High Tide Flooding Products from NOAA CO-OPS

High tide flooding is the overflow or excess accumulation of water that covers typically dry coastal land and occurs during high tides. As relative sea levels rise, high tide flooding (HTF) is occurring more frequently, even on sunny days. HTF creates short term impacts like road closures, overflowing storm drains, and temporary business closures. Over the long term, recurrent HTF causes more severe impacts like damage to below-ground infrastructure and degraded wetlands.

Through NOAA's National Ocean Service (NOS), the Center for Operational Oceanographic Products and Services (CO-OPS) and its predecessors have gathered oceanographic data along our nation's coasts for over 200 years to protect life, property, and the environment. Serving both the public and other government agencies, CO-OPS is the authoritative source for accurate, reliable, and timely water-level and current measurements that support safe and efficient maritime commerce, sound coastal management, and recreation. As part of those roles, CO-OPS offers a suite of interactive products that helps communities understand when, where, and how often high tide flooding may occur along the coast to better inform their coastal flood planning and mitigation efforts. The suite includes a summary of historical HTF days as well as Monthly and Annual HTF Outlooks which are produced for specific stations.

Predicting the likelihood of future high tide flooding helps coastal communities and state resource managers plan for and mitigate the associated impacts. The Monthly HTF Outlook shows when and where above-normal astronomical tides and high tide flooding may be experienced for each day in the calendar year, up to a year in advance, for specific stations. Annual Outlooks provide the range of potential HTF days during the following meteorological year (May to April). Annual Outlooks support long-term planning by providing decadal projections of HTF frequency for each sea-level rise scenario out to 2100. NOAA also provides summaries of annual historical HTF days for both calendar (Jan-Dec) years and meteorological (May-Apr) years.

Together, the Monthly and Annual Outlooks replace what used to be published as Seasonal HTF Bulletins.

NOAA incorporates three severity thresholds of coastal flooding when issuing advisories or warnings: minor, moderate and major. Flood advisories are issued when minor thresholds could be breached, and warnings are issued when the higher thresholds are expected to be breached. The suite of HTF products produced by CO-OPS define a HTF flood event as a day when water levels exceed, or expect to exceed, the minor coastal flood advisory threshold.

The Monthly Outlook is based on a probabilistic model that incorporates tide predictions, sea level rise trends, and seasonal changes in observed coastal sea level to predict the potential that a higher than normal high tide may exceed established NOS flood thresholds at each water level station. Flooding likelihoods are categorized in three levels of percentages (0-5%: unlikely; 5-50%: possible; and 50-100%: likely). During periods when the model indicates a daily likelihood > 5%, high tide flooding may occur in flood-prone areas. The Monthly HTF Outlook model does not account for real-time or forecasted weather conditions. In the event of severe weather, coastal flooding may still occur outside of dates identified in the product.

The Annual Outlook is based on a statistical model fit to past annual HTF days at each water level station and the phase of the El Nino-Southern Oscillation (ENSO) through the Oceanic Nino Index (ONI). The model is run annually in May-June using ENSO forecasts from the NOAA Climate Prediction Center (CPC) and the International Research Institute at Columbia University. Since 2021, NOAA also provides decadal projections for high tide flooding (i.e., mean annual HTF days projected per decade) out to the year 2100. Decadal projections are based upon past long-term HTF day frequencies and mean sea levels observed at each water level station and generated for each of a range of relative sea level (RSL) rise scenarios produced by the US Interagency SLR Task Force of the US GCRP.

Monthly HTF products are updated monthly. Annual outlooks are updated annually.

Within NOAA's suite of HTF products, the primary output of the historical summaries and the Annual Outlook are annual counts of HTF days. The historical summary data are observed quantities for calendar and meteorological years whereas the Annual Outlook produces an upper and lower range for the next meteorological year. Data are integer values of 0 (no days that year) to 365 (every day that year) rounded to the nearest number of HTF days. The primary output of the Monthly Outlook is a probability of the likelihood that water levels will exceed the NOS minor threshold each day for the next calendar year. Probabilities range from 0% to 100%, with days below 5% noted as HTF unlikely to occur without a significant weather event.

