A coastal flood advisory is the lowest of three alerts the National Weather Service issues when ocean water threatens to push onto shore. Next comes the coastal flood watch, and the most serious is the coastal flood warning. Each level signals a different degree of danger and certainty, and knowing which is which tells you whether to stay alert, start preparing or act right away. In the Bay Area, king tides, winter storms and atmospheric rivers can combine to push water well above normal levels. These alerts matter for shoreline communities from Pacifica to the Embarcadero.
What Are the Three Levels of Coastal Flood Alerts?
The National Weather Service (NWS) uses three tiers for coastal flooding, as it does for most weather hazards. From least to most serious, they are the advisory, the watch and the warning. An advisory means minor flooding is expected or already happening. A watch means significant flooding is possible within about the next 12 to 48 hours. A warning means significant flooding is expected or already under way. The wording reflects both how bad the flooding could be and how confident forecasters are that it will happen.
A coastal flood advisory is issued when minor coastal flooding is expected. Minor flooding reaches places that are not normally wet, such as low parking lots, shoreline trails, harbor docks and parts of beach access roads. It is a nuisance and can cause minor damage, but it does not directly threaten lives. The NWS Bay Area office, based in Monterey and covering the coast from Sonoma County south through Monterey County, issues these advisories several times a year. They are most common during winter king tide periods and when storms arrive during higher-than-normal tides.
A coastal flood watch means moderate to major coastal flooding is possible within 12 to 48 hours. Forecasters are not yet confident enough to issue a warning. A watch is a planning alert that tells you to start preparing. It often goes out when a strong storm is approaching during a period of high predicted tides, while the storm's track or timing could still shift.
A coastal flood warning is the most serious alert. It means moderate to major coastal flooding is expected or already happening, and it calls for action now. Major coastal flooding can cover roads, damage buildings near the water, erode beaches and bluffs, and endanger anyone near the shore. On the Bay Area's outer coast, a warning usually means large swells, storm surge and high tides are arriving together.
What Causes Coastal Flooding in the Bay Area?
Coastal flooding usually comes from several forces stacking on top of each other. The most important are the tides, storm surge, wave runup and runoff from rivers and creeks. Tsunamis are a separate hazard with their own warning system, which our guide to whether a tsunami can hit San Francisco covers.
The tides set the baseline. The year's highest predicted tides, called king tides, arrive from about November through January. They come when a new or full moon lines up with the moon's closest approach to Earth. Earth is also closest to the sun in early January, which adds a little extra pull. At the San Francisco tide gauge (NOAA Station 9414290), king tides typically reach 6.5 to 7 feet above the average lower low tide. That is about a foot higher than a typical high tide. On their own, king tides can flood the lowest stretches of shoreline.
Storm surge raises the water above that baseline during a storm. When a strong low-pressure system arrives, low air pressure and steady onshore winds push the water higher than the predicted tide. During major Bay Area winter storms, surge typically adds 1 to 2 feet. When a storm lands during a king tide, low spots along the Embarcadero and the Sausalito waterfront are among the first places to flood.
Wave runup is the extra height breaking waves reach as they rush up the shore, and it matters most on the open coast. At Pacifica, Half Moon Bay and Stinson Beach, big swells from North Pacific storms can send water 5 to 10 feet above the still-water level. That is why coastal flood warnings on the outer coast often come with high surf warnings, since both hazards come from the same storm.
Inside San Francisco Bay, waves matter less, but rain runoff matters more. When an atmospheric river drops heavy rain on Bay Area watersheds, creeks swell just as the bay reaches its high tide. At high tide, those creeks cannot drain, so water backs up into nearby streets and neighborhoods. This compound flooding, where high tides, storm surge and runoff arrive together, produces the most dangerous flooding inside the bay.
Where Coastal Flooding Hits the Bay Area
The Bay Area's coastal flood risk is split between the open Pacific coast and the inner bay shoreline, and the two behave differently. On the outer coast, from Stinson Beach south through Pacifica and Half Moon Bay, the main threats are waves and bluff erosion. In Pacifica, eroding bluffs forced the demolition of apartment buildings along Esplanade Avenue in 2016. Waves still overtop the seawall along Beach Boulevard during big swells at high tide. At Ocean Beach in San Francisco, winter storms keep eating into the bluffs south of Sloat Boulevard.
Inside the bay, the most vulnerable places sit at or near sea level. They include the Embarcadero in San Francisco, parts of Alameda, Foster City, which is built largely on bay fill, and the South Bay community of Alviso. These areas flood from high tides raised by storm surge rather than from waves. King tides alone can push water onto Embarcadero sidewalks and into parking areas along the Sausalito waterfront. When a winter storm adds surge, the flooding spreads farther and lasts longer.
