Hurricane Ian slammed into the southwest coast of Florida on Wednesday as a Category 4 monster just miles from where the National Hurricane Center originally predicted it might strike.
In the days leading up to touchdown, the forecast moved the center of the runway as far north as Big Bend, Florida on Sunday and also hovered over Tampa Bay on Sunday and Monday. Potential track forecasts have raised fears that the densely populated Tampa and St. Petersburg region could take its first direct hit since a 1921 hurricane.
Ultimately, Ian made landfall over 100 miles to the south, very close to the first position estimate for a potential Florida landfall.
So were the weekend forecasts wrong?
“If we look at Ian’s forecast path with the 5-day cone, his future landing spot was pretty much always within the forecast cone, right on the edge,” said Colorado tropical meteorologist Phil Klotzbach. State University and lead author of its seasonal hurricane forecasts.
The forecast cone, also called the “cone of uncertainty”, is often misunderstood. It is a series of circles along the positions of the forecast centers. The size of each circle shows two-thirds of the official five-year forecast errors.
This is the area where the center of the storm is most likely to move over five days, said meteorologist Scott Spratt, who retired in December as a warning coordinator for the National Weather Service office in Melbourne, Florida.
“It’s a great snapshot that shows you every six hours where your geographic location is relative to the cone,” Spratt said. “It allows people to see trends and prepare.”
After::Hurricane Ian approaches Florida, threatening storm surge. Graphics explain the deadly weather event.
This does not indicate where all the impacts of the storm will be, or even guarantee that the center of the storm will remain within the cone.
“NHC forecasters were very consistent with the publicity that the uncertainty in the trajectory forecasts was greater than normal,” Klotzbach said. “It was a complicated mess of an upper trough over the western Gulf of Mexico, a ridge building in the Midwest, a trough over the east coast, along with Ian’s strength. “
Ian tended to track a bit faster than predicted by forecasting models, he said.
It’s important for people to understand the uncertainties in forecasts, said James Franklin, a retired hurricane scientist who has written numerous forecasts for the hurricane center. This is the purpose of the cone.
The track shifted west and north over the weekend because forecasters couldn’t say for sure how Ian would react to those other influences in the gulf, Franklin said.
“I think the track forecast numbers for (Ian) are going to be pretty good by historical standards,” Franklin said. “But again, when you have a storm that’s approaching at an oblique angle to the coast like Charley did, and like Ian does, it doesn’t take very big track errors to make a big difference in where those impacts are going to happen.”
“It’s a good illustration of where we are with runway forecast uncertainty,” he said. “They’ve improved but they’re not perfect yet.”
Four decades show great improvement
Today’s cone size shows the big improvements in track predictions over four decades. Between 1990 and 2020, the two-day track forecast error fell from about 200 nautical miles to about 50 miles, Franklin said. “72-hour errors dropped from an average of 300 nautical miles to 75.”
“Since Hurricane Charley (2004), the 48-hour error has been cut in half,” from 120 nautical miles to 50, Franklin said. In 2004 the 5 day error was around 300 miles and now it is around 150.”
As the cone gets smaller and smaller as track forecasts improve and tropical storm sizes remain the same, more tropical storm impacts are expected outside the cone than ever before. .
The fluctuations in Ian’s forecast track are a good lesson that at two or three days it doesn’t make sense to focus too heavily on a single community, especially on a coast where hurricanes come in at oblique angles, Franklin said.
Klotzbach also noted that because Ian was tracking parallel to the coast, slight changes “make big differences as to who ends up having the biggest impacts.”
Anyone in the cone shouldn’t be surprised to see any changes and should be prepared, not set up flaps or necessarily evacuate, but “be aware so they’re not caught off guard or surprised,” Spratt said.
He and Franklin said confusion about course changes often arises when people focus on individual computer models or forecasts from independent or amateur forecasters.
“They make their own predictions or model-based predictions,” Spratt said. “It makes a difference in people’s minds, and they may think they’re out of harm’s way.”
Dinah Voyles Pulver and Doyle Rice are national weather and climate reporters for USA TODAY. Contact Pulver at Dpulver@gannett.com and Rice at email@example.com