When the tide doesn’t match the prediction

A tide prediction is a calculation of what the moon and sun will do to the ocean. It knows nothing about the weather. Most days that hardly matters: the water arrives within a few inches and a few minutes of the forecast. But some days the sea runs half a foot or more above or below the table, and once in a while, far more. This page explains why, how to tell from this site when it is happening, and how to stay safe when the numbers and the ocean disagree.

The residual

The difference between what a tide gauge measures and what NOAA predicted for the same minute is the residual:

residual = observed water level − predicted water level

A residual of +0.4 ft means the water is running 0.4 ft higher than the table. On this site, the Sources line at the bottom of the console reports the latest residual for stations with a live gauge, the Now panel uses the measured level when the gauge has reported in the last hour, and the observed vs predicted chart on Tide Intelligence draws three days of measured water in amber over the predicted curve.

fig · predicted vs observed

On a calm day the residual wanders within a few inches of zero. When it holds steady at a few tenths of a foot for a day or more, something other than astronomy is moving the water.

What moves the water

Air pressure

Air presses down on the sea. Where the pressure is high the surface is pushed down a little, and where it is low the surface rises, about 1 centimeter per millibar (hPa), or roughly 0.4 inch. This is the inverted barometer effect.

Air pressure	Water level vs. prediction
1033 mb (strong high)	about 0.65 ft lower
1023 mb	about 0.33 ft lower
1013 mb (average)	about the same
1003 mb	about 0.33 ft higher
993 mb (strong winter storm)	about 0.65 ft higher
983 mb (deep storm)	about 1 ft higher

A deep winter low can add more than half a foot to every tide while it passes. A long stretch of strong high pressure in spring or summer can trim a few inches off the minus tides you planned a tidepool walk around.

Wind

Wind blowing toward the shore piles water against it (wind setup); wind blowing offshore lets it drain away. On California’s narrow continental shelf this effect is smaller than on the broad, shallow shelves of the Gulf Coast or the East Coast, which is why hurricane-style storm surges of many feet don’t happen here. But long, strong southerly winds ahead of a winter storm still raise water levels along the coast by several inches, and they arrive with the low pressure, so the two add up.

The coast’s spring and summer northwesterlies do the opposite. Blowing along the shore with the coast on their left, they push surface water offshore (upwelling) and lower coastal sea level slightly, while bringing up the cold water that makes Central Coast summers chilly.

Swell and wave setup

Breaking waves push water up the beach. In big surf the average water level at the shoreline rises by a noticeable fraction of the breaking wave height (wave setup), and individual waves run far higher (run-up). Tide gauges sit in harbors and on piers, sheltered from this, so a gauge can show the tide right on prediction while the beach, in a 12 ft swell, is flooding to the cliffs. Always read the tide together with the swell.

Ocean warmth, El Niño and Kelvin waves

Warm water takes up more room than cold water. When the ocean off California warms, mean sea level rises with it, and every tide runs high for weeks or months. During strong El Niño winters (1982–83, 1997–98, 2015–16), California sea level ran several inches to around a foot above predictions for months, and storms on top of those levels did serious damage to beaches, piers and harbors.

The warming often arrives as coastally trapped waves: pulses of high sea level that start in the tropical Pacific as equatorial Kelvin waves, run up the coast of the Americas, and reach California weeks to months later. The Journal’s Kelvin wave explainer follows one in detail.

Seasonal and long-term change

Mean sea level along California rises a few inches from spring to fall as the water warms, then falls again. NOAA’s predictions include the average version of that cycle, so only unusual years show up in the residual.

Sea-level rise is slower but relentless. At San Francisco, the long record shows sea level rising about 2 mm a year, close to 8 inches per century, and the rate is expected to increase. Because tidal datums are averaged over a fixed 19-year epoch (1983–2001 for the stations on this site; see MLLW and tide datums), the water now runs a little higher against those datums than it did during the epoch.

Rivers, seiches and tsunamis

• River runoff After heavy rain, rivers like the Eel, the Klamath and the Sacramento–San Joaquin raise water levels in their bays and estuaries.
• Seiches Harbors and bays can slosh back and forth like water in a bathtub, with periods of minutes, set off by long-period swell or sudden pressure changes. They show up as wiggles on the 6-minute gauge record and can surge through a marina.
• Tsunamis A tsunami is a series of very long waves, minutes to an hour apart, that the 6-minute gauges record as sudden oscillations of feet. Crescent City, with a harbor shape that amplifies them, has been hit hardest in California: the 1964 Alaska tsunami killed 11 people there, and the 2011 Japan tsunami destroyed much of its harbor and badly damaged the harbor at Santa Cruz. Tsunami warnings come from NOAA’s Tsunami Warning Centers (tsunami.gov) and local emergency alerts, never from a tide table.

How far off can a prediction be?

Situation	Typical residual	Notes
Calm summer day	±0.1–0.2 ft	Timing within a few minutes
Strong high pressure	−0.2 to −0.5 ft	Minus tides a little shallower than you hoped
Winter storm passing	+0.3 to +1 ft	Highest during the storm’s peak
Strong El Niño winter	+0.3 to +1 ft for months	Plus storm effects on top
Tsunami	Feet, in minutes	Dangerous currents in harbors for hours

Timing can slip too. A strong residual that is rising or falling across a turn shifts the moment of high or low water by several minutes. And at subordinate stations, where NOAA publishes only highs and lows, the hours in between are interpolated.

Not for navigation

Predictions on this site are for planning a walk, a paddle, a surf or a fishing trip. They are not a substitute for official navigation products, for three reasons: the water does not always follow the prediction; the depth that matters depends on the nautical chart, the sandbars and the swell, none of which a tide table shows; and harbor entrances and bars change after every storm.

For navigation and anything safety-critical:

• Use NOAA nautical charts (with their soundings measured from MLLW) and NOAA’s official tide and current predictions.
• Check the National Weather Service coastal waters forecast and any small-craft, gale or high-surf advisories.
• Check the U.S. Coast Guard’s Local Notice to Mariners and any bar restrictions. Humboldt Bay, Noyo River and Morro Bay bars can close to small boats in big swell.

Staying safe on the shore

• Never turn your back on the ocean. Sneaker waves, much larger than the waves before them, can arrive without warning on any California beach, at any tide.
• Check swell with the tide. A minus tide in a 10 ft swell is not a safe tidepool day. The console puts them side by side for a reason.
• Know your exit before the water turns. Rocky points that are easy to walk around at a low can be cut off within an hour of the turn. Plan to be back an hour before the next high.
• Watch harbor entrances on the ebb. An outgoing tide against an incoming swell builds steep, breaking waves on the bar.
• Treat a sudden drop or rise as a warning. If the water pulls back far and fast, or rushes in unexpectedly, get to high ground: it may be a tsunami.
• Cold water kills. Central Coast water is often in the 50s °F. Wear a wetsuit or a PFD as the activity calls for.

Using the residual

1. Open the console for your station and read the residual on the Sources line.
2. If it is above +0.3 ft, expect today’s highs higher and today’s lows less low than the table. Allow extra margin around points and on harbor bars.
3. If it is below −0.3 ft, expect slightly lower lows, but check the swell before heading out on the reef.
4. If it is changing fast, something is happening: a storm arriving or leaving, or a seiche. Look at the observed vs predicted chart to see the trend.
5. No gauge at your station? The Tide Intelligence page names the nearest station with one. Residuals are usually similar along a stretch of coast.