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Subject: F7) How is storm surge forecast?
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.
Last updated August 13, 2004
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