Why the tide is higher or lower than predicted

What a tide chart actually predicts

A tide chart isn’t a measurement. It’s an astronomical prediction, made like this:

1. A tide gauge records the water level at a station, ideally for years.
2. From those records, hydrographers work out the station’s harmonic constants: the tide is split into dozens of regular waves, each tied to a motion of the Moon or Sun, with its own period, height and timing at that spot.
3. Because the motions of the Moon and Sun are known far ahead, those waves can be added up for any future date, giving the time and height of every high and low tide.

That calculation is very precise for what it covers. But it only covers that. NOAA says it plainly: it can only predict the astronomical tide, and it cannot predict the effect of wind, rain, freshwater runoff or other short-term weather. Australia’s Bureau of Meteorology adds that predictions are prepared for average weather and seasonal conditions.

So the water you see is always two things added together: the astronomical tide, which is on the chart, and everything else, which oceanographers call the residual or non-tidal part. If you want a refresher on reading the chart itself, see how to read a tide chart.

How far off it usually is

NOAA checks its own predictions. For each reference station, it compares a full year of predicted and observed water levels and publishes the average difference. Here are some results for 2013:

Station	High and low tide times	Water level
Los Angeles, CA	about 7 minutes	2 in (5 cm)
Honolulu, HI	about 19 minutes	2 in (4 cm)
Seattle, WA	8 to 9 minutes	5 in (13 cm)
Charleston, SC	about 13 minutes	6 in (16 cm)
The Battery, NY	15 to 18 minutes	7 in (18 cm)
Boston, MA	8 to 9 minutes	8 in (21 cm)
Galveston, TX	52 to 62 minutes	7 in (17 cm)
Anchorage, AK	11 to 12 minutes	12 in (31 cm)

These are averages over a whole year, calm days and storms included. A few patterns stand out:

• California and Hawaii are the most predictable. In Los Angeles, the chart is typically within a couple of inches.
• Bays, rivers and estuaries are harder. Stations on the outer coast are more accurate than those farther inland, according to NOAA. In Baltimore, at the top of Chesapeake Bay, winds blowing along the bay have pushed water 1 to 2 feet above or below the prediction.
• Shallow places with small tides can be off in time. At Galveston, in a shallow bay with a small opening to the sea, NOAA says weather can make the tide arrive an hour or more early or late.

NOAA also notes that errors are generally larger where the tide is weaker. At many stations on the western Gulf Coast, short-term weather can completely mask the astronomical tide.

Weather: air pressure, wind and waves

Air pressure

Tide predictions assume average air pressure. When the pressure drops, the sea rises about 1 centimeter per millibar (roughly 0.4 inch), and when it rises, the sea drops by the same amount. The Bureau of Meteorology puts it as about 4 inches (0.1 m) for every 10 millibars. If your barometer reads in inches of mercury, that works out to about 1.3 inches of water for every 0.1 inch of mercury:

Barometer	Water level vs. prediction
30.50 inHg (1033 mb, strong high)	about 8 in (20 cm) lower
29.92 inHg (1013 mb, average)	no change
29.50 inHg (999 mb)	about 5½ in (14 cm) higher
29.00 inHg (982 mb, deep low)	about 12 in (31 cm) higher

The sea doesn’t respond instantly, and it reacts to the pressure over a wide area, so treat these as rough numbers. NOAA notes that strong high pressure can bring exceptionally low tides on clear, sunny days, while low pressure with cloudy, rainy weather brings tides much higher than predicted.

Wind

Wind is the other big driver. A strong onshore wind piles water against the coast, so high tides come in higher and low tides don’t drop as far; NOAA says it can virtually eliminate low tide exposures. An offshore wind does the opposite and exaggerates low tides. According to the Bureau of Meteorology, wind is also the main reason the times of high and low tide differ from predictions.

When strong wind and low pressure combine in a storm, the departure can reach several feet. That’s storm surge, which we cover in storm surge and storm tide.

Waves and swell

Big waves add to the water level right at the beach. Breaking waves push water toward the shore and raise the level slightly, and each wave runs up well above that. A long-period swell from a distant storm can do this even on a calm, sunny day, so the water’s edge sits higher than the tide chart suggests.

Rivers, seiches and slow changes

River flow

In tidal rivers, fresh water adds to the tide. In rivers like the Delaware and the Columbia, NOAA says, powerful spring river flows can severely alter or mask the incoming tide, and its accuracy report warns that differences grow when runoff is above normal. In Astoria, Oregon, near the mouth of the Columbia, keep that in mind during the spring runoff.

