The “cone of uncertainty” is the most recognized graphic released by the National Hurricane Center (NHC) during the tropical storm season. This visual tool communicates the probable geographic range where the center of a tropical cyclone is expected to travel over the forecast period. It represents the inherent imprecision in predicting the atmospheric steering currents that guide these storm systems. The cone depicts the margin of error in the official track forecast, helping establish the geographical area that should prepare for potential landfall or near-miss conditions.
Defining the Cone of Uncertainty
The Cone of Uncertainty is a shaded area drawn on a map around the forecast track of a tropical storm or hurricane. This area represents the likely path of the storm’s center, not the extent of its winds or other hazards. It shows the range of possibilities for where the eye of the storm could be at various points in time, typically spanning a five-day forecast period.
The cone is narrowest near the present time and progressively widens into the future, reflecting the increasing difficulty of long-range prediction. The NHC establishes the boundaries so that the center of the storm is expected to remain within the shaded area about two-thirds of the time (60 to 70 percent). Historically, this means there is still a one-third chance that the storm’s center will track outside of the cone’s boundaries.
The Science Behind the Cone’s Calculation
The size and shape of the cone are not determined by the characteristics of the individual storm being tracked. Instead, they are based on a statistical analysis of past forecasting performance. The National Hurricane Center bases the cone’s dimensions on the average track errors accumulated by its forecasters over the previous five hurricane seasons. This historical method provides a consistent, standardized measure of forecast uncertainty.
To create the cone, the NHC plots circles around each forecast point along the track (e.g., the 24-hour, 48-hour, and 72-hour marks). The radius of each circle is mathematically set to enclose two-thirds of the historical track errors that occurred at that specific forecast time. The circle for the five-day forecast will have the largest radius because forecast errors naturally increase with the length of time projected. Connecting the tangents of these circles forms the familiar cone shape.
Actionable Interpretation and Common Misconceptions
The most frequent misinterpretation of the graphic is assuming that the cone represents the size of the storm or the full extent of its impacts. The cone only predicts the path of the storm’s center. Dangerous conditions like high winds, storm surge, and torrential rainfall often extend hundreds of miles outward from the center. Consequently, areas outside the cone’s boundaries can still experience significant weather.
Preparation is necessary whenever a location is near the cone, not just directly within it. Impacts can precede the arrival of the storm’s center by many hours, sometimes starting with tropical-storm-force winds that make outdoor preparation unsafe. Individuals should focus on the specific wind speed probabilities and storm surge forecasts issued by the NHC. These products provide a more accurate picture of the hazards that will affect their area, as relying solely on the track cone can delay necessary safety actions.