Tides and Flooding

Be aware that sections of Point San Pedro Road tend to flood during high tide events, especially during periods of heavy rain that increase the normal tides. Please note that forecasting water levels is a mix of predictable astronomy and unpredictable weather. Click here for a more thorough explanation of tides, flooding and predictions.

Water Level Forecasts

Some people check the astronomical tide charts before heading out to see if high tides are high enough to cause flooding and when the flooding might occur. However, astronomical tide charts do not take into account other weather factors, such as winds, currents, rainfall, or atmospheric pressure that can affect both the timing and height of the tides. We recommend checking NOAA’s inundation dashboard for a better forecast of water levels. When viewing the NOAA chart, we recommend looking at the light blue line labeled “Forecast Guidance” because it It blends the pure astronomical predictions (darker blue line) with oncoming weather systems, combining tide cycles with winds, storm currents, and air pressure. When you see the forecast guidance exceed the roadway height, you can expect flooding.

Sandbags

Free sandbags are currently available in our area at these locations:

• Peacock Gap Park, 699 Biscayne Dr. Parking Lot
• San Rafael Rock Quarry Dutra Materials‘ sandbag station is open during business hours (7am-3pm Mon – Fri except holidays) and ready to assist our San Pedro Corridor in preparing for the upcoming weather. The sandbag station is located on the left-hand side of the entry road to the Quarry once you have passed through the gates.

More Detailed Explanation of Tides and Flooding in San Rafael

Tides are the daily rise and fall of the ocean’s surface. While they seem as regular as clockwork, predicting exactly how high the water will get—especially in low-lying areas like San Rafael—is a mix of predictable astronomy and unpredictable local weather. This section explains what drives our tides, why predictions can be tricky, and why standard tide charts only tell half the story.

Tides are primarily caused by the gravitational pull of the moon and the sun acting on the Earth’s oceans. As the Earth rotates through these gravitational fields, different coastal areas experience high and low water levels.

• The Moon’s Pull: The moon has the strongest influence because it is so close to Earth. It pulls the water toward it, creating a “bulge” of high tide on the side of Earth facing the moon, and another bulge on the exact opposite side.
• The Sun’s Role: While much larger than the moon, the sun is farther away, so its tidal impact is about half as strong.
• Spring Tides: When the sun, moon, and Earth align during a full moon or new moon, their gravitational forces combine. This creates Spring Tides, which feature the highest high tides and lowest low tides of the month.
• Neap Tides: When the moon and sun are at right angles to each other (during a quarter moon), their gravity fights each other. This creates Neap Tides, which mean lower high tides and higher low tides.

“King Tides” is a popular, non-scientific term used to describe the highest high tides of the entire year. They are not caused by storms or climate change. Instead, they are completely predictable events driven by a perfect cosmic alignment of the Earth, moon, and sun. Throughout the year, there are a few specific times when this pull becomes supercharged due to two natural phenomena happening at the exact same time:

“King Tides” is a popular, non-scientific term used to describe the highest high tides of the entire year. They are not caused by storms or climate change. Instead, they are completely predictable events driven by a perfect cosmic alignment of the Earth, moon, and sun. Throughout the year, there are a few specific times when this pull becomes supercharged due to two natural phenomena happening at the exact same time:

When a full or new moon aligns perfectly while the moon and sun are at their absolute closest distances to Earth, their gravitational grip on the ocean is at its absolute maximum, resulting in a King Tide. In California, this maximum gravitational pull happens twice a year in California:

• The Winter Window: November, December, and January.
• The Summer Window: June and July.

During both of these seasons, the purely mathematical astronomical pull is practically identical, which is why the 2026 chart predicts a 7.2-foot height for both January and June. Even though the peak summer astronomical prediction matches the peak winter prediction, local meteorologists, emergency officials, and the media almost exclusively use the term “King Tides” to refer to the winter events. They do this because of how the ocean behaves differently between the seasons:

• The Day vs. Night Shift: In the winter, the extreme high tides hit during the middle of the morning, right when people are commuting, working, and out on the roads. In the summer, those exact same extreme high tides hit in the middle of the night (often between 10 p.m. and midnight), meaning most residents are asleep and never even notice the water creeping up the docks.
• The Weather Threat: The most dangerous component of a winter King Tide is that it coincides with Northern California’s stormy rainy season. In June, the weather is warm, dry, and stable. A high tide in June peaks quietly. A high tide in January can easily trigger a historic flood disaster if it collides with a passing storm front, as we discuss more in the next section.

