Currents during mixed tidal patterns
The NOAA current charts represent data during semi-diurnal tide patterns. About 80 percent of the time on the West coast of the US, the pattern deviates either moderately or severely from this symmetry. NOAA provides daily information on both tidal levels and currents, however, the data are now actually published by private corporations, e.g. "Tidal Current Tables 2001, Pacific Coast of North America and Asia", published by International Marine, a division of McGraw-Hill.
For each day, the tables present the times of slack current and the times and velocities of the maximum flood and ebb currents. The entries for the first two days of January 2001 are:
| January | |||
| Slack | Maximum | ||
| h m | h m | knots | |
| 1 M | 0226 | 2.8 F | |
| 0612 | 0904 | 1.1 E | |
| 1139 | 1437 | 1.2 F | |
| 1704 | 2027 | 1.4 E | |
| 2322 | |||
| 2 Tu | 0315 | 2.8 F | |
| 0650 | 0952 | 1.4 E | |
| 1304 | 1545 | 1.4 F | |
| 1827 | 2151 | 1.3 E |
As you can see, the times of slack current are listed in the first column, and the times of the alternating flood, ebb, flood, ... are listed in the second column. Currents at intermediate times can be calculated by interpolation.
If you want to use the current charts during a mixed tide, NOAA suggests the following procedure to obtain a rough approximation:
1: choose the clock time you are interested in,
2: calculate the number of hours (rounding to the nearest hour) between this time and the closest slack, flood, or ebb current using the Tidal Current Tables,
3: find the chart closest to your time, e.g. ebb +1, or flood -2,
4; scale all the currents in the chart by the ratio of the maximum current at the Golden Gate in the chart to the maximum current at the Golden Gate in the Tidal Current Tables for the date in question.