Our guide to Bay Area flood zone maps shows which communities face the highest risk, including FEMA flood zones and sea level rise projections.
Deep Dive: How Does a Storm Raise the Sea?
A storm raises the water level in two main ways. Its low air pressure lets the sea surface bulge upward, and its winds push water toward the shore. Waves add a third effect right at the coastline. Together they produce the storm surge that forecasters add on top of the predicted tide.
How Does Low Pressure Lift the Sea?
The weight of the atmosphere presses down on the ocean. Where the pressure drops, the water rises slightly to balance it, like a liquid in a barometer turned upside down. Scientists call this the inverse barometer effect.
The rule of thumb is about 1 centimeter of rise for every millibar of pressure drop. A 1993 NOAA technical memorandum describes a 1-millibar rise in pressure as lowering the sea by about 1.01 centimeters. Satellite studies have found the real response in mid-latitudes is usually 80 to 100 percent of that ideal value.
Consider a strong winter storm with a central pressure of 980 millibars, compared with a normal 1,013. By our calculation, it could raise the sea beneath it by about 13 inches. That is a large share of the 1 to 2 feet of surge that major Bay Area storms typically add. On top of a king tide, it can be the difference between a wet sidewalk and a flooded street.
How Do Wind and Waves Add Water?
Strong winds drag the sea surface along with them. When they blow toward land for hours, water piles up against the shore and inside bays, a process called wind setup. The effect is largest over broad, shallow water, because the shallow bottom gives the piled-up water less room to flow back out underneath.
That is why the shallow South Bay and San Pablo Bay can see more surge than the deep Central Bay in the same storm. Wind blowing along the length of the South Bay can pile water up at the end it blows toward, including the low shorelines near Alviso.
Breaking waves raise the water level near the shore as well, an effect called wave setup. On the open coast, big winter swells add both setup and runup, the rush of water up the beach. That is why Ocean Beach, Pacifica and Half Moon Bay can flood during a storm that barely raises the water at the San Francisco tide gauge.
How to Respond to Each Alert Level
When the NWS issues a coastal flood advisory, stay aware and use common sense, because minor flooding is expected in the lowest shoreline areas. Avoid parking in lots right next to the water, and stay off jetties, seawalls and coastal rocks around high tide. Check the tide tables and plan beach visits for lower tides. If you live or work near the shoreline, move valuables above the expected water level.
A coastal flood watch calls for more active preparation. Move vehicles away from flood-prone areas, review evacuation routes if you live in a low-lying area and secure outdoor items near the water. Watch the forecast closely, because a watch may be upgraded to a warning as the event approaches and confidence grows. That upgrade often comes 12 to 24 hours before the expected flooding.
A coastal flood warning calls for immediate action to protect yourself. Stay away from the shoreline and flood-prone areas, and never drive through a flooded road, even if you think you know how deep it is. Just 12 inches of moving water can carry away most cars. If officials order an evacuation of a flood-prone area, leave promptly. Afterward, return carefully, because undermined roads, damaged seawalls and debris in the water remain hazards.
How to Track Coastal Flood Alerts in the Bay Area
The NWS Bay Area office in Monterey, known as WFO MTR, issues coastal flood alerts for the region. You can see active alerts on the office's website or through the NWS alerts service. The NOAA tide gauge at San Francisco (Station 9414290) posts real-time water levels at tidesandcurrents.noaa.gov, where you can compare actual levels with the predicted tide. When the water runs well above the prediction, the difference shows how much extra water the weather is adding. During a strong El Niño, part of that gap can come from the ocean itself. In October 2026, a Kelvin wave kept San Francisco about a foot above predicted tides for weeks, even between storms.
County emergency agencies also send flood alerts through their own notification systems. San Mateo County, where Pacifica and Half Moon Bay see the most frequent coastal flooding, uses SMC Alert. Marin County's Alert Marin covers the Stinson Beach and Sausalito shoreline. Signing up for your county's system is the most reliable way to get coastal flood alerts directly.
Coastal flooding in the Bay Area is becoming more frequent as sea levels rise. The San Francisco tide gauge has recorded about 8 to 9 inches of rise since 1900, and mid-range projections call for another 1 to 3 feet by 2100. Flooding that now takes a strong storm on top of a king tide will eventually happen on ordinary high tides. Learning the alert system now, while coastal flooding is still mostly tied to storms, will help you respond as these events become more common. Before a winter trip to the coast, check Bay Area ocean conditions and the tide forecast, and take every NWS coastal flood alert seriously.
