Towards the Development of Operational Total Water Level Storm Surge Guidance

Storm surge flooding affects vulnerable coastal communities across the U.S. due to severe tropical and extratropical storms. Strong winds and low atmospheric pressure cause surges which pile water up along the coast, with flood heights exacerbated by large waves, high tides, and river inflows. Forecasts require accurate model guidance that predicts the total water level from these events in order to calculate the height of flood waters above ground. The National Ocean Service (NOS) develops and maintains operational storm surge models which are part of the ensemble model guidance used by National Weather Service (NWS) forecasters and partners to predict the flooding from strong coastal storms. NOS relies on the Advanced Circulation (ADCIRC) unstructured grid finite element model to develop storm surge guidance that combines the effects of surge, tides, and rivers.

The Extratropical Surge and Tide Operational Forecast System (ESTOFS) operates four times per day to predict storm surge and tides along the coastline. ESTOFS has been operational in the Atlantic since 2012 and in 2014 this system was implemented for the Eastern Pacific, including the West Coast, Gulf of Alaska, and Hawaii. This model uses a coarse coastal resolution of 1 to 3 km in order to operate at low cost while providing, for the first time, storm surge and tidal guidance for the West Coast and Hawaii. The Pacific implementation of ESTOFS was determined to have a root mean square error (RMSE) of less than 0.20 m for storm surge predictions for nearly the entire domain.

NOS has been experimentally testing the use of an ADCIRC storm surge model ensemble for hurricane predictions over the past three years. This experimental real-time system uses the ADCIRC Surge Guidance System to generate a five member ensemble water level prediction based upon the National Hurricane Center's Official Forecast Track. Ensemble members are generated by shifting the track location to the left and right and increasing or decreasing size and intensity, and by using output from ensemble simulations from the Hurricane Weather Research and Forecasting (HWRF) model. Output is disseminated via servers using the Open-source Project for a Network Data Access Protocol (OPeNDAP). This model simulates overland flooding caused by surge and tides by using an unstructured model grid with approximately 500 meter resolution for large portions of the East Coast and Gulf of Mexico.

NOS is now developing an operational implementation of ADCIRC for hurricane storm surge prediction. This model simulates overland inundation by providing resolution down to 200 meter from Texas to Maine and for Puerto Rico, and has been validated for the simulation of tides and storm surges from ten historical tropical and extratropical storms. This model will be implemented operationally with an ensemble of forcing and will be used to upgrade ESTOFS for the East Coast. This latest model is also being tested with river inflows from hydraulic models used in NWS operations in order to predict the total water level from these events.