(1) Event description: This 12-day event is actually two separate, but very similar events ("sub-events") which occurred only a few days apart (see Figures 3c(1) and 3c(2) for wind speed, direction, and wave height trends). Because of their similar characteristics and close separation, they are described together. Each of these sub-events was characterized by rapid increases in wind speed (up to12.2 knots/3 hours; 17.3 knots/6 hours) and sustained build-up of wave heights over periods of about 48 hours (with maximum rates of increase of up to 3.3 feet/3 hours and 4.6 feet/ 6 hours), followed by similarly rapid decreases. Each sub-event was about 3 days in duration. The first sub-event began late on day 1, with wind speeds rising from 2 knots late in the afternoon and through about 0600Z (2300 PDT) on day 2. Winds temporarily decreased during the following morning, then rose again, peaking at 28 knots during the afternoon and evening hours of days 2-3 (peak gusts to 36 knots). Winds then dropped sharply through about 1200Z (0500 PDT) on day 3, to 9-12 knots. Although another rise occurred later on day 3 (to near 20 knots), the overall trend in wind speeds was downward, falling back to around 3 knots early on day 4.

Wave heights, already 6 to 8 feet on the first day, built further -- peaking at 15.4 feet (with dominant periods of about 9 seconds) during the afternoon and evening hours on days 2-3. As the wind decreased overnight into day 3, wave heights also subsided rapidly, to near 5 feet by late on day 4.

Wind speeds remained lower on days 5 through 7 (maximum sustained wind speeds about 13-17 knots -- strongest late night and early morning hours), while wave heights continued to slowly subside from 5 to 3 feet. Sustained wind speeds then began another steady increase on day 8, rising more or less continuously from about 2 knots to a peak of 29.5 knots (gusts to 37.6 knots) late in the afternoon and early evening of day 9.

Wind speeds were strongest when from the north to north-northwest. When winds decreased (typically around 1200Z, or 0500 PDT), directions tended to turn to the north-northeast. Between the two major wind/wave peaks in this case, winds turned to the south-southwest and rose to 15-20 knots on days 5-6 as the thermal trough shifted off the coast.

Wave heights in this second sub-event (beginning on day 8) initially rose abruptly, then more steadily to 11-12 feet by 0000Z (1700 PDT) on day 9, then more gradually to a peak near 14 feet by 0300Z (2000 PDT) on day 10 (with dominant periods of 8.3 seconds, slightly shorter than the previous peak on days 2-3). Winds and waves then again fell rapidly, to about 2 knots and 3-4 feet on day 11. A third buildup of wind and waves apparently began on day 12, but is not described here.

(2) General Weather Situation: Figures 3c(3) through 3c(14) show the surface and 500 mb analyses for this event. As mentioned above, this case is really two separate events, and this is also clearly seen in the changing nature of the associated weather patterns. On day 1, an upper level trough was located just off the Pacific Northwest and far northern California coasts, with surface high pressure centered west of Washington state, and a weak thermal trough inland and near the coast over California. On days 2-3, the upper trough shifted inland and was replaced by a strongly amplified upper ridge over the northeast Pacific. As the upper ridge built, an associated surface ridge developed over the Pacific Northwest, while the thermal trough remained nearly stationary. Because of the surface ridge to the north, pressure gradients increased strongly over the far northern California and southern Oregon coasts. The upper ridge weakened on days 4-5, with the thermal trough moving to the California coast, and also northward off the Oregon and Washington coasts, thus relaxing the pressure gradients and ending the first of these two events. On days 5-6, another upper trough moved near the coast, then inland by day 8, and was again followed by a strongly amplified upper ridge over the northeast Pacific. The surface pattern for this second event also followed a similar sequence (compared to the first event).

(3) Harmonic Analysis of Wind Speed and Wave Height: Table 1 and Figures 3c(15) through 3c(16) show the harmonic analysis results (wind speed and wave height, period = 24 hours) for this event. Similar to Case #2, this pair of events was not characterized by strong diurnal trends overall, due to the changing weather patterns described above. For this case overall, the percent-variance explained by the first harmonic (wind speeds; period = 24 hours) was near zero. Again, though, individual days showed stronger diurnal character, between 60 and 80 percent on days 3, 7, 9, and 10 (September 11, 15, 17, and 18, respectively), and less than 50 percent on the other days. Overall, harmonic analysis showed no particularly favored time of day for stronger/weaker wind speeds -- again, likely due to the constantly changing weather pattern. Similarly, harmonic analysis of wave heights for this case showed no dominant trends. The percent-variance explained by the first harmonic was between 60 and 80 percent on days 3, 4, 8, and 10-12, and less than 40 percent on other days.

