Hurricane Season 2013 The “Cone of Uncertainty”

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June 1st marks the first day of the six-month long hurricane season in the Florida Keys.  Through November, both residents of and visitors to the fabulous Florida Keys will keep an eye to the east and south for any signs of “tropical development”.  With the advent of 24/7 media coverage of hurricanes and the growth of the Internet, the forecast “cone of uncertainty” has become an iconic graphic during tropical cyclone threats.  The graphic has been referred in the mass media as “cone of uncertainty”, “cone of concern”, “cone of error”, “cone of probability”, and “cone of death”, among other phrases.  But, what exactly does the cone mean?

The tropical cyclone track forecast cone is produced by the National Hurricane Center’s Hurricane Specialists in Miami, Florida.  The National Hurricane Center is the arm of NOAA/National Weather Service that specializes in the prediction of tropical cyclone formation, structure, intensity, and track.  Your local Florida Keys National Weather Service meteorologists, on the other hand, focus on Florida Keys communities and specialize in the smaller-scale aspects of the tropical cyclone like timing of individual spiral rainbands, tornado potential, rainfall amounts, wave heights, and storm surge inundation.  In addition, we provide impact-based decision support services to Federal, State, and local agencies and elected officials prior to, during, and after the tropical cyclone landfall.  The track forecast cone (the solid, white and stippled areas of the official forecast graphic) first appeared in 2002, and it is intended to illustrate the potential geographic range of the track of the tropical cyclone center.  The cone is constructed based on the simple principle that tropical cyclone forecasts can be in error.  Therefore, the cone conveys forecast uncertainty.  The solid white area depicts the track forecast uncertainty for days one through three of the forecast, while the stippled area depicts the uncertainty on days four through five.  Historical data indicate that the entire 5-day path of the center of the tropical cyclone will remain within the cone about 60 to 70 percent of the time.  To form a cone, a set of imaginary circles are placed along the forecast track at the 12, 24, 36, 48, 72, 96, and 120 hour positions, where the size of each circle is set so that it encloses 67% 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.

Seems simple enough, right?  However, consider the following caveats and implications:

  • Because of the way the cone is defined, the tropical cyclone center will move out of the cone area about a third of the time.  So, the fact that your specific location is not in the cone does not indicate necessarily that you are not at risk for impact by a given storm.
  • The hazardous weather associated with a tropical cyclone extends across a much greater area than just the “center”, so it would be wise to plan and prepare for storm impacts even if you are “near” the cone.
  • *The forecast track cone is produced for all tropical cyclone intensities (depression, tropical storm, hurricane categories 1–5).  Yet, the cone itself says nothing about intensity for a specific storm.  The cone only illustrates where the center may travel, based on the average error of track forecasts of all tropical cyclones over the previous five years.  Therefore, it is important to pay attention to the evolution of a tropical cyclone, especially its intensity, which is still very difficult to predict.  When under a tropical cyclone threat, check forecasts for changes regularly each day.
  • Since the cone is constructed based on historical track forecast errors, it contains no information from current prediction models, their ensembles, or spread.  Also, the cone is not tailored to the size, severity, or previous behavior of each individual storm.
  • Finally, do not focus on the “skinny, black line”, the forecast track line in the center of the cone.  Such a practice will lead to an inaccurate assessment of the actual hazardous weather threat associated with a particular storm.

The best course of action is to heed the advice of your Monroe County and community emergency managers.  They have in-depth knowledge of the local population, infrastructure, transportation systems, and community vulnerability.  In addition, they are consulting frequently with your Florida Keys National Weather Service meteorologists during a tropical cyclone threat.

Preparation is the key to wisely dealing with the hurricane problem.  Develop a hurricane plan now for yourself, your family, and/or your business.  Doing so is time well spent.

The men and women of the Florida Keys National Weather Service maintain a continuous surveillance of potential hazardous weather threats across the Florida Keys and adjacent coastal waters 24 hours per day, seven days a week, year-round.  If you have any questions regarding tropical storm or hurricane impacts in the Florida Keys this season, please give us a call at (305) 295-1316.

Hurricane Season is here – remember to be weather-ready, and stay safe!


2013 Tropical Cyclone Names
(Atlantic Basin)





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