What does Hurricane Irma’s ‘cone of uncertainty’ mean for SC? 6 things you need to know

With Hurricane Irma threatening the Lowcountry, you have likely seen the cone of uncertainty, the ubiquitous image splashed across the web, television and print to show you where a tropical cyclone is going and how strong it is expected to be when it gets there. On the surface it seems deceptively simple, but it can be harder to understand than you might think. Here are six things to help you make sense of it.

Tropical weather track

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Source: National Hurricane Center

What is the cone, exactly?

“Essentially, the cone of uncertainty is a graphical representation of the average error in the position of the center,” said James Carpenter, meteorologist with the National Weather Service in Charleston. “It looks at how wrong the NHC track has been historically at a certain point in time. That’s why it is wider the further out in time it goes, because there is more error.”

How do you read the cone?

The further into the future a forecast looks, the less reliable it is. That means that something whose location you are certain of today might not be where you expect it to be tomorrow. The cone exists to account for this unpredictability. When you see a cone getting wider it does not mean the storm is getting larger, it means that it is less likely that the Hurricane Center knows where it will be.

How is the cone created?

Hurricane Center meteorologists draw up each track forecast after carefully considering several different forecast models, said Carpenter. They do not necessarily blend models or use all of them in each forecast, but use their knowledge of which models have worked best for certain kinds of storms in the past to decide which ones should be paid attention to.

How reliable is the cone?

The further into the future the cone goes, the less reliable it is. According to The Weather Channel, the average track error for Atlantic storms from 2011 to 2015 was 9.1 miles after 12 hours. At 120 hours, or five days, that number increased to 221.7 miles. This may sound unreliable, but the cone exists to account for that track error. Hurricanes only stay within the boundaries of the cone about 66 percent of the time according to Carpenter.

Can a hurricane go anywhere within the cone?

Not only can a hurricane go anywhere within the cone of uncertainty, but the 66 percent accuracy rate of the cone means that up to a third of the time a hurricane can actually be found outside the cone.

How certain are the intensity forecasts within the cone?

“They do run analysis on how good intensity forecasts are and I will say that we’re not particularly good at them,” said Carpenter. “Intensity forecasting, by its nature, is much more difficult than location forecasting, so the errors on intensity forecasting are mich higher usually.”

Michael Olinger: 843-706-8107, @mikejolinger