What Does the "Fan-Shaped Area" on Typhoon Forecast Tracks Mean? Interpreting the Cone of Uncertainty
On forecast maps, the future path of a typhoon appears as an expanding fan shape. This does not mean the typhoon is growing larger; rather, it represents the range of uncertainty in its position. The further into the future, the greater the uncertainty.
Fan-Shaped Area ≠ Typhoon Size
On typhoon forecast maps, the future track is often depicted as a gradually widening fan shape (known internationally as the "forecast cone"). This represents the area of uncertainty for the future position of the typhoon's center, not the area affected by the typhoon, and certainly not an indication that the typhoon is expanding.
Each forecast time step has a "probability circle": there is approximately a 70% probability that the typhoon's center will fall within this circle. Connecting these circles across different time steps forms the fan-shaped cone.
How Large Are Forecast Errors?
Taking the recent average levels of the China Meteorological Administration (CMA) as an example, the forecast errors for the typhoon center's position are approximately:
| Forecast Lead Time | Average Error |
|---|---|
| 24 hours | Approx. 60-70 km |
| 48 hours | Approx. 100-120 km |
| 72 hours | Approx. 150-200 km |
| 120 hours | Over 300 km |
In other words, when looking at a 5-day forecast track, the actual landfall point could deviate by hundreds of kilometers. This is why forecasts from different countries often disagree when a typhoon is far away, but gradually converge as it approaches.
How to Correctly Use Forecast Tracks
- Do not focus solely on the landfall point: The wind and rain impact of a typhoon can extend hundreds of kilometers. Areas on both sides of the landfall point will be affected, with stronger winds and rain typically found on the right side of the path (the dangerous semicircle).
- Focus on trends rather than single forecasts: Forecasts are updated every 6 hours. If consecutive forecasts adjust in the same direction, it indicates that a trend is being established.
- Compare forecasts from multiple agencies: Agencies in China, Japan, the US, and others use different models. Significant divergence indicates high uncertainty, while close agreement suggests higher reliability. TyphoonSays provides overlaid comparisons of tracks from multiple agencies, allowing you to instantly gauge the level of disagreement.
- Rely on official warnings as the event approaches: Forecasts within 24 hours are relatively reliable. At this stage, you should strictly follow the defense measures and instructions issued by local authorities.