North Spit (9418767) Tide Station

Sea-Level Synthesis Plot

About the plot:

The plot displays each station's available historical water-level record in three panels with a common time axis and vertical scale. It opens with the most recent two years. Use the 1 yr, 2 yr, 5 yr, 10 yr, or All buttons, the slider below the plot, or click and drag across the plot to select a time period. The vertical scale adjusts to the displayed data. Double-click the plot or use the home button to return to the initial view. Use the English (ft) and Metric (m) buttons to switch units, and hover over the plot to view values.

Daily values use local Pacific calendar days, with Pacific Standard Time (PST) or Pacific Daylight Time (PDT) as applicable. Monthly anomalies represent calendar months.

Top panel: Monthly sea-level anomaly

Monthly mean sea level after removing the long-term relative sea-level trend and average seasonal cycle, as described on the Sea-level Anomaly pages. Anomalies reflect sustained changes in base sea level, including those associated with El Niño. Solid steps represent NOAA verified monthly means; dashed steps identify preliminary monthly values calculated from NOAA 6-minute data.

Middle panel: Tidal residual

The difference between vertical land motion (VLM)-adjusted observed water levels and regional sea-level (ReSL)-adjusted predicted water levels, indicating how much higher or lower the observed water level was than predicted. Bars show the daily mean residual for each local day with at least 80% of the expected observations. A centered 7-tidal-day (approximately 7.2-day) running mean highlights sustained non-tidal influences, including persistent winds, atmospheric pressure, and oceanographic conditions. Because the running mean is centered, it ends approximately 3.5 days before the latest data.

Middle panel: Skew surge

The difference between the highest observed water level during a tidal cycle and the corresponding predicted high water level, even when the peaks occur at different times. The plot shows the daily maximum skew surge, providing a measure of non-tidal influences on high water levels that is particularly relevant to coastal flooding.

Seasonal cycles: 

Monthly anomalies remove the average seasonal cycle observed at each station, while tidal residuals reflect the seasonal cycle represented in NOAA's tidal predictions. The difference between these seasonal adjustments is expected to be small.

Hourly and 6-minute data:

The analysis uses NOAA verified hourly water levels before January 1996 and NOAA 6-minute water levels beginning in January 1996. A dotted line identifies the hourly-data period.

For hourly observations, we estimate each high-tide peak by fitting a parabola through the highest hourly value and its two neighboring observations. This adjustment helps account for peaks that hourly sampling may miss by as much as 30 minutes. Even with this adjustment, hourly data tend to produce slightly lower skew surge values than 6-minute data, which also capture short-period water-level oscillations (seiches), particularly at Crescent City.

Tsunami days (grey bars):

Grey bars in the skew surge panel identify days when tsunamis reached the station, based on NOAA's National Centers for Environmental Information (NCEI) Global Historical Tsunami Database.

Tsunami waves can increase observed peak water levels independently of weather-related surge and background sea-level conditions. To prevent large tsunami peaks from compressing the vertical scale, the plot excludes tsunami days when determining its vertical range. Consequently, large events, such as the March 2011 Japan tsunami or July 2025 Kamchatka tsunami at Crescent City, may extend beyond the displayed range.

Hover over a grey bar to view the calculated skew surge, tsunami event name, and tsunami height reported by NOAA NCEI. Because tsunami waves often have periods of approximately 10–40 minutes, hourly observations may miss their peaks. As a result, tsunamis before 1996 may show little or no effect in the plotted record.

Data gaps:

Blank periods indicate gaps in the NOAA record when observations were unavailable or NOAA had not published the data.

Preliminary data (shaded):

Preliminary NOAA data have not undergone quality assurance or quality control (QA/QC) and may not meet NOAA's standards for official data. NOAA may revise these data; use with appropriate caution.

Daily data download:

The daily values shown in the plot are available for download as a ZIP file containing annotated CSV files in feet and meters, along with a short README describing the data columns. The files update with the plot. See the station-specific Record section below for the download link.

Data sources:

• NOAA CO-OPS (Center for Operational Oceanographic Products and Services): Verified hourly water levels, 6-minute water levels, tidal predictions, and monthly means.

• NOAA NCEI (National Centers for Environmental Information): Global Historical Tsunami Database.

• Vertical land motion (VLM) and regional sea-level (ReSL) change rates: Patton et al. (2023); NHE (2026).

North Spit, Humboldt Bay (9418767):

The record includes NOAA hourly water levels from August 1977 through December 1995 and 6-minute water levels beginning in January 1996. Data are available for approximately 98% of days.

Developed by: Jeff Anderson (NHE)