National Hurricane Center Debuts New Look for Cone of Uncertainty

You know you’ve seen it. A hurricane is approaching and the National Hurricane Center issues a forecast cone in their report to indicate the possible path and impact area of the tropical cyclone. 

Maybe you have wondered about this “cone of uncertainty.” 

It turns out a lot of people have wondered about what exactly the cone means. The problem is that the public’s understanding of the its meaning is usually wrong. So forecasters are trying to improve the tool. 

But first, just what is the cone? 

As mentioned, the illustration represents the probable track of a tropical cyclone's center. The shape of the cone is formed by a set of circles, each representing the predicted storm center at a different 12-hour interval. The marked forecast track of the storm is shown along these intervals. The sizes of the circles are particularly important; they're set at two-thirds of the historical forecast error over the previous five-year period. That's right, the cone actually represents the NHC’s average track error over the past five years. And as the forecasts get better, the cone shrinks. 

The forecasters at the NHC pick the middle path down the center of the cone; that’s their best estimate, based on computer models, satellite data and the trends of a particular storm. And the forecast can be extended out for five days. 

However, that still means there is a one-third probability that the center of the storm could track outside of the cone. 

It's important to remember that the cone is designed to convey uncertainty in forecast of the storm’s center, not the areas that will experience impacts. It’s very likely the storm’s impact will be felt far to the right and left of the cone. 

Now, for the 2017 hurricane season, the NHC is making some changes to the iconic cone to improve communication of hurricane information. 

First, the cone will be smaller. That’s because meteorologists are getting better at forecasting hurricanes. The NHC is also inserting a shaded area overlaid beyond the cone, which indicates the farthest reaches of possible tropical storm and hurricane force winds. 

Because the public tends to focus on the path of the storm (the cone), they don’t take into account the high winds, waves and other effects of a hurricane that happen outside the path. Wind fields, for example, are much larger than the cone. 

The NHC hopes to alert people not to let down their guard, even if they are not exactly in the direct path of a storm.

—Frank Graff 

Frank Graff is a producer/reporter with UNC-TV, focusing on Sci Tech Now North Carolina, a weekly science series that airs Tuesdays on UNC-TV. In addition to producing these special segments, Frank will provide additional information related to his stories through this North Carolina Science Now Reporter's Blog!

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