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If it were just the sun and the moon, tides would be perfectly consistent. However, the real world gets in the way. Tides are incredibly hard to calculate with pinpoint accuracy across different coastal towns because water is shaped by the physical environment:

• The Shape of the Coastline: Continents block the movement of water. As a tide enters a narrow bay like the San Francisco Bay, the water gets compressed and funneled, often making the tide higher or changing its timing compared to the open ocean.
• Underwater Geography: The depth of the ocean floor, underwater canyons, and shallow mudflats act like speed bumps or ramps for moving water.
• The Timing Delay: Because water has mass and takes time to move, high tide doesn’t happen the exact moment the moon is overhead. It can be delayed by hours depending on how the water has to navigate into local waterways like the San Rafael Canal.

When you look at a standard tide chart, you are looking at an astronomical prediction. It calculates only what the sun and moon will do under perfect, calm conditions. Tide charts do not factor in the weather because weather cannot be predicted months or years in advance when tide tables are printed. In San Rafael, three major non-astronomical factors regularly alter the actual water level:

1. Atmospheric Pressure: Air has weight. During a storm, a low-pressure system acts like a vacuum, allowing the ocean surface to dome upward and rise higher than predicted. High-pressure systems do the opposite, pressing the water down.
2. Wind: Strong, sustained winds can physically push massive amounts of water toward the shore. In our area, specific wind directions can pile water right into the bay, driving up the tides.
3. Storm Surge and Runoff: When heavy rain hits Marin County, local creeks and storm drains empty into the San Rafael Canal. If a heavy rainstorm hits at the exact same time as an astronomically high tide, the local water has nowhere to go, causing sudden localized flooding.
4. Sea Level Rise: Standard NOAA tide charts are calculated using historical monthly sea level data averages from roughly the 1990s. However, sea levels in the San Francisco Bay Area have already risen by approximately 3 inches since that time, a baseline change that is not natively factored into the standard astronomical tide tables.
5. Thermal Expansion: During a strong El Niño cycle, consistently warmer ocean temperatures can cause the water to thermally expand.
6. Freshwater Runoff: Astronomical tide gauges measure ocean water, but they cannot predict the volume of heavy riverine runoff rushing downstream. The torrent of rainwater flowing out of full inland watersheds met the incoming tide, acting like a bottleneck and forcing water levels even higher in low-lying areas like Marin County.

As we mentioned earlier, “King Tides” in the Bay Area do occur in the winter (typically November, December, and January) during our peak storm season. If a low-pressure winter storm or heavy rain runoff hits San Rafael at the exact same hour as a King Tide peak, the water levels can rise drastically, transforming a routine high tide into a severe, destructive flood event.

You may recall the sudden and severe flooding in Marin County between January 1 and 4, 2026, which was caused by a “compound, multi-hazard flooding incident” where record-breaking King Tides collided with a massive winter storm system. Local emergency officials described it as the most severe tidal flooding the area had seen in over two decades. It submerged parts of Highway 101, knocked out local 911 services, and completely swamped low-lying coastal neighborhoods from Sausalito to San Rafael. In the chart below, you can see that on Jan 3rd at 10:48am, the astronomical tide forecast was 7.31 feet, while the actual water level was 8.71 feet. This is why traditional tide tables often do not accurately predict flooding.

The City of San Rafael recommends using NOAA’s inundation dashboard as the best available forecast of water levels. The Richmond tide gauge (because it will have a bit more information linked to it) is representative of water levels in our area. There isn’t a meaningful difference between Richmond and the other locations (Pt. San Quentin or Pt. San Pedro). These other locations don’t have an actual tide gauge with weather monitoring/water level sensors.

Here’s an example of the chart for August 8, 2026:

Here’s what the different lines mean on the chart:

• Predictions: This dark blue line is the “astronomical forecast”. This line uses physics formulas based entirely on the mathematical orbit of the moon and sun. This is the standard “tide chart” data calculated years in advance, assuming completely perfect, calm weather.
• Forecast Guidance: This light blue line is NOAA’s best guess (but imperfect). Generated by NOAA’s automated computer models, this line looks 48 hours into the future. It blends the pure astronomical predictions with oncoming weather systems, combining tide cycles with winds, storm currents, and air pressure. Notice that the forecast seems to be overpredicting the high tides over the past two days (i.e., the light blue line is higher than the red line). Still the forecast (light blue line) seems more accurate than the astronomical “Predictions” (dark blue) line.
• Observed: This red line is the physical reality recorded by the water gauge’s acoustic or microwave sensors in real time. It is the exact height of the actual water right now, showing how the combined forces of nature are physically pushing the tide up or down.

