A flood advisory means minor flooding is happening or expected, so stay alert. A flood warning means serious flooding is happening or about to happen, so take action right away. A flood watch means flooding is possible, but it has not started yet. The National Weather Service office in Monterey, which covers the Bay Area, issues all three. Each step up signals more danger and less time to act. Knowing what each one means matters most for people who live in the Bay Area's flood-prone areas.
What Does Each Flood Alert Level Mean?
The Weather Service uses a similar ladder of alerts for river and rain flooding as it does for coastal flood alerts. The earliest signal is a flood watch, which means the weather pattern could cause flooding, usually within the next day or two, but the flooding has not begun. In the Bay Area, a watch often goes out before a strong atmospheric river arrives. At that stage, forecasts show heavy rain aimed at the region, but the exact totals and timing are still uncertain, so the watch is your window to prepare.
A flood advisory means flooding is happening or expected soon, but the effects should be minor. Think of water pooling on roads, flooded low spots and small creeks rising toward their banks. An advisory is not a reason to ignore the situation. It means the flooding is not expected to threaten lives if people use normal caution.
Advisories are common during the wet season from November through March. They are especially likely in cities such as San Francisco and Oakland, where heavy rain can overwhelm storm drains and flood streets.
The next step up, a flood warning, signals significant flooding that is happening, about to happen or very likely. In the Bay Area, a warning usually means a creek or river has reached flood stage or soon will. Homes and roads in the floodplain are then at risk. Forecasters watch stream gauges that measure water levels in real time. Key gauges sit on Coyote Creek in San Jose, the Napa River in Napa and San Francisquito Creek on the Peninsula. When a warning covers your area, be ready to leave or move to higher ground.
How Are Flash Flood Warnings Different from River Flood Warnings?
The Bay Area gets two main kinds of flood warnings, and the difference affects how fast you need to move. A regular flood warning covers flooding that builds over hours or days, as steady rain fills creeks and rivers. That kind of flooding gives people some time to react, though not always as much as they expect.
The Coyote Creek flood in San Jose on February 21, 2017 showed how quickly that time can run out. After weeks of rain, water poured over the spillway at Anderson Reservoir upstream, and the creek rose into neighborhoods that many residents did not know were at risk. About 14,000 people had to evacuate, many with little notice. The flood led the city and the county water district to overhaul how they warn residents along the creek.
A flash flood warning works on a much shorter clock. It signals dangerous flooding that is happening now or will start within minutes to a few hours. It usually comes from very heavy rain that overwhelms small creeks, storm drains or recently burned hillsides. Flash flood warnings give you minutes, not hours, to act.
In the Bay Area, flash flood warnings are most common in steep terrain, such as the Santa Cruz Mountains and the hills of Marin and the East Bay. Burn scars from recent fires are especially risky, because scorched soil sheds water and can send mud and debris down a canyon. Intense bursts of rain on already soaked ground are the main trigger, and the region's wettest days often bring several flash flood warnings at once.
Where Are Flood Alerts Most Common in the Bay Area?
Flood warnings tend to focus on the same creeks and rivers. The Napa River through Napa has a long history of flooding, though a decades-long flood control project has greatly reduced the risk downtown. Strong atmospheric rivers can still push the river above flood stage.
Coyote Creek in San Jose remains a frequent concern because it runs through crowded neighborhoods with limited room in its channel. Several other creeks also have a history of flood warnings during strong winter storms. They include Corte Madera Creek near Corte Madera, San Francisquito Creek between Palo Alto and East Palo Alto, and Alameda Creek in the East Bay.
Flood advisories are more common on flat ground in the cities. Much of San Francisco's sewer system, which carries both rain and wastewater, was built more than a century ago. Low spots in the Mission District and SoMa often flood during heavy rain. Along the coast, towns such as Pacifica and Half Moon Bay can face creek flooding and coastal flooding from high surf at the same time.
How Do You Receive Flood Alerts in the Bay Area?
Wireless Emergency Alerts are the loud alarms that phones in a threatened area receive automatically. Flash flood warnings trigger these alerts, but regular flood warnings, advisories and watches usually do not. Waiting for your phone to blare is therefore not enough to keep up with a flooding storm.
The Weather Service posts every watch, advisory and warning for the region at weather.gov/mtr. Counties run their own opt-in alert systems with more local detail. Examples include AlertSCC in Santa Clara County, AlertMarin, AC Alert in Alameda County, SMC Alert in San Mateo County and AlertSF. NOAA Weather Radio also broadcasts updates around the clock and keeps working on battery power when the electricity goes out.
To see how high a creek is running, check the USGS National Water Dashboard, which shows live readings from stream gauges across the Bay Area. Valley Water in Santa Clara County and the flood control districts in other counties also post gauge readings online. During a strong storm, the gauge nearest your home is the most direct way to see whether your creek is nearing flood stage.
What Should You Do at Each Alert Level?
When a flood watch goes out, check your flood zone status and plan your route out. Move cars away from creeks and out of low garages, charge your phone and keep a go bag handy. If you live near a creek with a stream gauge, bookmark its page so you can follow the water level as the storm goes on.
When a flood advisory is in effect, avoid driving through standing water. Six inches of moving water can knock a person off their feet, and about a foot can carry away a small car. Stay off roads near creeks and low crossings as well. The flooding is expected to be minor, but "minor" can still mean damaged property and closed roads.
When a flood warning is issued, act right away. If you are in the warned area, be ready to leave, and do not wait until water reaches your door. Floodwater can rise faster than most people expect, especially along narrow creeks that collect runoff from steep hills. Never try to drive or walk through floodwater, and follow evacuation orders from local officials without delay.
The gap between an advisory and a warning is the gap between an inconvenience and an emergency. Steep hills, uneven rainfall, aging storm drains and homes built close to creeks mean every wet season brings flooding somewhere in the Bay Area. Knowing which alert applies to you, and what to do about it, is some of the most useful weather knowledge you can have from November through March.
