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Hurricane Irma track: Which forecasting model is most accurate?

USA TODAY logo USA TODAY 9/8/2017 Joseph Baucum
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Video by the Associated Press

PENSACOLA, Fla. — Despite the bevy of Hurricane Irma forecasts circulating on social media, it is still too early to predict the Florida communities that could endure the worst of the storm's winds and rainfall.

Experts urged residents in potentially impacted areas to monitor Irma's predicted paths but to avoid latching onto any specific forecast track.

Ryan Rogers, a meteorologist with the National Weather Service in Mobile, Ala., said the agency is confident the Category 5 storm will turn to the north at some point after making landfall in South Florida, but it largely remains unknown exactly when the shift will happen.

What does the National Hurricane Center say?

The National Hurricane Center's best-known report is an official five-day forecast, which depicts a storm's center of circulation and its surrounding "cone of uncertainty."

Hurricane Irma 5 a.m. Sept. 7, 2017 © GRAPHIC CONTRIBUTED BY SOUTH FLORIDA WATER MANAGEMENT DISTRICT Hurricane Irma 5 a.m. Sept. 7, 2017 The cone contains the probable path of the hurricane's center, but does not show the storm's size. Dangerous weather conditions can also transpire outside of the cone's boundaries.

Rogers said the center generates the forecast through a combination of several small-scale and global models. It is a conglomeration of all available information. By far, he stressed, it is the most trustworthy and reliable source for predicting Irma's path.

"If people were to focus on the five-day forecast from the National Hurricane Center, we would have a lot less hype and misinformation out there," he said.

What about 'spaghetti plot' models?

But people who want to see forecasts beyond five days can look at "spaghetti plots." These models, which the National Hurricane Center does not use, show a range of tracks and offer a larger view of a storm's potential path than a single model.

Among the most notable spaghetti plots are the European Center for Medium-Range Weather Forecasts, which has a model more simply known as the European model. The Global Forecast System also provides the American model.

A man walks among the destruction left by Hurricane Irma at the Phillipsburg Town Beach on September 11, 2017 in Philipsburg, St. Maarten. In photos: Hurricane Irma The models have splayed-out lines that predict the path of the storm. The European model is widely considered the superior model for predicting the path several days in advance and contains a substantial amount more computing power than the American model. But, Rogers said, people should not rely solely on one model over others.

Are there different kinds of spaghetti plot models?

Ensemble plots are among the different types of spaghetti plots. These feature the same forecast run multiple times over but with slightly different initial data input, such as a half-degree difference in ocean temperature or a slight change in the solar radiation, according to Rogers. The European and American ensemble models can be found at weathernerds.org.

Rogers said if the lines of the plot in an ensemble are clustered tightly, the model is more trustworthy.

"If all of those runs are clustered together within the time period you're interested in, you can kind of get a sense on how stable or fragile that solution is," he said. "If it's tightly clustered together, there can be more confidence in it."

More: Hurricane Irma could create one of the largest mass evacuations in U.S. history

More: Hurricane Irma: Where is the Category 5 storm now and where is it headed next?

The European and American ensembles both show a tight cluster of lines for Irma's first few days traveling across the Caribbean. But as the storm's track reaches Florida, the lines diverge wildly across the state and into the Atlantic. Rogers warned even the clustered areas can create erroneous expectations.

"There are a lot of pitfalls that you can fall into and think this one model is completely clustered right here, but it could be clustered around the wrong solution," he said. "Sometimes putting data out there that can be misinterpreted is not always the best idea."

Pedestrians walk by a flooded car on a street as Tropical Storm Irma hits Charleston, S.C., Monday, Sept. 11, 2017. Photos: Hurricane Irma strikes the U.S. So should you stay or go?

Unfortunately, because of the lack of certainty in where Irma will travel once it hits the mainland, Rogers said, spaghetti plots and other forecasts cannot definitively determine if residents throughout the Florida peninsula and up through the Carolinas should evacuate.

A primary consideration should be the risk of storm surge. Rogers said a number of properties can provide shelter to protect people from the wind, but rising water is unavoidable. Still, with so much still left unknown about Irma's exact direction, the decision to evacuate remains a hard choice.

"There are so many considerations," he said. "We advise people to listen to their local officials and heed their advice. But the nature of this beast is uncertainty, especially after the three-to-four-day time frame. It's a tough call."

 

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