Tropical Storm Bonnie’s Cone of Uncertainty

I have heard a few people remark that the forecast track of Bonnie was off the mark.  Let’s take a look at that.

According to the National Hurricane Center (NHC) verification report, the NHC track forecast cone depicts the probable track of the center of a tropical cyclone, and is formed by enclosing the area swept out by a set of circles along the forecast track at various time periods.  The size of each circle is set so that two-thirds of historical official forecast errors over the most-recent 5-year sample fall with the circle.  The cone circle radii (in statute miles) for 2010 for various time periods are as follows:

Forecast Period (hours)

Track Forecast Cone Two-Thirds Probability Circles (statute miles)











The radii expand approximately 60 to 70 miles each 24 hours.

TD#3 Cone from 11 am Thursday July 22, 2010

The above graphic shows the cone of uncertainty that was available with the 11:00 am EDT Thursday, July 22nd Advisory # 1 on Tropical Depression Three (which became Tropical Storm Bonnie).  Focusing on the South Florida portion, we can see the forecast position valid at 8 am on Friday is between Cuba and South Florida with the northern edge of the cone extending into the extreme southern part of Miami-Dade County.

TS Bonnie Cone from 11 am EDT Friday July 23, 2010

The above graphic shows the cone of uncertainty that was available with the 11:am EDT Friday, July 23rd Advisory # 5 on Tropical Storm Bonnie.  It shows the center position of Bonnie just as the center moved onshore near Cutler Bay in Miami-Dade County.  That position was only slightly north of the cone issued 24 hours earlier.

I doubt that the forecasters were surprised that the center ended up near the edge of the cone.  The point of the cone is to help describe the uncertainty associated with the forecast track of the center of the tropical cyclone.  In the case of Bonnie, the center was pretty insignificant because Bonnie was barely a tropical storm when it moved onshore in Miami-Dade.  But the lesson needs to be learned that the center of the tropical cyclone, no matter what the intensity, will not always stay in the middle of the cone.

It is also important to note that the track cone is not an impact graphic.  It is not intended to show areas forecast to be impacted by winds, storm surge, rain or tornadoes.  The cone’s main purpose is to simply display the uncertainty associated with the track.


2 thoughts on “Tropical Storm Bonnie’s Cone of Uncertainty

  1. Max
    I watched the 11 PM news.You definitely said the storm was 1. weak 2.disorganized and 3.forming North of earlier projections.Whie my storm shutters were in place I was far less confident they would be tested after listening to you.Keep up the great work.Unfortunately,we have a lot of season left to go.Also,remember us down in the Keys,we are not the biggest part of viewers but we are loyal.
    Hope to see you down here soon and have dinner again


    John and Carol

  2. I am a 8th grade teacher in NC and came across your site while researching some information about tropical storms for my class this year. I just wanted to thank you first of all for the great information and articles about tropical storms, and second let you know about a site we are putting together for teachers that might have some useful information for your site.

    We would love it if you could write a few articles for us, or link to some of the current articles to help us spread trusted resources to other teachers. I have included a link to the site below in hopes you might want to write some articles for us or link to it.

    Thanks and keep the great resources coming 🙂

    Bre Matthews

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