Tropical Storm Chantal's cone of uncertainty

By Max Mayfield - Hurricane Specialist
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MIAMI - One of the most misused tropical cyclone graphics is the track forecast "cone." The track forecasts have improved dramatically over the past several decades due, in large part, to better observations and computer modeling. However, the forecasts will never be perfect and there is always uncertainty associated with the forecasts.

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The National Hurricane Center conveys the track forecast uncertainty by the track forecast "cone." It is designed such that the entire five-day path of the center of the tropical cyclone will remain within the cone about 60 to 70 percent of the time. 

To form the cone, a set of imaginary circles are placed along the forecast track at the 12, 24, 36, 48, 72, 96 and 120-hour forecast positions, where the size of each circle is set so that it encloses 67 percent of the previous five years official forecast errors. The cone is then formed by smoothly connecting the area swept out by the set of circles.

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The 5 p.m. Tuesday's cone graphic for Tropical Storm Chantal kept South Florida (above) in the cone. But it is important to keep in mind that the cone only accounts for the uncertainty in the track. It does not address uncertainty in the intensity or size or, more importantly, the impacts. Chantal is expected to move over the mountainous areas of Hispaniola, which should weaken the tropical cyclone. There is considerable uncertainty on how strong the system will be when and if it emerges intact on the north side of Hispaniola. The forecasters usually say there is more confidence in the track than the intensity.

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It is also important to realize that a tropical cyclone is not a point. The impacts from wind, rain, storm surge and tornadoes can extend well beyond the center. We will talk more about those impacts, if necessary, as Chantal gets closer to South Florida.

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