Hurricanes: Their Nature and Impacts on Society - Climate Science ...

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HURRICANES: THEIR NATURE AND IMPACT ON SOCIETY
intensity forecasts from the different models are available for discussion. The
final decision on the forecast results from discussion among these officials,
although the NHC has final authority for the official forecast decision.
With the forecast in h<strong>and</strong>, discussion focuses on the potential impacts,
timing, <strong>and</strong> locations of hurricane l<strong>and</strong>fall. At this point, officials identify
areas to post hurricane watches <strong>and</strong> warnings, which are issued by the NHC.
Using the forecast: hurricane watches <strong>and</strong> warnings
Hurricane watches <strong>and</strong> warnings are based on the official hurricane forecasts.
A hurricane watch is an announcement for a specific coastal area that hurricane
conditions pose a possible threat within 36 hours (Sheets 1990). A
hurricane warning is an announcement that sustained winds of more than 74
mph associated with hurricane conditions are expected to affect a specific
coastal area within 24 hours (Sheets 1990). A hurricane warning may remain
in effect even if winds are less than 74 mph but a threat still exists due to high
water or waves.
Upon l<strong>and</strong>fall, hurricane force winds typically impact about a 50-mile wide
swath of l<strong>and</strong>. However, because of uncertainties in forecasts, hurricane
warnings are generally posted for l25-mile segments of the coast: in a storm
moving from east to west, the warning area would thus include about 50 miles
south <strong>and</strong> 75 miles north of the expected point of l<strong>and</strong>fall. The additional 25
miles to the north of the expected point of l<strong>and</strong>fall are necessary because a
storm is typically more intense to the right of center, with respect to its
forward motion. A consequence of the need to post warnings for an area
greater than the size of the storm is that people along about 75 miles of the
coastline (or about 60%, on average) are warned of a hurricane but do not
suffer the storm's greatest effects (i.e. 125 miles of warning area minus 50
miles of actual impact). This is called "overwarning" <strong>and</strong> is of concern
because of the potential loss of public faith in forecasts <strong>and</strong> warnings <strong>and</strong> the
practical problems of preparing a large community for the impacts of a
hurricane.
The hurricane watch <strong>and</strong> warning process typically involves three groups:
(1) NWS meteorologists, who bear statutory responsibility for watches <strong>and</strong>
warnings; (2) relevant local <strong>and</strong> state officials who determine what response is
needed; <strong>and</strong> (3) the media who are responsible for communicating watches
<strong>and</strong> warnings to the public. Working together, the three groups try to satisfy
three interrelated criteria. First, sufficient lead time must be provided for the
community to protect life <strong>and</strong> property effectively. Second, care must be taken
not to warn communities unnecessarily, which can potentially threaten the
credibility of hurricane warnings -in other words, don't "cry wolf". Third,
care must be taken to optimize response to the warning; that is, the warning
should be timed <strong>and</strong> spatially distributed in such a manner as to evoke desired
social actions <strong>and</strong> responses.