Adjusted Tide-Station Water Levels, Tidal Residuals, and Skew Surge

This webpage and its subpages present adjusted water levels, tidal residuals, and skew surge for three NOAA tide stations along the Northern California coast: Crescent City, North Spit (Wigi or Humboldt Bay), and Arena Cove. The adjustments account for vertical land motion (VLM) and regional sea-level (ReSL) change, providing a consistent basis for comparing observed and predicted water levels across the three stations.

NOAA reports observed water levels relative to local tidal datums, which reflect relative sea-level changes, including the effects of vertical land motion. NOAA's tidal predictions are referenced to tidal datums established during the 1983–2001 National Tidal Datum Epoch, centered on 1992.5, and do not explicitly account for subsequent sea-level change. Consequently, differences between observed and predicted water levels can reflect long-term changes in land elevation and regional sea level, as well as short-term oceanographic and meteorological conditions.

To account for these changes, we adjust preliminary observed water levels for VLM and tidal predictions for ReSL change using station-specific rates from Patton et al. (2023) and NHE (2026). Both adjustments use 1992.5 as the reference year and account for changes through the current date. We express the adjusted water levels relative to the North American Vertical Datum of 1988 (NAVD88), providing a common vertical reference for evaluating coastal water levels.

An automated R script retrieves the latest NOAA data every 15 minutes, applies the adjustments, calculates tidal residuals (adjusted observed minus adjusted predicted water levels), and updates the plots and summary tables. The plots display yesterday's water levels and today's ongoing tide, including tidal residuals that reflect non-tidal influences such as winds, atmospheric pressure, storm surge, and longer-term oceanographic conditions associated with El Niño.

The summary tables also report the daily maximum skew surge calculated from adjusted observed and predicted water levels. Skew surge is 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. Because coastal flooding often occurs near high tide, skew surge provides a useful measure of how much non-tidal processes elevate high water levels. In contrast, the largest instantaneous tidal residual may occur near low tide, where differences in tidal timing can produce large residuals. For today's ongoing tide, the tables include only completed high tides and mark preliminary daily values with an asterisk (*) until all high tides for the day are complete.

The station pages provide interactive plots and summary tables in English or metric units. Users can save individual plots as images using the camera button. Supporting documents describe the VLM and ReSL rates used in the adjustments.

For longer-term context, including monthly sea-level anomalies and El Niño effects, visit the Sea-level Anomaly or Sea-level Synthesis pages.