Methods and Sources

NOAA tide predictions

The high and low tides on YepTidal are predictions from NOAA's Center for Operational Oceanographic Products and Services (CO-OPS), retrieved from the CO-OPS Data API for each station. They predict the astronomical tide: the rise and fall of the water caused by the Moon and the Sun. As NOAA's FAQ notes, they don't include wind, rain, freshwater runoff, or other short-term weather, and NOAA's guide to tide predictions adds storm surge and sea level rise. The actual water level can differ from the prediction. These are predictions, not measured water levels and not a forecast.

NOAA has two kinds of tide prediction stations. At a harmonic station, the predictions come from the analysis of the station's own long-term tide record. A subordinate station has only a short record, so NOAA takes the high and low tides predicted for a nearby harmonic reference station and adjusts them for the subordinate station. YepTidal shows the predictions NOAA gives for each station and names the reference station of each subordinate station. It doesn't make the adjustment itself. Of the 2,343 stations on YepTidal, 815 are harmonic and 1,528 are subordinate.

Heights and the MLLW datum

Heights are in feet above mean lower low water (MLLW), the datum NOAA uses for US nautical charts. MLLW is the average of the lower of the two low tides of each tidal day over NOAA's 19-year National Tidal Datum Epoch. A low tide below that average has a negative height, often called a minus tide.

Heights are shown to a tenth of a foot. YepTidal rounds the size of the value NOAA gives and keeps the minus sign, and never shows -0.0. The low tide search and the comparisons use the values before rounding, so a low tide shown as 0.0 ft may be just above a 0.0 ft limit. Times are shown to the minute, as NOAA gives them.

Times and daylight saving time

YepTidal retrieves the predictions in UTC and shows them in each station's local time, including daylight saving time, which starts at 2 a.m. on the second Sunday in March and ends at 2 a.m. on the first Sunday in November. The time zone comes from the station's offset from UTC in NOAA's station list. A station is left out if that offset doesn't match the legal time zone of its county.

On the days daylight saving time starts or ends, every tide time that day shows its time zone, such as PDT or PST. When daylight saving time ends, the hour after 1 a.m. happens twice, and the zone tells the two apart. Dates and weekdays are the station's local dates.

For some stations and periods, NOAA's predictions come with times on 6-minute steps instead of to the minute. YepTidal shows those times as NOAA gives them.

Sunrise and sunset

YepTidal calculates sunrise and sunset for each station's latitude and longitude with the method of the NOAA Global Monitoring Laboratory Solar Calculator, which is based on equations from Jean Meeus's Astronomical Algorithms. Sunrise and sunset allow 0.833° for atmospheric refraction and the size of the Sun. NOAA's calculation details say the results are theoretically accurate to within a minute between 72° north and south, and that the weather can change when the Sun actually appears to rise and set. YepTidal doesn't account for elevation or for hills and buildings on the horizon.

Daylight is from the minute of sunrise to the minute of sunset, with no extra margin. A low tide in the same minute as sunrise or sunset counts as in daylight.

The low tide search

The search lists the predicted low tides on the station's local dates in the period you choose that are at or below the height you enter, and, if you ask for them, in daylight and on weekends (Saturday and Sunday in the station's local time). High tides aren't included. Each match is the time of a low tide, not a window of time when the water is low.

By default, the search looks for low tides at or below 0.0 ft, in daylight only, on any day, for 3 months from today. The period can be 1 to 12 months, and the height from -10.0 to 30.0 ft. A period of N months ends the day before the same date N months later. The search stops where YepTidal's predictions end, and the list shows up to 200 low tides.

The comparison runs the same search at up to 4 stations and puts each station's low tides in the row of its own local date.

The tide curve

The tide curve joins each predicted high and low tide to the next with a smooth cosine-shaped line, over one local day from midnight to midnight, with daylight shaded from sunrise to sunset. The line is an approximate shape. It isn't a prediction of the water level between the highs and lows, so YepTidal never shows a height read from the line. NOAA's own station pages describe their line between the highs and lows the same way, as a fit that approximates the water level in between. At the ends of the predictions, the line stops at the first or last high or low tide.

What YepTidal leaves out

• Measured water levels, currents, waves, wind, water temperature, and weather.
• Water levels between the highs and lows, the range of the tide, moon phases, and solunar tables.
• Fishing scores, safety ratings, and time windows for low water.
• Twilight, moonrise, and moonset.
• Stations outside the states YepTidal covers so far (Alabama, California, Connecticut, Delaware, Florida, Georgia, Louisiana, Massachusetts, Maryland, Maine, Mississippi, North Carolina, New Hampshire, New Jersey, New York, Oregon, Rhode Island, South Carolina, Texas, Virginia, Washington), and stations in Alaska, Hawaii, US territories, and the Great Lakes.
• Stations whose predictions or details don't pass YepTidal's checks.

How the data is updated

Once a month, YepTidal retrieves NOAA's station list and, for each station, the predictions for whole calendar years (in UTC) from the start of the current month to at least 13 months ahead. NOAA publishes its official tide tables on October 1 for the following year and updates predictions every quarter, so a new retrieval can change earlier predictions.

Before a new retrieval goes live, it's checked: highs and lows must alternate, times must increase, each day must have a plausible number of highs and lows, heights must be in range, and each station must have at least 12 months of predictions. A station that fails is left out, or, if it was already on YepTidal, keeps its previous predictions with a note while YepTidal checks NOAA's latest update.

Data versions

Sources and citation

Tide predictions: NOAA National Ocean Service, Center for Operational Oceanographic Products and Services (CO-OPS), retrieved October 15, 2026 from the CO-OPS Data API. Daylight filters, curves and comparisons are calculated by YepTidal from these predictions and are not official NOAA products.

Station details come from the CO-OPS Metadata API.

Disclaimer

The tides on YepTidal are NOAA's predictions, not measured water levels or a forecast. YepTidal isn't for navigation.

NOAA provides its data as is and asks that it be credited when it's shared. YepTidal's calendars, curves, searches, and comparisons are calculated from NOAA's predictions and aren't official NOAA products. See NOAA's disclaimers. YepTidal is an independent site and isn't affiliated with NOAA.

Related pages

• Alabama tide stations
• California tide stations
• Connecticut tide stations
• Delaware tide stations
• Florida tide stations
• Georgia tide stations
• Louisiana tide stations
• Massachusetts tide stations
• Maryland tide stations
• Maine tide stations
• Mississippi tide stations
• North Carolina tide stations
• New Hampshire tide stations
• New Jersey tide stations
• New York tide stations
• Oregon tide stations
• Rhode Island tide stations
• South Carolina tide stations
• Texas tide stations
• Virginia tide stations
• Washington tide stations
• Compare low tides at NOAA stations