thousands of customers.
Most watches in the Bay Area turn into warnings, but not all of them do. Sometimes the wind comes in weaker than expected, or humidity recovers sooner. Having both a watch and a warning lets forecasters give early notice while saving the stronger alert for when the danger is clear.
When Are Fire Weather Warnings Most Common in the Bay Area?
Bay Area fire weather alerts cluster in the fall, from about September through November. By then, grass and brush have gone months without rain and reach their driest point of the year. Fall is also when offshore wind events become more common, as high pressure starts to build over the Great Basin and the summer sea breeze weakens.
The number of warnings changes a lot from year to year. October 2017 brought back-to-back wind events, including the one that drove the Tubbs Fire. October 2019 brought the Kincade Fire during another long Red Flag period. In years when the first big storm comes early, the fire weather season can be cut short.
Summer warnings are less common, because the marine air usually keeps humidity up near the coast and the bay. They do happen, though, especially during heat domes, when the marine layer shrinks and inland areas bake for days. Early July 2024 brought one example, when a long heat wave led to Red Flag Warnings for the hills and inland valleys. Areas such as Concord, Livermore and the upper Napa Valley can see summer warnings in those conditions.
What Should You Do During a Fire Weather Warning?
During a Red Flag Warning, the most important step is staying informed. Sign up for your county's alert system, such as AlertSCC in Santa Clara County, SoCoAlert in Sonoma County or AC Alert in Alameda County. If you live near open hills, keep a go bag packed with documents, medicine, chargers and other essentials. Know at least two ways out of your neighborhood, because a fire can block the obvious route.
Avoid anything that could start a fire during a warning. Many parks close trails or fire roads during Red Flag Warnings, and for good reason. A spark from a lawn mower or a dragging trailer chain can start a fire. In strong wind, that fire can spread faster than a person can run. Wildfire smoke can also travel far, so check the AQI before heading outside, even when the nearest fire is 50 or 100 miles away. The wildfire smoke guide explains how to protect yourself.
For many Bay Area residents, the fires of 2017 through 2020 made these warnings personal. The fires that burned through Santa Rosa neighborhoods in 2017 moved through suburban streets, not remote wilderness. Knowing what a watch and a warning mean, and having a plan for each, is one of the simplest ways to stay safe when the dry winds arrive.