Latest detailed information can be found at: https://www.tidesandcurrents.noaa.gov/publications/techrpt86_PaP_of_HTFlooding.pdf and at https://www.frontiersin.org/articles/10.3389/fmars.2022.1073792/full

In the event of severe weather, coastal flooding may still occur outside of dates identified in this tool. This tool should not be used for real-time situations. To monitor water levels in real-time and track storms, visit our Coastal Inundation Dashboard (the URL is included in the Distribution Information Section, but is https://tidesandcurrents.noaa.gov/inundationdb/).

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In the event of severe weather, coastal flooding may still occur outside of dates identified in this tool. This tool should not be used for real-time situations. To monitor water levels in real-time and track storms, visit our Coastal Inundation Dashboard (the URL is included in the Distribution Information Section, but is https://tidesandcurrents.noaa.gov/inundationdb/).

Observed data are recorded and transmitted; then go through a quality control procedure and are loaded into a database. For HTF Products, verified monthly data are used to generate products.

Observed water level data are recorded and transmitted; then go through a quality control procedure and are loaded into a database.

The Monthly Outlook is updated monthly, and incorporates stations with continuous verified hourly water level data since at least 1997. The linear relative sea level trend for each station is calculated using a minimum of 40 years of verified water level data, and the plotted values are relative to the 1983-2001 mean sea level datum for most stations. Several stations have separate trends determined pre- and post- earthquake time periods, and their values are plotted relative to the station's most recent epoch. Sea level anomalies are also incorporated into the model by the climatology produced autocorrelation factor for various month lead times. The choice of type and time period of the mean sea level trend, and the robustness of the autocorrelation, will affect the reference water level the predictions are based on. Although a climatology of past non-tidal contributions to observed water levels are incorporated in the current methodology, weather forecasts are not. Hence, the frequency of storms and other surge/wave producing atmospheric and oceanic circulation patterns have a large influence on the accuracy of the HTF outlooks. A retrospective skill assessment of the HTF likelihoods is run routinely to evaluate how well the likelihoods compare to the occurrences of daily maximum observed water levels exceeding the established NOS minor threshold (i.e., HTF days).

Annual Outlooks are updated yearly based on observed HTF days and Oceanic Nino Index (ONI) values. The statistical model uses linear and quadratic least squares methods to produce a range of values, plus/minus one unit of the root mean squared error (RMSE) of observations. Model accuracy depends on observed interannual variability of HTF days and length of data records. Smaller variability and longer record lengths enhance model robustness. Annual HTF predictions for the upcoming meteorological year are heavily based on ENSO forecasts, El Nino or La Nina. Accuracy of decadal HTF projections out to year 2100 are dependent on the frequency and magnitude of climate modes impacting coastal flooding (e.g., ENSO) during a specific time period, and how well observed mean sea levels match projected scenarios.

The Monthly and Annual Outlooks, as well as the annual summaries of the Historical HTF days, are produced for locations where NOS has collected continuous water level data since at least 1997 and have defined NOS flood thresholds. A subset of these stations are disseminated in the Monthly Outlooks based on the results of a retrospective skill assessment evaluating the model performance for the most recent 20 years of observations.

In the event of severe weather, coastal flooding may still occur outside of dates identified in this tool. To monitor water levels in real-time and track storms, visit our Coastal Inundation Dashboard.

CO-OPS updates high tide flooding likelihoods monthly using a methodology based on Dusek et al. 2023 (see URL 1 above). Flooding likelihoods are derived from a probabilistic model that incorporates tide predictions, sea level rise trends, and seasonal changes in coastal sea level to predict the potential that a higher than normal high tide may exceed established NOS flood thresholds. During these periods, high tide flooding may occur in flood-prone areas.