The Tropical Cyclone Track Forecast Cone

My previous blog discussed the errors associated with the official National Hurricane Center (NHC) tropical cyclone (TC) track forecasts.  Knowing these errors exist, the NHC issues several products in an attempt to communicate the uncertainty in track forecasting.  One of the products is the Track Forecast Cone, sometimes referred to as the Cone of Uncertainty.  These cones are often seen on television and in newspapers.  An example on Tropical Storm Fay is shown below.

Initial 5-Day Track Forecast Cone on TS Fay

Initial 5-Day Track Forecast Cone on TS Fay

The NHC definition of the cone is as follows:  “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 2008 for the Atlantic are given in the table below.

Radii of NHC Atlantic basin forecast cone circles for 2008, based on error statistics from 2003-2007:

Forecast Period (hours)

2/3 Probability Circle (nautical miles)

12

39

24

67

36

92

48

118

72

170

96

233

120

305

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.

While the Track Forecast Cone graphic is useful for determining the most likely track area of the center, it says nothing about the TC intensity or size.  Intensity is important because the damage increases exponentially as the winds increase.  And a TC is not just a point.  The tropical storm and hurricane force winds can extend well beyond the cone enclosing the most likely track area of the center.

Future blogs will deal with the uncertainty related to intensity and size.

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One thought on “The Tropical Cyclone Track Forecast Cone

  1. Hi Max–
    As a Keys resident I understand the notion of the cone and of the difficulties of forecasting intensity. Fay ‘s track prompts me to ask whether there is a relationship between track uncertainty and intensity modeling, and if so, what is it?
    Many thanks!

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