Sep 19, 2007 9:03 pm US/Eastern
Hurricane Minute: The Forecast Cone
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MIAMI-DADE (CBS4) ―
CBS4 Hurricane Expert Bryan Norcross says the cone often illustrated in a graphic showing the path of a hurricane has become the standard way of communicating its forecast track. It's important to understand exactly how it's drawn and what it means.
For example, the National Hurricane Center believes the track of a hurricane's eye will be located near Great Abaco, northeast of the Bahamas in two days. These scientists would draw a circle around that point, and by averaging the last 5 years of forecasts, two-thirds of the time, the storm's center would have stayed a certain point close to the forecast point 2 days out.
Forecasts are also done in the same manner for the 12, 24, 36, and 72 hour forecasts, drawing circles of the appropriate size based on the average error for each of those times. The circles get bigger as you go further out in time. After connecting all those circles with where the storm is at the current moment, fill it in and you get the forecast cone.
About two-thirds of the time, the center of the hurricane will stay in the cone and about one third of the time, it will venture outside. But the good thing to know is that forecasts are getting better, and so the cones are shrinking.
In fact, the cones that are used to illustrate a hurricane's direction was based on the forecast for Hurricane Andrew on the Saturday before it hit.
But using today's forecast errors, one can see how much bigger it would have been in 1992 if cones were used back then, about 50% wider and even more so when three days out. Modern technology has made a tremendous difference.
These days, we have a five-day cone, but the rules are the same. About two-thirds of the time, the center stays within the cone.
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