Not an official NOAA app

Tide Alerts is not an official NOAA product. We do not speak for NOAA, and we do not claim to be more accurate than NOAA everywhere.

Most tide apps show the same NOAA tables

When people say they “already have a tide app,” they are usually looking at NOAA’s published high and low times. That table is the same in almost every consumer tide app. It is an educated guess from historical measurements — a long-term average of how the water has behaved — not a live reading of the water in front of you today.

Those forecasts often do not include near real-time levels. Wind, surge, and local weather can move the actual water well away from the printed tide. If the app has no observed trace, you cannot see that gap.

What a subordinate station is

NOAA harmonic stations have a long record and a full tide-curve model.

A subordinate station usually does not have its own full model. NOAA publishes it as a time and height offset from a distant harmonic gauge — for example, “high water here is typically 40 minutes later and a few tenths of a foot different than at the reference station.” That offset is averaged from history. It is not a sensor in your creek, and subordinate stations often cannot provide a full curve the way a harmonic station can.

Nearby is not the same waterway

Many tide apps extrapolate your location to the nearest NOAA station — harmonic or subordinate. Tides can vary significantly from bay to bay. A station a few miles away can be a different inlet, a different constriction, or a different river. Pinning you to that station looks convenient and is often wrong.

What Tide Alerts shows

• Hyperlocal Sentient Things gauges: near real-time measurements, and our own predictions built from those local observations (NOAA-style harmonic techniques on local data — not a borrowed subordinate offset). That is what fills the chart, tide table, and calendar at those sites. Even if NOAA publishes a table for a nearby station, ours is more accurate for this location because it is based on hyperlocal measurements here.
• SECOORA, NERACOOS, and USGS: NOAA typically publishes no tide tables for these stations. We ingest raw observations from those networks and compute our own predictions. That gives those locations tables, calendars, and predicted curves that typically have not existed before — other tide apps usually have nothing here except, at best, a pin to the wrong NOAA station. Even if NOAA does publish a table for that waterway, ours is more accurate for that location because it is based on hyperlocal measurements at that sensor — not an offset from a distant harmonic gauge.
• Public NOAA stations only when they provide live or recent readings. For those stations we use NOAA’s predictions, plus the observed water those sensors report. We do not fill the map with subordinate-only estimates that have no live gauge.
• A chart that can show observed vs predicted, so wind and weather are visible instead of hidden.

Seeing a NOAA, SECOORA, NERACOOS, or USGS name in the station list does not make Tide Alerts that agency’s app. For partner networks it means we show that sensor’s raw water levels and predictions we compute, because NOAA typically has no table there — including monthly tables and calendars at sites that usually had none. Even when NOAA does publish something nearby, our curve is from measurements at that station, so it fits that waterway. For NOAA sensors it means that station reports live water, and the predicted curve is NOAA’s — not a generic offset from the next bay.

Where we have hyperlocal measurements at the gauge, that prediction is typically a better fit for that location than a NOAA table that only offsets a distant station. We still do not claim to beat NOAA on every shoreline.

Data is provided as-is for awareness. Use official marine forecasts and your own judgment for navigation and safety.