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Cone Construction
This is the raw track forecast that was issued for what later became Hurricane Charley, which struck Punta Gorda, Florida, as a Category-4 hurricane in 2004.
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....TROPICAL STORM CENTER LOCATED NEAR 16.0N 72.8W AT 11/0300Z POSITION ACCURATE WITHIN 40 NM.... PRESENT MOVEMENT TOWARD THE WEST-NORTHWEST OR 295 DEGREES AT 22 KT ....FORECAST VALID 11/1200Z 17.1N 75.7W.... ....FORECAST VALID 12/0000Z 18.8N 78.8W.... ....FORECAST VALID 12/1200Z 20.7N 81.1W.... ....FORECAST VALID 13/0000Z 22.9N 82.3W.... ....FORECAST VALID 14/0000Z 26.5N 83.0W.... EXTENDED OUTLOOK. NOTE...ERRORS FOR TRACK HAVE AVERAGED NEAR 250 NM ON DAY 4 AND 325 NM ON DAY 5... ....OUTLOOK VALID 15/0000Z 31.0N 82.0W.... ....OUTLOOK VALID 16/0000Z 35.7N 77.0W...INLAND.... |
As explained in this advisory text, the last two lines - those corresponding to the 96h (4-day) and 120h (5-day) forecast periods - are labeled EXTENDED OUTLOOK because of the significant decline in track forecast skill after 72 hours. Hurricane forecasts outside three days were started only in 2001 and began to be issued publicly only in 2003. The 3-day cone was nonetheless preserved, so now there is a 3-day cone and a 5-day cone disseminated with each forecast advisory. The 3-day cone that corresponds to this text forecast for Charley is below.
Each point on the graph that becomes the cone corresponds to its own forecast period, beginning with 0h (zero-hour, the storm's estimated current position) at the bottom of the picture and extending forward to 72h for the 3-day cone, or out to 120h for the 5-day cone.
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In this particular forecast, the southeasternmost point, labeled 11 PM TUE , is for the 0h time; it is the official estimate of the storm's current position. Even here there will be a degree of error, however small (usually just under 10 miles), as it is not possible to pinpoint exactly the current position of the center of an active hurricane. The point labeled 8 PM WED is for the 24h (1-day) forecast, and 8 PM THURS describes the 48h (2-day) forecast. The unlabeled points in between denote the 12 and 36 hour forecasts.
The outermost point, farthest away from the storm and labeled 8 PM FRI , corresponds to the outermost forecast period to be represented in the cone, which in this case is the 72h (3-day) forecast, since this is for a 3-day cone. A 5-day cone would have two additional points plotted to represent the 96 (4-day) and 120h (5-day) forecast periods, respectively. All the points together represent the forecasted latitudes and longitudes of the storm for the first six forecast periods (3-day cone), or for all eight of them (5-day cone). The letters H and S on the plotted points signify whether the cyclone is expected to be a hurricane or tropical storm at the end of the specified forecast period; this is part of the intensity forecast and is addressed in the next section.
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Circles are drawn around the forecast points to show that there is uncertainty in those forecasts, and to enable users to see how large the statistical uncertainties associated with each forecast period are relative to one another. The uncertainties, of course, will differ in their size depending on the forecast period. This is done for all six of the standard times, including the current position estimate (0h).
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At the time of Charley, the radii (measured in nautical miles) of the circles were scaled to be equal to the average of the forecast errors (likewise measured in nautical miles) for all other storms from the last ten hurricane seasons. This procedure was changed in 2007. Today, instead of using an average and instead of going back an entire decade, the radii are now scaled to include two-thirds of the forecast errors from the last five seasons.
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The circles are filled in and shaded together in white to form the cone. In addition, a line is drawn connecting the forecast points in order to establish a smooth transition from one to the next. This is sometimes called the "forecast track line," which might convey the wrong idea, because it is only the series of discrete position points that are actually forecast, not a continuous storm path.
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