Historically there have been no NOAA predictions for the tidal current speed in the Swinomish Channel flowing past La Conner, WA, but local mariners know it can be strong—over 2 kts at times.
Years ago we noted that this current should be predictable from the tide difference at the north and south ends of the channel—a driving force called a hydraulic head. The current flows from the high-tide end toward the low-tide end, and the bigger that difference, the stronger the current.
There is a NOAA tide station at La Conner (#9448558), 1.4 nmi into the 6-nmi long channel from the south and another right at the north end of the channel (#9448682). Typical channel widths are 300 to 400 ft, but navigable waters are narrower—dredged to minimum of 12 ft over 100 ft width. I note that the only chart of the Channel (US5WA31M) apparently has the dredge depth wrong at 6 ft. We have reported this to NOAA.
Historically mariners did not have digital tides on board, so it was tedious to copy the tides from tidesandcurrents.noaa.gov and transfer them to a spread sheet to make the current forecasts. I am not sure that method ever caught on with local mariners; we originally worked on this for a specific Seattle to Bellingham kayak race.
Whether or not that procedure ever became popular does not matter, because we have now an all new way to get presumably good forecasts for the channel current with the click of a button, thanks to the newly available results of the Operational Forecast System (OFS) model along with also new developments in how to read that data.
The driving force of the current and the way to forecast it remains true, but we have much better data now on the actual tide heights. The Salish Sea and Columbia River OFS model (SSCOFS) forecasts the current in the channel every hour out 3 days, and the model is recomputed every 6 hr to take into account changes in local environmental factors that can have major effects on tides and currents. The big advantage of the model forecasts is they take into account local values of wind, pressure, and river runoff, which the static harmonic NOAA station predictions do not account for.
For the moment, we can see these new current forecasts online two ways. One is the NANOOS presentation, shown below. This works great for the Salish Sea data but there are not as good IOOS presentations for the other 14 regions around the US where we have OFS predictions.
The above source is established and dependable, but NOAA has a new online viewer called OceansMap and it promises to include the currents and a lot more information, with versatile display options. As of now, it is still in beta form, so sometimes not all data are available. Also for now we can only see the time scale in EST (PST +3h).
Historically there have been several local guidelines for predicting the channel currents based on a single value of the tide height at La Conner or Seattle. Google "Current speed in Swinomish Channel" to see a few of them. And indeed some may work in some average conditions, but they cannot be dependable because of the wide range of environmental changes plus the area frequently has unusual tide patterns, such as the above example taken at random with high tide nearly all day long—essentially a diurnal pattern where it is normally semi-diurnal.
In the picture below we used the SSCOFS data viewed in OceansMap and stepped through a days data, one hour at a time, and recorded the tide heights at the north and south ends, as well as the current near La Conner. Then we did the old school method of subtracting the tides and plotted that along with the current.
So we see very nicely that this is the driving force of the current, which just confirms our original approach with modern data.
But we no longer need this analysis. We just open one of the two apps above and get the current. Eventually the OceansMap will be the working tool as it has that neat meteogram optional display at the bottom that could be copied and saved in a phone.
Below are a couple more of these meteograms that compare currents in the channel with tide height at La Conner. Sometimes there is a correlation; other times not.
Earlier models of the Victoria Clipper, traveling from downtown Seattle to Victoria, BC, did sometimes transit the Channel when conditions in Puget Sound were very bad. This OFS data should make that planning better in those cases where it might come up again. The new Clippers are larger, so such careful planning is even more crucial.
The channel has a mean tide height of 6.1 ft, with mean high water of 9.4 ft and mean low water of 2.7 ft. With dependable tide and current predictions, the Channel might be a more frequent option to Deception Pass for low-powered vessels, which then have the bonus of a visit to La Conner, which is a popular NW destination for good food, good art, and friendly people.
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For completeness, let me add that the wonderful GRIB versions of the OFS data we can get from Expedition and LuckGrib are fantastic for optimum routing over inland waters, but they must compromise on the grid size when converting the NetCDF to the GRIB format, and the resulting resolution is not adequate to use for the narrow Swinomish Channel. For this we need the two resources cited above. Also it seems Panoply which does load the full original data files, cannot resolve the data to that level either.
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