Evaluating the Regional and Temporal Variation of Skill of the NOAA High Tide Flooding Monthly Outlook

High Tide Flooding (HTF) is occurring more frequently every year across the United States and is increasingly disrupting coastal community life, causing significant damage to private property, public infrastructure, and our natural resources. HTF describes the flooding of low-lying areas occurring at high tides, usually referring to minor flood levels that in the past had required a coastal storm but now can occur due to abnormally high tides, a change in ocean currents, or a stiff onshore breeze. Because of rising seas, land subsidence, and changing natural barriers, high tide flooding is now twice as frequent in U.S. coastal communities as it was 20 years ago, a rise that is expected to continue in the future. In response, NOAA has invested significantly in 1) research to better understand observed trends and underlying tidal, oceanic, atmospheric, and other processes that influence increased sea levels and coastal flooding, and 2) operationalizing the latest science into multiple reports and web-based products to disseminate and visualize past and future HTF events. Results of this work aid in improving our ability to predict potential occurrence of coastal flooding and provide coastal communities more accurate and longer advance warning of its associated hazards.

This talk will focus on recent research aimed at improving the subseasonal to seasonal (S2S) predictions of HTF included within the Monthly Outlook product. The Monthly Outlook is based on a novel statistical model that relies on hourly astronomical tide predictions, sea level trends, monthly sea level anomaly persistence and climatological non-tidal residuals at tide gauge locations. Over each day, these components are added together to determine the likelihood that water levels will equal or exceed the defined minor flood impact threshold. Published results show the method to be skillful at predicting flooding up to 12 months in advance, particularly at locations where flooding is more tidally driven than storm surge driven. An analysis of the model skill assessment over the seasonal cycle, across years, and within geographic regions will be presented. Model skill is also evaluated against extratropical and tropical cyclone frequency and prevalence of climate modes (e.g., El Nino vs La Nina years ).

NOAA’s suite of interactive HTF products and datasets help communities understand when, where, and how often HTF may occur to better inform coastal flood planning and mitigation efforts across climate time scales.