The Cone of Uncertainty (Cone of Probability)

The cone of uncertainty is the most important part of a hurricane forecast map. The cone shows you where the center of the storm may wind up at any point within the forecast period (usually 5 days).

Weather forecasting is still an inexact science, and hurricane forecasting is even less certain. The cone of uncertainty accounts for the historical margin of error in the National Hurricane Center's forecast track. The points on a forecast map are timed out every 12 hours, so the margin of error increases in 12-hour increments. The margin of error is measured with circles radiating out from the forecast location of the storm's center, and then smoothed, creating the "cone" that everyone is familiar with.

The easiest way to understand the cone of uncertainty is to remember that, historically, the center of a tropical cyclone stays inside the cone of uncertainty 66% of the time. The center moved so far away from the forecaster's predicted path that it fell outside of the cone 33% (well, I guess it'd be 34%) of the time.

For example, at the 24-hour mark, the National Hurricane Center's historical margin of error is 52 nautical miles. This means that 66% of the time, the actual location of the center of the storm wound up within 52 nautical miles of the NHC's predicted location. This also means that 34% of the time, the center wound up outside of that 52-nautical-mile circle.

Last night when Tropical Storm Bertha formed, I mentioned that there was a slight threat that the storm could impact the eastern United States, as the cone of uncertainty covered about a hundred miles of coastline across North Carolina. As the map at the top of this post shows, that forecast has since shifted, steering the storm out to sea, but it's still worth keeping an eye on.

For a historical example, Hurricane Isaac from 2012 shows us the importance of the cone of uncertainty, as well as a great reminder that the center winds up outside of the cone about 33% of the time.

Isaac was a notoriously hard storm for forecasters to predict. I lived in Mobile at the time and the center's predicted path came straight over us for several days, but the storm wound up making landfall in Louisiana.

Speaking of which, that brings us to the next major aspect of hurricane forecasts.