What is NAVD? (The y-axis in the chart above)

When measuring how high a hill is, or how high a flood might reach, everyone needs to agree on where “zero feet” starts. If one agency uses local sea level and another uses the ocean floor, the numbers won’t match. NAVD88 stands for the North American Vertical Datum of 1988. It is a highly accurate, permanent mathematical blueprint used by scientists, surveyors, and FEMA to measure elevations across the continent. Think of it as a giant, invisible grid locked in place over North America.

It is a common mistake to think NAVD88 is exactly the same as “mean sea level” at your local beach. They are slightly different for a few reasons:

• The Ocean Isn’t Flat: Due to currents, winds, water temperature, and Earth’s gravity, the actual surface of the ocean humps and dips. Sea level in San Francisco is physically different than sea level in New York.
• Sea Level Changes: Climate change and shifting tides mean the actual ocean level is constantly moving.
• NAVD Stays Put: NAVD88 was established by picking one highly precise fixed point on land (in Father Point, Quebec) and calculated mathematically across the continent. Because it is anchored to the solid earth, NAVD88 never changes, giving engineers a reliable baseline that doesn’t bounce around with the waves.

Here’s why NAVD Matters for San Rafael Flooding. When you look at flood maps or tide gauges for the San Rafael Canal, you will often see elevations listed “relative to NAVD88.”

• Building Safety: If FEMA says a home in San Rafael needs to be elevated to 10 feet NAVD88 to stay dry, a surveyor can use that exact continent-wide benchmark to ensure the house is built safe from floods.
• Comparing Apples to Apples: Using NAVD88 ensures that the height of a concrete floodwall, the peak of a high tide, and the elevation of your living room floor are all being measured by the exact same ruler.
• Modern Maps (Published after 2009): The U.S. Geological Survey (USGS) benchmarks all its current “US Topo” maps to NAVD88 for vertical elevation.
• MarinMap: Locals and property owners frequently use MarinMap, the county’s public geographic information system, to check property lines, zoning, and building permits. All the elevation contours and public LiDAR terrain data embedded into Marin County’s official mapping systems are anchored to NAVD88. When the county models sea-level rise or builds sea-level viewers (like the BayWAVE project), they translate the ocean’s tides into NAVD88 numbers so the water data matches the land maps.

The year 2026 provided the perfect, real-world demonstration of why astronomical tide charts cannot be looked at in isolation:

1. The Dangerous January Peak: When the 7.3-foot astronomical King Tide arrived on January 3, 2026, it unfortunately crossed paths with a powerful Pacific storm front. The storm’s low atmospheric pressure and driving winds added an extra 1.4 feet of storm surge. Combined with torrential runoff, the actual observed water levels obliterated the chart’s 7.3-foot baseline and pushed the bay closer to 8.7 feet, triggering major county-wide emergency flooding.
2. The Quiet July Peak: Conversely, when the bay hit that exact same 7.3-foot astronomical height during the summer King Tide cycle on July 12, 2026, it happened under clear skies. Despite some record-high summer ocean temperatures causing minor “thermal expansion” swell of 0.6 feet there was no storm or rain runoff. The 7.9-feet water level caused minor, brief splash overs on a few low-lying docks and parking lots, but it resulted in zero widespread street damage.

A 7.3-foot tide in July is a scenic photo opportunity; a 7.3-foot tide in January can be a community disaster. The astronomical math was exactly identical—the weather made all the difference. Note that the winter flooding in January 2026 was due to a “perfect”perfect storm” of flooding factors:

• Record King Tides: The alignment of the sun, moon, and Earth created a massive gravitational pull that brought historic, extremely high ocean tides.
• Winter Storm & Storm Surge: A heavy winter storm system struck simultaneously, with high winds and a low-pressure storm surge pushing the rising ocean water even further inland.
• Riverine Runoff: Weeks of prior heavy rain meant that water basins were already full, creating massive precipitation-driven runoff that overwhelmed local drainage systems.
• Levee Failure: The extreme high waters caused a critical breach in the levee protecting the Santa Venetia neighborhood, letting the bay flow directly into residential streets.

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Tsunamis

Tsunamis can also affect areas along the Pt San Pedro Road corridor.

Click here for more information on Tsumani Risk including an interactive map. In the map below, areas toward the water from the solid olive-green line are considered Tsunami Hazard Areas.