In SF Bay each day there are two current cycles (two times of maximum flood and maximum ebb, one of which is substantially stronger than the other). The image below compares TT and NOAA current speed and direction for the larger flood on September 7, 2010, occurring at 10:19 at the Golden Gate Entrance. The image shows a large discrepancy in the direction of the TT-predicted current (red steamers) from the NOAA direction (yellow streamers) at most locations, and a systemmatic and significant underestimate of current strength at almost all locations. For example, at the Golden Gate the NOAA current strength is 4.2 knots, whereas TT predicts 3.5 knots.
For comparison, the image below shows LK and NOAA values at the same points and times on September 7, 2010. Again, the NOAA values are shown by yellow streamers, which can be seen underlying the red (LK-predicted) streamers with identical strength and direction.
The image below compares TT and NOAA current speed and direction for the larger ebb on September 7, 2010, occurring at 3:36 at the Golden Gate Entrance. As with the flood, the image shows a large discrepancy in the direction of the TT-predicted current (red steamers) from the NOAA direction (yellow streamers) at most locations, and a systemmatic and significant underestimate of current strength at almost all locations. For example, at the Golden Gate the NOAA current strength is 5.0 knots, whereas TT predicts 3.7 knots.
NOAA also specifies the time of minimum current at the current stations. In most locations, current flows in a constant direction during flood, at various strengths, decreasing to zero (slack water) before increasing during ebb, when it flows in another (approximately opposite) direction. At a few points, current direction changes continuously as the magnitude drops, and never goes to zero, flowing in a different direction at some minimal value. This is called a "rotary current," and can occur at the minimum before flood, the minimum before ebb, or both. The image below shows TT predictions at the times of the NOAA minimum current (red streamers). A box showing "0" indicates that the NOAA value is zero at that point; at these points the red streamers (showing non-zero current) indicate a difference in timing of slack water between TT and NOAA. There is rotary current at the stations without a "0"; at these points the red and yellow streamers show a substanial difference between TT and the NOAA values of current at minimum (the example is the minimum before the larger ebb on September 7, 2010).
The image below shows the same comparison between NOAA (yellow streamers) and LK (overlying red streamers) predictions, an exact match.
Local Knowledge v. Tidetech Currents: Comparisons at NOAA Locations
All well-designed mathematical current models compute current at every point, and can be used to produce graphic current displays of any desired resolution - regardless of the underlying resolution of the data used to construct the model. What you see in a plot is interpolated values from the model, not measured current. The only real test is to compare model predictions with data. We are fortunate in San Francisco Bay because there have been many measurements over the years, including current sensors which have operated daily at several points (off and on since 1995), and a complete revision of NOAA predictions in 2004 at more than 50 "current stations" in San Francisco Bay. This information is very useful in building a model and in testing model predictions. On this page we compare values at NOAA stations in the Central Bay, all of which are reproduced exactly by the Local Knowledge ("LK") model, with the predictions of the new "Tidetech" (or "TT") current model. The images below use a current "streamer" to indicate current flow away from a circle marking the location of the station; the orientation of the streamer shows the direction of flow, and its length is proportional to the strength of the current. The yellow streamers show the NOAA (and LK) values, and the red streamers the TT predictions. NOTE: in these images the currents shown are the maximum (or minimum) values at the various locations, at the times specified by NOAA, not simultaneous current values around the Bay (the values shown are computed at somewhat different times, corresponding to the predicted max or min at the given point). On other pages we compare timing and include images that show the comparative current distribution at a given time (see links at bottom of page).