As Tropical Storm Erin moves west toward the Caribbean islands and the official forecast from the National Hurricane Center shows the storm likely beginning to curve — potentially even away from the United States — the center’s forecasters have exercised caution, consistently warning that there is a “greater than normal uncertainty” about where the storm will go after the weekend.
But why?
The uncertainty has to do with quite a few factors, but a main one is the output of the computer weather models that help meteorologists make their forecasts. Specifically, it’s a type of map called a spaghetti plot showing where a computer simulation predicts the center of the storm will be five, seven or even 14 days in the future.
These mapped model outputs get their name from their resemblance to long strands of pasta.
The closer the lines are together, the more confidence forecasters have in what the storm might do, because it means many of the models agree with one another. For the next few days, there is a pretty reliable consensus that the storm will track northwest. But how much and when it might turn north or even northeast are still big questions.
“The confidence in the predicted location is less at, say, five days, than at two days, said Richard Pasch, an expert at the Hurricane Center. “In the case of Erin, the model spread increases beyond five days, but not unusually so.”
“It should be noted,” he added, “that the typically greater uncertainty in the forecasts at six and seven days is the main reason why the National Hurricane Center does not currently issue official forecasts at these longer time ranges.”
In 2017, Hurricane Irma, which formed in a similar position to Erin’s, was forecast to make a similar curve, but instead it kept moving west, hitting the Caribbean islands and eventually disrupting the west coast of Florida. And Erin is currently in a similar position in the Atlantic Ocean to where Hurricane Lee was in 2023, which did make a turn, allowing it to pass over cooler waters and weaken before hitting Canada. Then there was Hurricane Gloria in 1985, another storm with a similar path to Erin’s, which knocked out power to millions of customers and disrupted the lives of millions of people from North Carolina to New England.
“I thought a Gloria track might be possible, but it is looking pretty unlikely that we will see a U.S. landfall from Erin,” Phil Klotzbach, a hurricane expert at Colorado State University, said on Thursday afternoon.
One of the hardest parts of any hurricane forecast is trying to predict when a storm will turn and how much of a turn it might make.
Forecasters like Mr. Klotzbach look for a pattern to help build their confidence around where a storm is most likely to go. The computer models typically run four times a day. After each, forecasters look to see if there have been changes from the last one. Consistency between outputs is one thing forecasters watch for. But if there is a significant change, they usually wait for another model run to make substantial adjustments to the guidance they turn around and share with the public.
They aren’t just worried about where the storm will go — they are also concerned about how intense the storm might become. While forecasters can use one model to monitor intensity, they usually prefer to look at all of them. To do this, they often plot different forecast intensities on a chart that progresses over time. This line plot gives forecasters an idea of what could happen.
Similar to the map plots, the tighter the lines are together between models, the more confident the forecasters are about the intensity. Sometimes they might say the official forecast is below the model guidance, or, as happened Thursday morning, they may say the official forecast was below some of the guidance. This suggests that some models indicate Erin could strengthen more than expected over the next five days. As of Thursday afternoon, the Hurricane Center is saying Erin could grow at least as strong as Category 3 by next week.
Judson Jones is a meteorologist and reporter for The Times who forecasts and covers extreme weather.
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