Six-Day High Sea Level Forecast : Apra Harbor, Guam
Attention: The forecast is not accurate when a tsunami, tropical storm or cyclone watch/warning is in effect. For these events, please seek information for either tsunamis or tropical storms/hurricanes.
The Dark Blue Curve displays the Observed Sea Level at Apra Harbor for the previous 3 days. The measurement is relative to Mean Lower Low Water (MLLW, a typical NOAA datum).
The Cyan Curve displays the Forecast Sea Level at Apra Harbor for the next 6 days. The forecast is also relative to MLLW, and includes tides plus multi-day sea level variations. The forecast is updated every 4 hours.
The Red Line indicates the sea level height (3.15 ft.) that is exceeded by 2 percent of the observed daily Higher High Waters (HHW) based on a 19-year historical record (excluding tsunamis). When this sea level height is exceeded, flooding of the lowest lying lands begins.
The Six-Day High Sea Level Forecast is a tool to predict higher than usual sea level at locations such as protected harbors and atoll lagoons. The forecast provides information that contributes to safe and reliable operations by harbor users, and provides a benefit to residential and commercial property owners in surrounding low-lying areas. This forecast does not incorporate the effects of wind-generated gravity waves, either locally or remotely forced, and therefore does not indicate the magnitude of short-period surges that also impact harbor operations (see the description of the PacIOOS Harbor Surge Forecast). This Six-Day High Sea Level Forecast tool should also be used cautiously in assessing high sea level danger on exposed coasts and shoreline areas outside the harbor or lagoon, since, as just noted, the forecast does not include the significant impact of wind-generated gravity waves on coastal sea levels (see the description of the PacIOOS Wave Run-Up Forecast).
PacIOOS currently offers the Six-Day High Sea Level Forecast for 9 harbor locations throughout the Pacific Islands region. While observed sea level heights and tide predictions are readily available elsewhere for these locations, there have been periods of time (exclusive of tsunamis and storm surges) when the sea level exceeds the predicted tide enough to produce unexpected flooding of low-lying land near these stations. These events are usually due to the forcing of higher sea level, on top of that due to the tides, by common, but strong, local winds and/or by ocean currents evolving offshore. The Six-Day High Sea Level Forecast was developed to enhance preparedness by providing advance notice of such events.
In case of possible high sea levels, please consult with local authorities to seek further information and direction. The PacIOOS High Sea Level Forecast does not serve as a warning system.
For more information, please contact Martin Guiles. The Six-Day High Sea Level Forecast Tool was developed by M. Guiles, D. Luther, and M. Merrifield within the Department of Oceanography in the School of Ocean and Earth Science and Technology (SOEST) at the University of Hawaiʻi at Mānoa.
This example from 2012 shows observed sea level at Apra Harbor (dark blue line) periodically exceeding the threshold (red line) for the initiation of flooding of low-lying lands. The highest sea levels are a combination of the predicted tide (pink line), which by itself would not produce flooding, and a multi-day, non-tidal sea level event (black line) related to large-scale regional ocean variability.
Components of the Six-Day High Sea Level Forecast
Please also view the detailed technical report to learn more about the methods applied to produce the PacIOOS High Sea Level Forecasts.
The Dark Blue Curve shows the Observed Sea Level (SL), in feet, at Apra Harbor for the 3 days (in hourly increments) prior to the time this plot was created (indicated by the orange yellow vertical dotted line). The Observed Sea Level is measured relative to Mean Lower Low Water (MLLW, a typical NOAA datum).
Observed SL = Calculated Tide + Observed Residual SL
The Cyan Curve shows the Forecast Sea Level (SL), in feet, at Apra Harbor for the next 6 days (in hourly increments). The forecast is also made relative to MLLW and is updated every 4 hours.
Forecast SL = Calculated Tide + Predicted Residual SL
The Red Line indicates a high sea level value of 3.15 ft. that is calculated as follows. First, tsunami signals were edited out of the Observed Sea Level for the 16-year period 1996-2011. Then, the linear trend of the 16-year record was estimated and subtracted. The Higher High Water (HHW) value for each day was extracted. The sea level height that was exceeded by only 2% of these HHWs was determined. This 2% exceedance value was added to the mean sea level based on the 2010-2011 period of data to yield the 3.15 ft. sea level value plotted as the red line in the top plot. Field verification has shown that when this sea level height is exceeded, flooding of the lowest lying lands begins.
Tide Component of Observed and Forecast Sea Level
The Magenta Curve is the Calculated Tide component of sea level for all 9 days displayed. Tidal variations of sea level are calculated from analyses of prior observed sea level data. For Apra Harbor, approximately 9 years of data, from January 2001 through April 2010, were employed to determine the tidal constituents. From these constituents, the tidal component of sea level can be very accurately predicted many years into the future.
Non-Tidal Component of Observed and Forecast Sea Level
The Filled Black Curve in this plot shows the Non-Tidal components of both the Observed Sea Level and Forecast Sea Level. In general, Sea Level variations are dominated by the tides. However, there can be significant Sea Level height changes (occasionally, up to 1 foot or more) due to either persistent winds, strong offshore currents (eddies), surface wave-driven setup, or, of course, tsunamis which are forecast at the Pacific Tsunami Warning Center. A recent example of high (about 1/2 foot), slowly varying, non-tidal sea level can be found here.
Observed Non-Tidal Sea Level (SL) = Observed SL – Calculated Tide
The Forecast Non-Tidal Sea Level is based on a mathematical projection into the future of the Observed Non-Tidal Sea Level from the past 6 days. The main components of the Observed Non-Tidal Sea Level that are specifically accounted for in the forecasts are as follows:
- The daily and semi-daily variations in Sea Level that result from, for instance, forcing by the wind, by solar heating, and by set-up due to surface gravity wave swell; and,
- Changes in Sea Level that occur over several days or more due, for instance, to persistent winds and to slowly changing open ocean currents around the island.
The Vertical Red Lines in this plot represent the confidence intervals for the Forecast Non-Tidal Sea Level. The length of these lines (from end to end) represents twice the standard deviation of the difference between the Observed Non-Tidal Sea Level and the Forecast Non-Tidal Sea Level for the year from October 2008 to October 2009. The true Non-Tidal Sea Level, once measured, can be expected to fall within the limits of the red lines approximately two times out of three. Doubling the length of the red lines will ensure that approximately 19 out of 20 true, observed values will fall within those confidence limits.