Seiches and meteotsunamis

A seiche is a standing wave sloshing back and forth in a lake, bay or harbor, like water in a bathtub. Strong winds and sudden pressure changes set it off. NOAA describes Lake Erie as a classic case: with strong southwest winds, water piles up at one end of the lake. In 1844, a 22-foot seiche broke through a 14-foot seawall there and killed 78 people.

The Great Lakes are a special case. According to NOAA, their tide is less than 2 inches, so their level changes with wind, pressure and seiches on top of slow changes in how much water the lakes hold. And on ocean coasts, a fast-moving line of thunderstorms can set off a meteotsunami, a quick rise and fall that NOAA says has reached 6 feet or more on the Atlantic and Gulf coasts and the Great Lakes.

Seasonal and long-term sea level

Mean sea level isn’t fixed either. It rises and falls a little with the seasons as water warms, cools and shifts. Predictions include the average seasonal cycle, but not unusual years: the Bureau of Meteorology notes that during strong El Niño or La Niña events, sea level can stay up to about 1.5 feet (0.5 m) above or below normal for months. Sea level rise adds to this over decades.

Before you blame the ocean: station, time and datum

Sometimes the difference isn’t in the water but in the chart you’re reading.

The station

A prediction is for one spot. A few miles away, especially up a river or inside a bay, high tide can come earlier or later and higher or lower. Many NOAA stations are subordinate stations, whose times and heights are worked out from a reference station with fixed corrections. In Savannah, for example, the tide comes from the Fort Pulaski reference station with NOAA’s official corrections.

The time zone

NOAA’s predictions can be shown in local standard time, in local time with daylight saving time, or in GMT. If you read a table in standard time during daylight saving time, every tide will seem an hour late, and a table in GMT is off by four to ten hours depending on your time zone.

The datum

Tide heights are measured from a reference level. US charts use mean lower low water (MLLW), but a tide staff, a flood gauge or a map can use another one, such as mean sea level or the land survey datum NAVD 88. Mix them up and you’ll see differences of a foot or more that don’t exist. Our guide to chart datum and MLLW explains how they fit together.

How to check the real water level

To see what the water is doing right now, look up a real-time tide gauge near you:

• NOAA Tides and Currents plots the observed water level on top of the prediction for each of its stations.
• In the UK, the National Tidal and Sea Level Facility publishes real-time gauge data.
• The IOC Sea Level Station Monitoring Facility, run under UNESCO, gathers tide gauges from around the world.

Look at the gap, not just the height. If the water is running a foot above the prediction and the low is still sitting offshore, the next high tide will probably come in high as well.

What TidePlanet shows

Our tide pages show the astronomical tide, like any tide chart. We calculate it from each station’s harmonic constants. Compared with NOAA’s official predictions at 29 US stations over October 2026, our high and low tide times differ by 3 minutes on average and heights by 0.6 inch (1.6 cm). So when the water doesn’t match, the difference almost always comes from the weather or the place, not from the math.

To help you judge how far to trust it, every place says where its tide comes from:

• A · Official station: the tide from a nearby gauge, as in Savannah or Astoria. If it’s a NOAA subordinate station, the page says which reference station it comes from.
• B · Nearby station: the place has no station of its own, so we use the nearest one. On the open coast the difference is usually a few minutes and inches; inside estuaries, rivers and enclosed bays it can be much larger.
• M · Global model: where there’s no station nearby, the tide comes from a global model, with heights above mean sea level. It’s approximate and not for navigation.
• C · No station nearby: if the model isn’t reliable at that spot either, we don’t show a tide for now.
• No significant tide: in places with a tiny tide, the page notes that wind and air pressure move the water more than the tide does.

Next to each day’s tide curve we remind you that it’s an astronomical prediction: wind and air pressure can raise or lower the water by a few inches, and more in a storm. Times are local, with daylight saving time included, and heights are above chart datum (MLLW in the US), except at level M. We don’t show live gauge readings, but at places with a station (levels A and B), if a storm may raise the sea in the next 48 hours, a notice tells you how much and when.

How to use a tide chart, knowing all this

• Treat the chart as the tide in average weather, and leave a margin.
• With low pressure and onshore wind, expect higher water; with high pressure and offshore wind, lower.
• Use the closest station that’s similar to your spot, and be wary of times inside bays and rivers.
• Check that the times are in local time and that the heights use the datum you think they do.
• For navigation, always use official predictions and nautical charts.