Establishing a Community-Based Extratropical Storm Surge and Tide Model for NOAA's Operational Forecasts for the Atlantic and Gulf Coasts

Impacts of extratropical storm surge can be far-reaching and catastrophic along the East Coast of the United States. In order to improve National Oceanic and Atmospheric Administration (NOAA) forecasts, a new operational coastal ocean model, the Extratropical Surge and Tide Operational Forecast System (ESTOFS), was developed for the Atlantic and Gulf coasts using the community-based ADvanced CIRCulation (ADCIRC) model. ESTOFS is designed to model the combined water level caused by extratropical storm surge and tides.

The ESTOFS delivers predictions of (1) combined surge and tide, (2) astronomical tides, and (3) sub-tidal water level (the isolated surge) throughout the U.S. Atlantic and Gulf coasts. ESTOFS uses an unstructured grid with a coastal resolution of down to 3 km that extends up to the coastline. The ESTOFS models the surge and tides together; a parallel simulation is made solely with tidal forcing to generate the tidal prediction and these tides are subtracted from the combined water level to compute the subtidal water level. By modeling surge and tide together, important nonlinear interactions are captured by the model, and by separating them via post-processing, users such as forecasters can view and adjust the surge prediction. This capability serves the needs of National Weather Service forecasters at the National Centers for Environmental Prediction/Ocean Prediction Center (NCEP/OPC) and the NCEP/National Hurricane Center/Tropical Analysis and Forecast Branch (NHC/TAFB) for providing offshore marine forecasts, and at Weather Forecast Offices for coastal inundation forecasts.

ESTOFS also provides critical water level boundary conditions for coastal wave predictions from the Nearshore Wave Prediction System* currently in development by the NCEP/Environmental Modeling Center (EMC). Therefore, ESTOFS's set-up is designed to mimic that of NCEP/EMC's operational WAVEWATCH III® (WW3) wave model implementations: it uses the same surface forcing, has the same forecast cycle, and length.

ESTOFS was put through a comprehensive statistical skill assessment that included a full year's hindcast and real-time forecast tests. Operational implementation of ESTOFS Atlantic occurred in August 2012 at NCEP. The model is forced by 10 m winds and sea level pressure from the NCEP Global Forecast System (GFS) to create predictions out to 180 hours four times daily. Tides are forced at the ESTOFS open ocean boundary using a TOPEX/POSEIDON global tidal model (TPXO, version 6.2) and refined further within the domain via increased resolution and tidal potential body forces. Hourly output from ETOFS is delivered in two file formats, including General Regularly-distributed Information in Binary, version 2 (GRIB2) format on the standard 2.5 km structured National Digital Guidance Database (NDGD) grid for the Atlantic and Gulf coasts, and the 1.25 km NDGD grid for Puerto Rico. Hourly output on the ESTOFS native unstructured grid is also provided in Network Common Data Format (NetCDF). In addition, ESTOFS produces NetCDF files of six minute water level records at gauge locations.

The ESTOFS leverages the community-developed and supported ADCIRC finite element model. The ADCIRC hydrodynamic model is supported by a broad group of federal, academic, and private sector developers and users. It has features beneficial for ESTOFS, such as tide and wave components and has been shown to be very effective at predicting tidal circulation and storm surge propagation in complex coastal systems. Additionally, its unstructured grid methodology can resolve the propagation of storm surges from offshore, across the shelf, and to the coast, and can represent irregular shorelines, barrier islands, rivers and waterways. The grid used in ESTOFS was originally developed for tidal simulations and was applied here.

The Coast Survey Development Laboratory (CSDL) of the National Ocean Service (NOS) and NWS/NCEP/EMC collaborated to develop ESTOFS and transition it to operations in conjunction with NCEP Central Operations (NCO). ESTOFS is now being developed for the U.S. Pacific coasts with operational implementation planned for 2014.