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Contributed by the National Hurricane Center

Storm surge, the abnormal rise of ocean water on land due primarily to strong onshore winds, is primarily forecast with the SLOSH computer model. SLOSH (Sea, Lake and Overland Surges from Hurricanes) is run by the National Hurricane Center (NHC) to estimate storm surge heights and winds resulting from historical, hypothetical, or predicted hurricanes by taking into account five factors: the winds, the central pressure, the size, the forward speed and the track direction of the hurricane.

This output from the model displays color coded storm surge heights for a particular area in feet above the model's reference level, the National Geodetic Vertical Datum (NGVD), which is the elevation reference for most maps.

The calculations are applied to a specific locale's shoreline, incorporating the unique bay and river configurations, water depths, bridges, roads and other physical features. If the model is being used to estimate storm surge from a predicted hurricane (as opposed to a hypothetical one), forecast data must be put in the model every 6 hours over a 72-hour period and updated as new forecasts become available.

The SLOSH model is generally accurate within plus or minus 20 percent. For example, if the model calculates a peak 10 foot (3.0 m) storm surge for the event, you can expect the observed peak to range from 8 to 12 feet (2.4 to 3.6 m). The model accounts for astronomical tides (which can add significantly to the water height) by specifying an initial tide level, but does not include rainfall amounts, riverflow, or wind-driven waves. However, this information is combined with the model results in the final analysis of at-risk-areas.

The point of a hurricane's landfall is crucial to determining which areas will be inundated by the storm surge. Where the hurricane forecast track is inaccurate, SLOSH model results will be inaccurate. The SLOSH model, therefore, is best used for defining the potential maximum surge for a location.