Tropical cyclone forecast cone shrinking

Continued improvements in forecasting the track of tropical cyclones in the Atlantic Basin will lead this year to a slightly smaller cone representing the probable track of the center of a tropical depression, tropical storm or hurricane.  

The National Hurricane Center in Miami, a division of the U.S. National Weather Service, announced the change last month. Its forecasts are used by the Cayman Islands National Weather Service for the purposes of tropical cyclone tracking. 

“The cone … is formed by enclosing the area swept out by a set of imaginary circles placed along the forecast track – at 12, 24, 36 hours, etc.,” the National Hurricane Center said. “The size of each circle is set so that two-thirds of historical official forecast errors over the previous five years (2010-2014) fall within the circle.” 

Since the forecast errors for the last five years are fewer than previously, the size of the circle radii will be smaller for the 2015 Atlantic Basin hurricane season, which starts June 1 and ends Nov. 30. The measurements are defined at 12, 24, 36, 48, 72, 96 and 120 hours. 

In the Atlantic Basin, the reductions are rather small, with the biggest reduction of size being three nautical miles at 72 hours out. The sizes of the circle radii at 24 hours and 96 hours remain the same, while at all other forecast times they reduce in size by one or two nautical miles.  

The size of the forecast circle radius in 2015 increases steadily between 12 and 120 hours and will range from 32 nautical miles at 12 hours and 52 nautical miles at 24 hours, to 225 nautical miles at 120 hours. 

An even more significant reduction in the size of the forecast cone will also occur in the Eastern North Pacific Basin, which the National Hurricane Center also monitors, because the forecast errors have been less in that area over the past five years. Reductions in circle radii sizes in that basin range from 4 to 10 nautical miles, with the circle radius showing the biggest reduction at 48 hours, shrinking from 79 to 69 nautical miles.  

Other changes  

In addition to a reduction in size of the forecast cone, there will be a couple of other changes on the National Hurricane Center website for the 2015 Atlantic Basin hurricane season. 

There will be a change in the category labels that indicate the chance of tropical cyclone formation over 48 and 120 hours. These categories are a feature of the Hurricane Center’s Tropical Weather Outlook, which is issued every six hours during the hurricane season. The two-day Tropical Weather Outlook discussion also features a graphic that is color coded, with yellow representing a low chance of tropical cyclone formation, orange representing a medium chance of formation and red representing a high chance of formation. 

Previously, if an area had a zero to 20 percent chance of formation, it was categorized as a low chance of formation; between 30 and 50 percent chance of formation was categorized as a medium chance; and a 60 to 100 percent chance of formation was categorized as a high chance. In 2015, the categories will indicate zero to 30 percent is a low chance of formation; 40 to 60 percent is a medium chance of formation; and 70 to 100 percent is a high chance of formation. 

“The original category definitions were established to best reflect forecasters’ ability to assess formation potential,” the National Hurricane Center said in explaining the change. “Unfortunately, the original definitions did not correspond with some users’ expectations of what the everyday terms “low,” “medium” and “high” mean. For example, a 30 percent chance of occurrence falls in the lowest third of possible outcomes, yet was described as being “medium.”  

This mismatch led some users to overestimate the chance of formation when they heard that a system had entered the medium or high categories.” The Hurricane Center added that improvements in forecast accuracy over the past few hurricane seasons now allow for a more natural assignment of the three categories.  

In addition to the two-day Tropical Weather Outlook, the National Hurricane Center will issue a five-day graphical outlook in 2015. That graphic was experimental in 2014 but will become a fixture in 2015. This graphic will contain the same color coding as the two-day graphic but will show larger areas where the potential tropical cyclone formation could occur to take into account the distance an area of disturbed weather could travel over five days.  


This five-day graphical depiction of the chance of tropical cyclone formation issued by the U.S. National Hurricane Center was experimental in 2014 but will become a fixture on the Hurricane Center’s website in 2015. – Graphic: NOAA.