Frequently Asked Questions
Are the predictions accurate?
Tide Alerts™ predictions use the same techniques that NOAA uses and are typically more accurate than NOAA predictions at subordinate stations.
NOAA's subordinate stations use a nearby harmonic station's tide predictions and apply an offset to generate predictions for the subordinate station's location. These stations can only provide high and low tide predictions.
Tide Alerts stations are typically located where NOAA does not have harmonic stations or does not provide predictions.
Why do the observed and predicted values sometimes differ?
A number of environmental factors can contribute to differences between observed and predicted tide levels.
Wind speed and direction can have a major effect on actual water levels. Winds can force water levels higher or lower depending on the direction of the wind. Wind driven higher levels are often referred to as storm surge and can cause flooding.
If a bay freezes during the winter, ice can cause the water levels to be consistently lower than normal.
How far away are predictions valid for?
Tide levels and predictions are typically very local.
For example, in a bay with a narrow inlet the tides inside the bay are likely very different to the tides in the ocean outside the bay.
Different bays close by are also likely to have different tides because the tides are highly dependent on the size of the inlet and the depth, size, and shape of the bay. Long narrow bays will also have different tides at different parts of the bay.
What is MLLW?
MLLW stands for Mean-Lower-Low-Water. It is the long-term average of the daily lowest tide level. Tide levels are shown relative to MLLW, which is the zero level by definition. Other common datums are:
- MLW (Mean Low Water)
- MHHW (Mean Higher High Water)
- MHW (Mean High Water)
- MSL (Mean Sea Level)
For more information, see the NOAA tidal datums definitions.
How do I install the Tide Alerts App?
Click the Install App button in the footer of this website. The button will automatically detect your browser and device, then show you step-by-step installation instructions tailored to your specific setup.
Note: The Install App button only appears if the app is not already installed on your device.
What do the events on the Tide Calendar mean?
Most tide calendars show the time and heights of the high and low tides each day. For some stations, shallow events are also shown.
The shallow events mark when boats are likely to get stuck during low tides at locations near the tide gauge. In other words, a shallow event is the period when the tide is lower than a boat would need to navigate the area.
How do I get the Tide Calendar in my calendar app?
- Navigate to the station's page and click on the Daily Tides tab, then click on the Tide Calendar section
- Click the Subscribe button (with the calendar icon) at the top of the calendar
- Choose your calendar application from the dropdown menu
- Follow your calendar app's prompts to complete the subscription
Note: The Tide Alerts Calendar creates many events. If you don't want to see them, you can turn off the display of the Tide Alerts Calendar in your calendar app. The events will still be updated in the background.