Cone of Uncertainty: Facts and Myths About This Tropical Forecasting Tool

Tom Moore
Published: July 12,2016
It's a staple of every hurricane season: the "cone of uncertainty," which extends out from the storm on forecasting maps to warn the public of the projected path of a tropical system. But there are some things you might not know about the cone that you'll want to learn before the next storm is named.
Every year, mainly during the hurricane season, the National Hurricane Center issues various information and forecast products for current tropical cyclones. The forecast track of the storm is represented by a white "cone" on their graphic.
The NHC's forecast cone isn't used by all weather forecasters; some will vary, although the cone is universally accepted as the format for projecting a storm's path.
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There can also be variations in the color of the cone for media outlets that are not affiliated with the NHC. The Weather Channel and weather.com, for example, present their forecast cone in a dark red color. Time stamps can also vary across various media and track point positions are not always depicted.

Example of a forecast track graphic (cone in dark red) as seen on weather.com.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

What Does The NHC Cone Mean?

The NHC issues graphical track forecasts for all tropical depressions, tropical storms and hurricanes in the Atlantic and eastern Pacific basins, extending out for five days. A forecast time is also indicated for each track point along a line. The line is solid for a three-day forecast and broken for days four and five. To be clear, the track points and lines are not indicative of the exact forecast path. These are forecasts based on computer models, satellite data and trends of the particular storm, and they can change rapidly.
Needless to say, the further out the forecast is projected, the greater the track error will be.
The white "cone" that you see on the NHC graphical forecast represents the probable track of a tropical cyclone's center and is formed by enclosing the area swept out by a set of circles (not seen on the graphic) along with the forecast track at 12-hour intervals.
The size of each circle is set at two-thirds of the historical forecast error over the previous five-year period. Based on those previous forecast errors, the entire track of the tropical cyclone can be expected to remain within the cone about 60 to 70 percent of the time. The cone becomes wider over time as the forecast uncertainty increases.
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Due to better computer-model and satellite data, forecast errors are gradually shrinking but remain fairly substantial, especially farther out in time. The cone is a much better guide for the track forecast than the line.
The chart below indicates the average forecast track error for Atlantic tropical systems, up to 120 hours, from 2011 to 2015.

Forecast Period (hours)	Average NHC Forecast Track Error (miles)
12	9.1
24	26.3
36	42.0
48	56.9
72	73.8
96	109.7
120	221.7

Tropical storms and hurricanes are steered by winds aloft, and the overall track can be a function of a storm's intensity. These days, intensity forecasts still have some limitations, but there is a concerted effort to make improvements.
The National Weather Service has issued a sample NHC hurricane forecast graphic and lists five important points to help you to better understand the "forecast cone:"

• The cone represents the probable track of the center of the tropical cyclone.
• The size of the cone is drawn so that about two-thirds of the time, the center of the storm will remain in the cone.
• The cone does not take the size of the storm into account.
• A hurricane is not a point; impacts often occur well outside of the core.
• The cone indicates the forecast up to five days out from the last recorded position of the storm.

The NHC cone is bright white for 72 hours and "hatched" for the 96- and 120-hour forecasts. Keep in mind that some tropical storms and hurricanes are more difficult to forecast than others.
The cone only indicates a forecast track of a tropical cyclone. It doesn't indicate other information like potential storm surge, rainfall and any watches or warnings that may have been issued. Some impacts from tropical storms and hurricanes can be experienced hundreds of miles from the center of the storm.
As we move into the future, forecasters hope tropical storm and hurricane forecasts will continue to improve and the "cone of uncertainty" will shrink even more.
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