The links below provide hurricane storm surge inundation maps for the U.S. Atlantic coast, Oahu, the Bahama Islands, Puerto Rico, and the Virgin Islands. They were generated using the primary computer model used by the National Hurricane Center (NHC) to forecast storm surge--the Sea, Lake, and Overland Surge from Hurricanes (SLOSH) model. The accuracy of the SLOSH model is advertised as plus or minus 20%. There are two sets of images available. The first set, titled "Maximum Water Depth", shows the water depth at each grid cell of the SLOSH domain. Thus, if you are inland at an elevation of ten feet above mean sea level, and the combined storm surge and tide (the "storm tide") is fifteen feet at your location, the water depth image will show five feet of inundation. The second set of images, titled "Maximum Storm Tide", shows how high above mean sea level the sum of the storm surge plus the tide reaches. Over the ocean, the storm tide and water depth images will show the same values. The storm tide images contain no information about how deep the water will be inland, and are generally less useful than the water depth images. All of these Maximum of the "Maximum Envelope of Waters" (MOM) images were generated for high tide, and thus show worst-case inundation scenarios for mid-strength hurricanes of each Saffir-Simpson Category (Category 1, 2, 3, 4, and 5). Category 5 hurricanes have never occurred in the Mid-Atlantic or New England regions, so there are no Category 5 images shown there. No single storm will be able to cause the level of flooding depicted in the SLOSH storm surge images along the entire coast.
Maximum of the "Maximum Envelope of Waters" (MOM) Storm Surge Imagery
Figure 1. Sample water depth inundation image (left) and storm tide image (right), created using NOAA's SLOSH model. These Maximum of the "Maximum Envelope of Waters" (MOM) plots are for Tampa Bay, Florida, for a composite maximum surge for large suite of possible mid-strength Category 4 hurricane (sustained winds of 143 mph) hitting at high tide.
How to interpret the storm surge images
A sample set of storm surge images for a Category 4 hurricane hitting Tampa Bay is shown in Figure 1. Black lines mark the coastline, and also delineate the grid the SLOSH model used. There may be storm surge present outside the boundaries of the grid, so pay attention to where the grid boundaries are. Also, if you see a high surge modeled for a narrow waterway that goes right up to the edge of the grid boundary, don't believe it. The model puts an artificial barrier at the grid boundary, and the surge is piling up against this non-existent barrier. Empty brownish grid cells with no coloration show where no inundation is computed to occur. St. Petersburg becomes two islands in a worst-case scenario Category 4 hurricane, as shown by the brown areas surrounded by colored areas of storm tide (this did occur during the Great Gale of 1848, a Category 4 hurricane that hit the city). The tide level is marked at the bottom of the color legend, and is 1 foot in this example. The left "maximum water depth" image shows how high above each grid cell the storm tide reaches. The storm tide--the combination of the storm surge plus the 1 foot high tide--reaches as much as 27 feet above mean sea level (pink colors) near downtown Tampa (right-hand "maximum storm tide" image). The amount of inundation inland is controlled by the elevation of the land. Some of the inland regions near downtown Tampa being inundated by the 27-foot storm tide are at an elevation of 19 feet, so as much as 8 feet of inundation will occur at those locations (dark blue colors in the left-hand "maximum water depth" image). Interstate highways are the thick grey-green lines, and smaller highways are shown as dark green and light green lines. If a road is inundated by storm surge, it will not appear. County boundaries are shown in red.
Understanding SLOSH model output: MEOWs and MOMs
The primary computer model used by the National Hurricane Center (NHC) to forecast storm surge is called the Sea, Lake, and Overland Surge from Hurricanes (SLOSH) model. This model divides the U.S. coast up into 20 or so separate grids (called basins) that storm surge simulations are performed for. If one takes the maximum the water reaches at any point in time at every grid cell in a SLOSH basin, a composite "Maximum Envelope of Water" (MEOW) plot can be made. MEOW plots are created for every category of storm moving in a particular direction, usually stratified by forward speed and tide elevation. Simulations are run using a variety of storm sizes. If one takes the maximum storm surge height for all the MEOW plots at every grid cell, one can generate a worst-case storm surge for the coast for each Saffir-Simpson hurricane category: 1, 2, 3, 4, and 5. These so-called "Maximum Of the MEOWs", or "MOMs" are what are plotted in the SLOSH storm surge images on wunderground, and are the composite worst-case scenario storm surges from about 15,000 different hypothetical hurricanes for each SLOSH basin. All of the MOM images we provide are for high tide, and were performed using the 2009 version of the SLOSH Display Package provided to wunderground by the National Oceanic and Atmospheric Administration (NOAA). The colors show either the water depth either as pure surge above the vertical datum level (at mean tide), or the storm tide--the height above the vertical datum level of the storm surge plus an additional rise in case the storm hits at high tide.
Vertical Datum Levels: NGVD 1929 and NAVD88
SLOSH does not tell you how high above Mean Sea Level (MSL) the surge will rise. Rather, SLOSH is referenced to one of two vertical datums, NGVD 1929 or NAVD88, developed by the National Geodetic Survey. These are two separate vertical datums that are used as references to estimate elevation. "NGVD" is a geodetic reference system that is based on surveys taken in 1929. Due to subsidence of the land and the global sea level rise of 7 inches over the past century, the 26 coastal stations used for these surveys have changed in their elevation relative to true MSL considerably since 1929. NOAA is undertaking a major effort to replace SLOSH runs done using the "NGVD 1929" reference system with runs done using the "NAVD88" reference system, which is of superior accuracy in most locations. In the Mid-Atlantic states, "NGVD 1929" thinks mean sea level lies 0.5 - 1.9 feet below present-day mean sea level (PDF File). According to the U.S. Climate Change Science Program, NAVD88 is in error by about plus or minus 0.5 feet along the mid-Atlantic coast.Most of the Texas coast, plus New Orleans, North Carolina, Tampa, and Lake Okeechobee are done using the newer NAVD88 reference system. There is a color-coded legend provided on the SLOSH images, with the tide level listed at the bottom of the legend. At its top, the legend will say either "NGVD 1929" or "NAVD88". It is important to know the elevation of your property, and whether this elevation is relative to NGVD 1929 or NAVD88, to properly use storm surge forecasts from SLOSH. Many people mistakenly assume that NGVD29 and even NAVD88 are rough approximations to MSL. Such is not true and NOAA is trying hard to debunk that myth.
IMPORTANT: The MOM storm surge maps on wunderground are a worst case scenario for the entire coast, regardless of where the storm makes landfall. An actual worst-case hurricane will not cause a worst-case storm surge along the entire length of coast portrayed in these images. A hurricane's maximum surge typically occurs over a region about 20 - 50 miles wide near where the right side of the storm's eyewall comes ashore. To see what kind of surge historical hurricane have generated at your coastline of interest, consult our Storm surge animations of historical storms page.
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