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As part of the Framework, the extent of coastal flood hazard was completed by using readily availableSea, Lake, and Overland Surge from Hurricanes (SLOSH) modeling conductedby NOAA. SLOSH modeling of hurricane intensities is categorized by the Saffir-Simpson hurricane wind scale and includes other characteristics of hurricanes that can vary considerably along the coast, such as angle of approach to the shoreline, width and slope of the continental shelf, astronomical high tide level, and local geographic features (FEMA 2011). The SLOSH model outputs inform hurricane evacuation studies. The inundation zones identified by the SLOSH model depict areas of possible flooding from the maximum of maximum (MOM) event within the five categories of hurricanes by estimating the potential surge inundation during a high-tide landfall. Although the SLOSH inundation mapping is not referenced to a specific probability of occurrence (unlike FEMA flood mapping, which presents the 0.2-percent- and 1-percent-annual-chance flood elevation zones) nor does it include wave heights, the flooding inundation from a Category 4 hurricane making landfall during high tide represents an extremely lowprobability of occurrence but high-magnitude event.
The intent of the NACCS is to generate a spatially comprehensive, but first-order approximation of flooding vulnerability across the entire northeastern Atlantic coastal region. The use of the SLOSH model MOM was necessary based on the very large spatial extent of the study area and the fact that it is currently the most advanced storm surge modeling available for the entire study area. The extent of the Category 4 MOM represents the maximum storm tide levels caused by extreme hurricane scenarios across the region, and, therefore, provides a reasonable approximation of the most extreme flooding extent. The State and District of Columbia Appendix presents the SLOSH hydrodynamic modeling inundation mapping associated with Categories 1 through 4 hurricanes used for evacuation modeling.