(4) Pressure Gradients: Figure 3c(17) shows the time-series of wind speeds at buoy #46022, along with pressure gradients (i.e. absolute magnitudes) between buoys #46027 and 46014, about 80 miles north and 120 miles south, respectively. The 46027-46014 pressure gradients rapidly increased during the first event (days 2-3), but lagged behind the increase in wind speed by several hours, peaking at 7.3 mb at 0700Z (midnight PDT) on day 3, or about 5-7 hours after the wind peak. In contrast, 46027-46014 pressure gradients during the second event began rising a few hours before the wind speed began rising, and continued to rise strongly through days 8-10, peaking at 9.3 mb at 0600Z (2300 PDT) on day 10. Despite the initial tendency for the 46027-46014 pressure gradient increases to precede the increases in wind speed in the second event, the peak pressure gradient eventually occurred about 5 hours after the peak in sustained wind speed, similar to the first event. It is also noteworthy to mention that the 46027-46014 pressure gradients observed during the second event in this case were among the strongest observed for all the cases analyzed in this study. For reasons noted previously, harmonic analysis of the pressure gradients was not conducted for this case.

(5) Upstream/Downstream Conditions: During the period of this case, strong winds also generated large waves at buoys 46027 (80 miles north) and 46014 (120 miles south). At buoy 27, winds were even stronger than at buoy 22 (this case); average sustained wind speeds were 15 knots, with peak sustained winds close to 35 knots, and peak gusts of more than 40 knots. Wave heights at buoy 27 averaged 6.5 feet, with peak wave heights of 11.8 feet. Given an average period of 8.3 seconds at buoy 27 (with waves 10 feet), this would indicate waves decaying only about 2 feet between buoys 27 and 22 (i.e. up to 8-10 foot decayed waves arriving at buoy 22). Assuming a 200 nautical mile fetch and a sustained wind speed of 25 knots (probably excessive for this case, but useful for illustration), the theoretical fully developed wave height would be about 10 feet after 24 hours, still about 5 feet less than the peak observed at buoy 22 for this case. It is possible that winds between buoys 27 and 22 could have been stronger, leading to larger waves upstream. It is also possible that long period swell from well outside the area could have contributed to the wave heights observed. Beginning on day 5 and continuing into day 6, a dominant period of around 14 seconds emerged, but the significant wave height at this time was only around 4 feet. Considering that, overall, wave heights rose as wind speeds rose (and vice-versa), it is probably reasonable to assume that the contribution from long period swell was negligible.

Downstream effects (contribution from waves generated near buoy 22 propagating south): Assuming a dominant period of 8.6 seconds at buoy 22 (average based dominant periods with wave heights 12 feet) and peak waves of 12 to 15 feet, waves up to 10-12 feet could be expected to arrive downstream at buoy 14. Sustained winds at buoy 14 during this period averaged slightly less than at buoy 22 (11.6 knots, with a peak of 29.1 knots, and peak gusts to 38.6 knots). Given average wave heights of 7.1 feet at buoy 14 (peak wave heights of 14.4 feet), the upstream contribution (from near buoy 22) would again help to explain the wave heights actually observed.

Figure 3c(1). Wind speed and wave height, buoy #46022 (9-20 September 1988).
Figure 3c(2). Wind speed and direction, buoy #46022 (9-20 September 1988).

Figure 3c(3). Surface analysis (left) and 500 mb analysis (right): 1200Z, 9 September 1988.

Figure 3c(4). Same as Figure 3c(3), but for 10 September 1988.

Figure 3c(5). Same as Figure 3c(3), but for 11 September 1988.


Figure 3c(6). Same as Figure 3c(3), but for 12 September 1988.

Figure 3c(7). Same as Figure 3c(3), but for 13 September 1988.

Figure 3c(8). Same as Figure 3c(3), but for 14 September 1988.


Figure 3c(9). Same as Figure 3c(3), but for 15 September 1988.

Figure 3c(10). Same as Figure 3c(3), but for 16 September 1988.

Figure 3c(11). Same as Figure 3c(3), but for 17 September 1988.


Figure 3c(12). Same as Figure 3c(3), but for 18 September 1988.

Figure 3c(13). Same as Figure 3c(3), but for 19 September 1988.

Figure 3c(14). Same as Figure 3c(3), but for 20 September 1988.

Table 3c. Time of peak amplitude and percent-variance explained by the first harmonics of wind speed and wave height (period = 24 hours) at buoy #46022 (9-20 September 1988).

Figure 3c(15). Harmonic analysis of wind speed data, buoy #46022, 9-20 September 1988.
Figure 3c(16). Harmonic analysis of wave height data, buoy #46022, 9-20 September 1988.
Figure 3c(17). Wind speed (buoy #46022) and pressure gradient (absolute magnitudes) for buoys #46027-46022, 46022-46014, and 46027-46014, 9-20 September 1988.