A Red Flag Warning is the National Weather Service's main alert for dangerous fire weather. It means the weather right now, or very soon, could let fires start easily and spread fast. One step below it, a Fire Weather Watch means those conditions are possible, usually within the next day or two, but they are not yet certain. In the Bay Area, both alerts come from the Weather Service office that covers San Francisco and Monterey. They are most common from September through November, when fire season peaks. The difference between them is the difference between getting ready and acting now.
What Is a Red Flag Warning?
A Red Flag Warning goes out when strong wind, low relative humidity and very dry vegetation are expected to come together. Each Weather Service office sets its own criteria based on local fuels and terrain. Bay Area forecasters weigh wind and humidity together, so very strong wind with somewhat low humidity can qualify, and so can lighter wind with extremely dry air.
When those conditions line up, almost any spark can start a fire that grows quickly. A downed power line, a trailer chain dragging on the road or a lawn mower blade striking a rock can all be enough. Wind-driven fires in dry grass and brush can grow by thousands of acres within hours.
A Red Flag Warning is not a forecast that a fire will happen. It is a forecast that any fire that does start will be very hard to control. Each warning covers specific areas, such as the North Bay mountains or the East Bay hills, for a set period, often a day or two.
In the Bay Area, the most common cause is a Diablo wind event, when dry offshore winds race down the slopes of the hills from the northeast. Gusts on the ridges can reach 40 to 70 mph, and humidity can fall into the single digits. The 1991 Oakland-Berkeley Hills Fire, the 2017 Tubbs Fire in Santa Rosa and the 2019 Kincade Fire all burned during dangerous offshore wind events.
What Is a Fire Weather Watch?
The watch is the early alert. Forecasters issue it when they see a real chance of Red Flag conditions, often 24 to 72 hours ahead, but are not yet sure. The ingredients look familiar, with strong wind, low humidity and dry fuels. Forecast models may still disagree, though, about the timing, the wind speeds or how dry the air will get.
As confidence grows, a watch is often upgraded to a warning, sometimes a day before the worst conditions arrive. Still, not every watch becomes a warning. The wind may come in weaker, humidity may stay a little higher or the event may hit a different part of the region. Forecasters can also skip the watch and go straight to a warning when dangerous conditions develop quickly. Either way, a watch is a valuable head start, so treat it seriously.
How Do These Alerts Differ in Practice?
The practical difference between the two comes down to urgency. While a watch is in effect, review your evacuation plan, charge your devices, find your go bag and keep an eye on forecast updates. It is also a good time to clear dry brush from around your home and park your car facing out of the driveway. Make sure you know the routes you would take if an evacuation order came.
Utilities such as PG&E start deciding during the watch phase whether to cut power to some areas, a step called a Public Safety Power Shutoff. They try to notify affected customers a day or two ahead, so watch for those messages too.
During a Red Flag Warning, the danger is either about to arrive or already here. Power shutoffs are most likely to be in effect, and fire agencies add extra crews. A single spark can become a large fire within hours. People in high-risk areas should be ready to leave on short notice. Those areas include the East Bay hills in Oakland and Berkeley, and wine country around Napa, Sonoma and Santa Rosa. The hills around inland valley towns such as Walnut Creek, Concord and Livermore are at risk too. If an evacuation warning or order comes during a Red Flag Warning, leave right away instead of waiting to see flames.
Why Bay Area Fire Alerts Matter More Than You Think
Bay Area fire risk follows the terrain closely. During fall heat events, the inland valleys and the hills east and north of the bay heat up and dry out. The coast, meanwhile, often stays cooler and damper. On a Diablo wind day, humidity in Livermore or Concord can drop below 10 percent while parts of San Francisco stay far more humid.
Because conditions vary so much, the Weather Service divides the region into fire weather zones. A Red Flag Warning may cover the North Bay mountains and East Bay hills but not the San Francisco Peninsula. When a warning goes out, check which zones are included and whether your home or plans fall inside them.
Smoke behaves differently from flames. Wildfire smoke can spread across the entire region, whatever zone the fire burns in. Air quality can turn unhealthy across all nine Bay Area counties within hours of a major fire. The flames themselves follow the terrain and the wind, but the smoke can reach everyone.
What Should You Do During Each Alert Level?
Once a watch is issued, focus on preparation. Make sure your household has a go bag with medicine, important documents, phone chargers and enough supplies for three days. Pick two ways out of your neighborhood in case one is blocked, and fill your car's gas tank. If you have pets, large animals or medical equipment, plan how you would move them.
Sign up for your county's emergency alerts if you have not already. AlertSCC covers Santa Clara County, SoCoAlert covers Sonoma County and AC Alert covers Alameda County. Other counties have similar systems, and many cities also post updates through Nixle.
During a Red Flag Warning, move from preparing to being ready. Keep shoes and car keys by the door and your go bag in the car. Follow local fire agencies and emergency alerts for updates. If you see smoke, call 911 right away rather than assuming someone else has reported it. In the 2017 North Bay fires, flames reached some neighborhoods within minutes of the first warnings. If you live in a high-risk area, leaving early, before you are told to, is often the safest choice you can make.
How Often Does the Bay Area Get Red Flag Warnings?
The number of Red Flag Warnings changes a lot from year to year, and most come in the fall. Some years bring several back-to-back events, and the fall of 2019, for example, saw repeated Red Flag periods across Northern California and several large power shutoffs. Other years are quieter, especially when an early storm soaks the hills in October. Watches are more common than warnings, because many forecast events weaken before they reach warning levels.
Fire seasons across California have grown longer, as warmer temperatures dry out vegetation sooner and the first soaking rains often arrive later. Two recent fires showed that disastrous fire weather can strike well after the traditional peak. The Thomas Fire burned in Ventura County in December 2017, and the Camp Fire destroyed Paradise in November 2018. For Bay Area residents, treating every Red Flag Warning as a serious event is simply realistic.
Together, the two alerts give you a timeline. A Fire Weather Watch is your signal to plan, and a Red Flag Warning is your signal to be ready to move. Bay Area fire history shows that the time between a warning and a life-threatening evacuation can be very short. Use the watch to prepare, so that when a warning comes, you can act without hesitation.
