Track Forecast Interpretation Guidelines

Storm Motion

The track forecast is a prediction about the motion of a hurricane during a fixed interval of time. It takes into account factors likely to affect the storm's direction of travel, that is, relative to its last known position, and its speed, understood in terms of the distance the hurricane is expected to cover in that amount of time, not necessarily on a straight path.

Storm direction and speed are combined in a single forecast of storm motion (across time and space), which in its barest form consists of pairs of latitude and longitude coordinates (hurricane positions) associated with fixed times in the future. The uncertainties associated with storm direction and speed are also combined as one, such that it becomes possible to think about the cone of uncertainty as portraying the uncertainty in the general motion of hurricanes.

A hurricane's motion may be characterized by its speeding up, slowing down, traveling at constant velocity, turning left, turning right, turning around in loops and circles, moving in a straight line, doing several of these things simultaneously or in quick succession, or stopping dead in its tracks - all depending on evolving atmospheric conditions.

As a result of its going through all these motions, a hurricane will, of course, end up at a particular set of coordinates at the end of the forecast period. In order to predict where that will be, forecasters have to first be able to predict all those intricate motions the storm will have to go through in order to get there and still actually be there at the specified time.

Uncertainty

The track forecast is, then, in a practical sense, a prediction that the eye of a hurricane will be in the process of passing through a spatial region, rather than through a tiny pinpoint, at the appointed time. It projects the future position of the storm center plus or minus a number of (nautical) miles on any of four sides of the storm - ahead, behind, left, or right - thereby encompassing both the storm's direction and speed.

A 3-day forecast is, on average, less accurate than a 2-day forecast, in that the statistical uncertainty in the storm's motion is greater three days out than two days out, a difference represented in the different circle sizes (radii) in the uncertainty cone.

The width of the cone and the thickness of the line have nothing to do with the size of the storm, or the extent (radius) of its fastest winds. The forecast points, not the line, represent the projected track of the center of the storm, the cone represents the fact that there is uncertainty in that motion, and the line through the middle of the cone is simply a series of connecting line segments drawn in to connect the discrete forecast points to one another.

The line, thus constructed, is of a standard arbitrary width and is the same on every cone. The cone's width changes from year to year depending on the documented forecast errors from the last five seasons, not depending on the size of the storm. Neither the cone nor the line should be taken as any kind of physical representation of the storm itself.

The forecast points themselves have no physical dimension or size. They are not physical representations even of the eye of the storm, much less of the entire storm. They are always the same size, from one uncertainty cone to the next, from one season to the next.

The track forecast pertains to the eye of the storm only and not to the hurricane- or tropical storm-force winds that surround it. Even if the storm's center is forecast to "miss" a particular coastal region, part or all of that region may still be subject to potentially damaging hurricane- or tropical storm-force winds as well as to the storm surges they are capable of generating. These winds can radiate outward tens of miles from the center. A different forecast product, the Wind Speed Probabilities product (addressed in its own section), covers the full range of hurricane- and tropical storm-force winds.

The white shaded area of the cone does not represent any kind of geographical "threat area" for damage from the hurricane; it stands for some of the possible alternate tracks of the center of the storm only. Should the storm center track along the edge of the forecast cone or come near its edge at any point, hurricane- or tropical storm-force winds, as well as any storm surge effects associated with them, will most likely extend to areas outside the cone.

The 5-day tracks of hurricanes remain entirely within their respective 5-day cones only between 60-70 percent of the time. The boundaries of the cone are not representative of any constraints imposed on nature, but of the limits of modern science to the understanding of its motions. Any storm may venture entirely outside the cone at any time; the entire cone is also likely to "jump" to the left or right, or become slightly longer or shorter, from one forecast to the next as models are updated to reflect changing atmospheric conditions.

Landfalling Hurricanes

The track forecast does not explicitly predict landfall events. However, a landfall forecast may be inferred from the appearance of land masses within the cone of uncertainty.

A correct landfall forecast with respect to the cone of uncertainty is landfall of the center of circulation anywhere inside a portion of the cone that includes land. The eye of a storm could therefore make landfall at the "wrong" city but still be a correct forecast, as long as that city is within the bounds of the cone.

A track forecast indicating a probable landfall near a location two days out is different from the exact same forecast issued 36 hours out and has to be interpreted differently, as the uncertainty in the hurricane's motion is, on average, going to be less for the forecast with the shorter time-frame. This makes the estimated time to landfall a critical piece of information that has to be taken into account in interpreting the forecast, not just for planning purposes, but more importantly, for the purpose of properly assessing how likely it is the storm will make landfall near one's location.

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