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Subject: F7) How is storm surge forecast at NHC?

Contributed by the NHC Storm Surge Unit

The Sea, Lake, and Overland Surges from Hurricanes (SLOSH) model is the computer model utilized by the National Oceanic and Atmospheric Administration (NOAA) for coastal inundation risk assessment and the operational prediction of storm surge.

The eastern seaboard and Gulf Coast of the United States, Puerto Rico, the Bahamas, the Virgin Islands, and Hawai'i, are subdivided into 39 regions or "basins." These areas represent sections of the coastline that are centered upon particularly susceptible features: inlets, large coastal centers of population, low-lying topography, and ports. The SLOSH model computes the maximum potential impact of the storm in these "computational domains" based on storm intensity, track, and estimates of storm size provided by hurricane specialists at NHC.

SLOSH model basin coverage

Currently, SLOSH basins are being updated at an averate rate of 6 basins per year. SLOSH basin updates are ultimately governed by the Interagency Coordinating Committee on Hurricanes (ICCOH). ICCOH manages hazard and post-storm analysis for the Hurricane Evacuation Studies under FEMA's Hurricane Program. Updates are driven by a number of different factors such as: changes to a basin's topography/bathymetry due to a hurricane event, degree of vulnerability to storm surge, availability of new data, changes to the coast, and addition of engineered flood protection devices (e.g. levees).

Sometimes these updates include higher grid size resolution to improve surge representation, increasing area covered by hypothetical tracks for improved accuracy, conversion to updated vertical reference datums, and including the latest topography or bathymetric data for better representation of barrier, gaps, passes, and other local features.

The SLOSH model can generate sevel different products :

  • Deterministic runs
    This is an operational product based on the official NHC track and intensity forecast of a tropical cyclone. Operational SLOSH runs are generated whenever a hurricane warning is issued, approximately 36 hours prior to arrival of tropical storm winds. It is run every 6 hours coinciding with the full advisory package. This is a single run product which can result in uncertainty because it is STRONGLY dependent on the accuracy of the storm track and timing. This product is intended to provide valuable surge information in support of rescue and recovery efforts.

    The three diagrams below illustrate a classic case in which Hurricane Ivan's (2004) actual track shifted 30 miles to the east of NHC's 12 hour advisory forecast track, well within the cone of uncertainty, and greatly shifted the storm surge impacts from Mobile Bay, AL to Pensacola, FL.

    Surge based on NHC 12 hour advisory

    Actual hurricane track 30 miles east of 12 hour advisory forecast track

    Surge based on NHC Best Track

  • Probabilistic runs (P-surge)
    This is a graphical product using an ensemble of many SLOSH runs to create a Probabilistic Storm Surge (P-Surge) product. This is intended to be used operationally so it is based on NHC's official advisory. P-Surge uses SLOSH-based simulations which are based on statistics of past performance of the advisories. These different SLOSH simulations are based on the distribution of :
    • Cross-track error (impacts landfall location)
    • Along-track error (impacts foreward speed and timing)
    • Intensity error (impacts pressure)
    • Size error (impacts size)
    P-Surge is available whenever a hurricane watch or warning is in effect. It is posted on the NHC webpage within approximately 30 minutes after the advisory release time.

    Example of P-Surge output

  • Maximum Envelope of Water (MEOW) runs
    This is an ensemble product representing the maximum height of storm surge water in a given basin grid cell using hypothetical storms run with the same:
    • Category (intensity)
    • Foreward speed
    • Storm trajectory
    • Initial tide level
    Internally a number of parallel SLOSH runs with same intensity, forward speed, storm trajectory, and initial tide level are performed for the basin. The only difference in runs is that each is conducted at some distance to the left or right of the main track (typically at the center of the grid). Each component run computes a storm surge value for each grid cell. For example, five parallel runs may yield storm surge values of 4.1, 7.1, 5.3, 6.3, and 3.8 feet. In this case, the MEOW for the cell is 7.1 ft. It is unknown (to the user) which track generated the MEOW for a particular cell, so it is entirely possible that the MEOW values for adjacent cells may have come from different runs. MEOWs are used to incorporate the uncertainties associated with a given forecast and help eliminate the possibility that a critical storm track will be missed in which extreme storm surge values are generated.
    MEOWs provide a worst case scenario for a particular category, forward speed, storm trajectory, and initial tide level incorporating uncertainty in forecast landfall location. The results are typically generated from several thousand SLOSH runs for each basin. Over 80 MEOWs have been generated for some basins. This product provides useful information aiding in hurricane evacuation planning.

    Example of MEOW output
    (NNW;Cat 3;20 mph;High Tide)
    Each parallel line represents a different track used in the creation of this MEOW

  • Maximum of MEOW (MOM) runs
    This is an ensemble product of maximum storm surge heights for all hurricanes of a given category regardless of forward speed, storm trajectory, landfall location, etc.. MOMs are created internally by pooling all the MEOWs for a given basin, separated by category and tide level (zero/high), and selecting the MEOW with the greatest storm surge value for each basin grid cell regardless of the forward speed, storm trajectory, landfall location, etc. This procedure is done for each category of storm. Essentially, there is 1 MOM per storm category and tide level (zero/high). MOMs represent the worst case scenario for a given category of storm under "perfect" storm conditions. The MOMs provide useful information aiding in hurricane evacuation planning and are also used to develop the nation's evauation zones.

    Example of Cat 3 MOM output

Strengths and limitations of SLOSH

The SLOSH model is computationally efficient resulting in fast computer runs. It is able to resolve flow through barriers, gaps, and passes and models deep passes between bodies of water. It also resolves inland inundation and the overtopping of barrier systems, levees, and roads. It can even resolve coastal reflections of surges such as coastally trapped Kelvin waves. However it does not model the impacts of waves on top of the surge, account for normal river flow or rain flooding, nor does it explicitly model the astromical tide (although operational runs can be run with different water level anomalies to model conditions at the onset of operational runs).

Last updated May 14, 2010

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