The Cone of Uncertainty: How is it Made?

(credit: NOAA National Hurricane Center)

DISCUSSION: A quick stop to check the calendar and low and behold, late May is upon us in the Northern Hemisphere. That could only mean that the start of the 2018 Atlantic hurricane seasons is are right on our doorstep. With the season beginning June 1st and a storm brewing already in the Caribbean, early preparations are being done at the National Hurricane Center (NHC) to facilitate procedures to best relay important information to the public about any potential storms this season. The approaching season also reintroduces us to the NHC forecast cone better known and coined as the “cone of uncertainty.” This cone is often made out to be a villain; a mighty force to be reckoned with. The cone itself has a striking significance, but the science of its creation often goes silent and is just as vital as the visual impression.

Each year, the National Hurricane Center revises their cone based on error statistics compiled over the previous five years of data. The observed positions for each storm’s center of circulation on all advisories (both standard 6-hourly and intermediate 3-hourly) are compared to the forecast positions at each advisory and the difference between the two creates the error distance. These calculations are performed for each projected position at 12-hourly intervals up to the maximum of 120 hours (5 days). The widths of the cone are then determined based on these errors which suggest that statistically, any storm’s center will have a 2/3 probability of being located somewhere in the cone. The remaining 1/3 probability accounts for centers lying outside of the cone. Outliers and poorly-skilled forecasts are just a couple of reasons that this 1/3 probability exists. When comparing the Pacific and the Atlantic Ocean basins, these numbers vary slightly but range from as little as 25 (26) nautical miles at a 12-hour lead time up to 162 (198) nautical miles at a 5-day lead time. Why are these numbers significant? The answer is that over the last few decades, the accuracy of forecast location has sharply improved! The current 3-day error is equivalent to the single-day error in 2000, and the current error at 5 days is equivalent to the 3-day error in 2001.

While the reduction in cone size is a great forecasting advantage, it must be constantly stressed that the cone of uncertainty is not a barrier or limit that dictates where impacts from a storm will be felt, nor does it imply that tropical cyclones are decreasing in size. As observed with storms such as Hurricanes Irma, Maria, and Jose from 2017, impacts from any tropical cyclone can be felt several dozen or hundreds of miles/kilometers away from the storm center. These impacts do not have to be weather-related as ocean waves and swells can amplify miles away from the storm leading to coastal erosion and dangerous surf conditions.

To see the tables and more information regarding the updated cone for 2018, click here. Also, click here to read some very useful hurricane track verification documentation by the NHC.

To learn more about other interesting tropical cyclone-related stories, be sure to click here!

© 2018 Meteorologist Brian Matilla