First published on OceanNavigator.com / updated here Feb 17, 2025
We had one tidal revolution in 2021 when NOAA announced that they were discontinuing the decades-long use of Table 2 lists of secondary station corrections, and that there will no longer be any sanctioned annual Tide and Current Tables. Plus there will no longer be any international tidal data published by NOAA. Going forward, the way we get official tide and current predictions is go to tidesandcurrents.noaa.gov and create a monthly or annual table for specific stations as PDFs and then print them. It takes four pages per year, per station. This is a superior system, as we rarely needed the global coverage in the historic annual tables, plus the use of Tables 2 (one for tides and one for currents) was tedious, and, indeed, we learn now, not accurate in many cases. We still see in 2025 the discontinued 2020 Tables 2 in some third-party tide or current books, but it is important to know that much of that content is wrong.
The USCG have also now recognized that historic tidal predictions have not been valid since 2021 and the new round of license exams have removed all Table 2 references in lieu of the modern approach of direct data from tidesandcurrents.noaa.gov. This is also now updated in all electronic navigational charts (ENC). All previous references to “NOAA Tide and Current Tables” have been removed.
But with that revolution still unknown to many mariners, we have a new one! NOAA’s new Operational Forecast System (OFS) now produces digital tide and current forecasts that are superior to the traditional NOAA predictions, which are based on harmonic constants for each station. We now have tidal current forecasts uniformly over the full waterways, out two or three days, in fifteen regions and two channels around the country.
The beauty of the OFS model forecasts is they take into account the local values of wind and pressure, as well as unseasonal river runoff. The models are updated four times a day to account for changes in these local environmental factors that affect tide height and current flow. The model data has a latency of about 2 hours, meaning a 3-day set of hourly forecasts run at 12z will be available to mariners at about 14z.
The other huge improvement are the OFS current directions. Traditional harmonic currents are presented as pure reversing currents with just two directions, being the average flood and average ebb directions. But most open water currents are rotating currents to some extent, which do not have just two directions. An example is in Figure 2.
Where to get OFS tide and current forecasts.
For the time being, NOAA presents the OFS forecasts as graphic animations such as shown in Figure 3. These animations are not a very precise way to access this very precise data, but they are working on other presentations. In the meantime, third party navigation apps have solved this problem for us, which we come back to shortly.
Figure 3. Sample OFS currents as presented by NOAA. To access these, start at tidesandcurrents.noaa.gov/models and choose a region on the left. Then scroll to the bottom of the page, and under currents click Forecast Guidance. If that link is not there, then click any subdomain indicated on the main image, and then look for the current link.
NOAA is also working on a new OceansMap web app that promises to be a sophisticated digital display that replaces the Figure 3 animations. A sample from the beta version is shown in Figure 4.
Figure 4. San Francisco OFS currents displayed in the forthcoming NOAA OceansMap web page. When completed, it will also show the harmonic predictions as well as the NDBC buoys that measure currents for direct validation checks. Please keep in mind that this is still a developing beta and all features may not work yet as intended.
In the meantime, we also have presentations from other agencies. A particularly nice one is from the Northwest Association of Networked Ocean Observing Systems (NANOOS) for the Salish Sea region shown in Figure 5.
These graphic presentations show us the general flow of the tidal currents, revealing patterns we would never know from the isolated harmonic station predictions alone, but for actual navigation underway we need the digital data in GRIB format. This way we can load it into navigation programs and compute optimum routing for all classes of vessels, but this is specifically crucial to sailors and low-powered craft. The problem is the official data are only published in NetCDF format, which most nav apps cannot read.
But mariners can be grateful to two marine navigation apps who have taken it on their own to convert this crucial data to GRIB format. They are Expedition and LuckGrib. The former is a popular racing and performance PC app, and the latter is a state of the art marine weather data source and display for Mac and iOS. Both can incorporate OFS currents into optimum inland routing computations. Both apps also allow users to export the OFS grib files they created. LuckGrib has a two-week, full-function demo period, so users can experiment with this OFS data and other features it offers.
Figure 6 shows the Salish Sea OFS grib file exported from LuckGrib and then loaded it into qtVlm, another nav app which is chosen here because it can display the OFS forecasts as well as the NOAA harmonic station predictions so we can compare the two current sources. qtVlm is a free app for Mac or PC.
Figure 6. Salish Sea OFS forecasts (black arrows) compared to the harmonic forecasts (colored arrows). In the gray labels, M is the OFS Model forecasts, and T are the Tabulated NOAA harmonic predictions. The yellow labels show the model values we would not know from harmonic predictions alone. We see that at the harmonic station locations (circled) the speeds are usually pretty close, which gives us confidence that the model data are right at other locations. The differences in directions between model and harmonic currents can be larger in between the peak and slack samples shown here. We are reminded in the slack data (bottom picture), that slack water is rarely still water — the model data makes this even more apparent.
Usually tidal currents affect our navigation more than the tide heights themselves, but there can be exceptions, and the OFS model includes tide heights that can help with this. One example would be predicting current flow along a narrow that has no harmonic predictions nor OFS current forecasts for the channel. Such cases are usually controlled by the tide height at each end, with current flowing from the higher-tide side toward the lower-tide side. An example is shown in Figure 7.
Figure 7. Salish Sea OFS tide height forecasts (background colors, mostly green) in the region of the Swinomish Channel that flows past La Conner, WA, displayed in qtVlm along with the harmonic predictions (small meters) at several locations. Local knowledge calls for the current to start flowing north sometime between 2.5 to 4 hours before high water (HW) at La Conner, and last till the same interval past HW — and even this broad prescription is known to be sensitive to the state of local river runoff and the range of the tide. The OFS tides include the effects of present and forecasted winds and river runoff, so it is likely that the OFS tide forecasts can be used for more precise predictions of these currents. In this example, there is a notable slope in tide height across the channel 6 hours before HW.
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When this post was first made, we could not read SSCOFS on the OceansMap and we could not see any details of the currents in Swinomish channel from the GRIB coversions as they have to average the grid and cannot go to that resolution.
But now we see that the OFS can indeed predict currents in Swinomish Channel and they look very promising.
The above is from the OceansMap viewer; below is from the NANOOSH viewer.
Now we are in a position to test the local knowledge guidelines, which do not account for variations in environmental factors, and likely create a more dependable way to know the currents. Could be as easy as just looking at one of the two sources above.
