According to the National Hurricane Center the “Cone of Uncertainty” used on forecast graphics is not based on calculations of uncertainty of that particular system but instead:
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 2015 for the Atlantic and eastern North Pacific basins are given in the table below.
Cliff Mass weather blog has a post about the cone here:Time to Replace the National Hurricane Center’s Forecast Cone
From that post:
Uncertainty in hurricanes tracks vary by storm, location, date, forecast situation and forecast period. One size does not fit all. And we now have much more sophisticated capabilities that can produce relevant forecast track uncertainty that shows a very different structure than the simple cone method.
To put it another way, the cone approach is out-of-date and should be dropped for next hurricane season.
Bottom line here is the uncertainty in the forecast can not be determined using NHC “cone of uncertainty”