Definition of the NHC Track Forecast Cone
Definition:
The cone represents the probable track of the center of a tropical cyclone, and is formed by enclosing
the area swept out by a set of circles (not shown) along the forecast track (at 12, 24, 36 hours, etc).
The size of each circle is set so that two-thirds of historical official forecast errors over a 5-year
sample fall within the circle. The circle radii defining the cones in 2011 for the Atlantic and eastern
North Pacific basins are given in the table below.
One can also examine historical tracks to determine how often the entire 5-day path of a cyclone remains
completely within the area of the cone. This is a different perspective that ignores most timing errors.
For example, a storm moving very slowly but in the expected direction would still be within the area of the
cone, even though the track forecast error could be very large. Based on forecasts over the previous 5
years, the entire track of the tropical cyclone can be expected to remain within the cone roughly 60-70%
of the time.
Radii of NHC forecast cone circles for 2011, based on error statistics from 2006-2010:
Forecast Period
(hours) |
2/3 Probability Circle,
Atlantic Basin
(nautical miles) |
2/3 Probability Circle,
Eastern North Pacific Basin
(nautical miles) |
| 12 |
36 |
33 |
| 24 |
59 |
59 |
| 36 |
79 |
79 |
| 48 |
98 |
98 |
| 72 |
144 |
134 |
| 96 |
190 |
187 |
| 120 |
239 |
230 |
Example Graphic of 5-Day Track Forecast Cone:
Click to Enlarge
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