Use of the Cone of Uncertainty

The National Hurricane Center (NHC) issues several different products to convey the uncertainty in its tropical cyclone forecasts.  One of the most familiar products is the track forecast cone, sometimes called the “Cone of Uncertainty.”  According to the NHC, this track forecast cone depicts the probable track of the center of the tropical cyclone, and is formed by enclosing the area swept out by a set of circles along the forecast track at various time periods.  The size of each circle is set so that two-thirds of historical official forecast errors over the most-recent 5-year sample fall within the circle.  The circle radii defining the cones in 2009 for the Atlantic basin based on error distributions for 2004-8 are given in the table below.

Atlantic Track Forecast Cone Two-Thirds Probability Circles for 2009

Forecast Period                                  (hours)

Radii of 2/3 Probability Circle        (nautical miles)

12

36

24

62

36

89

48

111

72

167

96

230

120

302

It is important to note that the NHC’s Cone of Uncertainty is not an impact graphic.  It says nothing about the tropical cyclone’s wind distribution, size, storm surge, rainfall, or tornadoes.  It simply tells you the most likely area for the center of the tropical cyclone to be within the next five days.

The cone graphic is often misused.  Note how large the circles are that form the cone at the longer time periods.  I know that the anxiety level really increases for some people once the cone moves over their location.  But the probability of experiencing the core of the tropical cyclone at the extended time periods is actually quite low for any given location.  The original intent was for people to simply start thinking about what they would do if the threat continues to increase when the cone first moves over your location at the five day time period.  That means that you need to keep up with the latest advisories.  As the tropical cyclone gets closer, you need to be able to implement your hurricane plan when told to do so by your local officials.

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