Staying or going, S.C.? Here’s why you should watch Irma’s track line (and cone) | Island Packet

S.C. Gov. Henry McMaster said Friday afternoon he would wait for the 5 p.m. update from the National Hurricane Center before announcing further plans for a potential evacuation.

And that begs the question: What can change in three hours? (McMaster’s afternoon news conference came just moments after the hurricane center issued its 2 p.m. update.)

Well, if you look at Thursday’s 2 p.m., 5 p.m. and 8 p.m. forecasts — the shift of Hurricane Irma’s track line from, basically, a near-direct hit on Beaufort County to a westward shift — a lot, perhaps.

“Yes, it’s possible, and it could certainly move back east at the (5 p.m. Friday) update,” said James Carpenter, a meteorologist with the National Weather Service in Charleston.

To be clear: Carpenter wasn’t foreshadowing that shift, but rather reminding South Carolinians that they need to pay attention to changing weather conditions as Irma nears the United States.

Throughout Thursday night and into Friday, Irma’s track line has continued to drift west. With it, the storm’s “cone of uncertainty” has also drifted in that direction — currently it barely includes South Carolina.

Of the famous (and controversial) cone, the Washington Post said the hurricane center’s forecasting can be considered “just one big game of ‘connect the dots.’”

“While the cone is an indicator of forecast confidence, it is based on the accuracy of past forecasts,” Post reporter Matthew Cappucci recently wrote. “It may or may not be an adequate reflection of current forecast uncertainty.”

While that can be interpreted as a criticism of the center’s forecasting, it’s also a reminder that the cone — and track line — incorporates past projections and previous error.

“Essentially, the cone of uncertainty is a graphical representation of the average error in the position of the center,” Carpenter said. “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.”

More important, a hurricane’s effects can be felt outside of the cone. (Regarding the cone, the average track error for Atlantic storms from 2011 to 2015 was 9.1 miles after 12 hours, according to The Weather Channel. At 120 hours, or five days, that number increased to 221.7 miles.)

And most important, hurricanes hit areas outside of the cone about a third of the time.

In short, it’s hard to tell exactly where a hurricane will hit.

Which means all of us — including McMaster, who will address the state at 6 p.m. Friday — have to continue to wait.

And watch.

Reporter Michael Olinger contributed to this story.