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Dreaded 'cone of uncertainty' will shrink for the coming hurricane season

The National Hurricane Center plans to shrink the dreaded "cone of uncertainty" during the upcoming season based on an improving forecast record.

The Miami-based center made the announcement Monday, along with a series of other changes intended to improve how hurricane forecasters convey warnings to the public. Along with the shrinking cone, forecasters will extend advisories, which include warnings and watches, to 72 hours in advance of a storm, providing a full additional day to prepare. Experimental graphics used last year to depict arrival times for dangerous winds will also become a permanent addition to forecasts.

"The changes are to improve information contained in the hurricane center products and to provide it in maybe easier to understand formats," said Dan Brown, a senior hurricane specialist in charge of warning coordination.

The forecast cone has long been a fixture of the hurricane forecasts but frequently debated because the public tends to focus on the center track, ignoring dangerous winds and other hazards that can extend for many more miles.

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The cone is calculated using the previous five-year record of forecast errors and drawn to reflect at least two-thirds of the inaccuracies. In other words, the cone provides a probable track about two-thirds of the time. Improved forecast tracks in recent years have allowed forecasters to narrow the cone, Brown said. Two years ago, they enlarged it to include 5-day forecasts and last year added tropical storm force wind projections.

But he warned the public should still consider the cone an imperfect estimate.

"A third of the time storms can still fall outside the cone," he said. "It doesn't provide you with any indication of potential impacts. It's only showing you where the center is likely to track."

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Forecasters also felt improved forecast tracks allowed them to extend warnings and watches to provide more preparation time, Brown said. More people were also poring over forecast graphics that included five-day estimates, so it made sense to incorporate warnings, he said.

Last year the center also began tracking and warning about tropical systems even before they became full-blown tropical depressions or storms if they threatened land within 48 hours, a move made to improve warnings about storms that form in the Gulf and provide little time to prepare.

The Weather Prediction Center, which tracks systems once they weaken below tropical depression intensity, will also begin issuing advisories to better warn the public about ongoing hazards like heavy rain. Last year, the center compiled the information as Hurricane Harvey meandered off the Texas coast and concluded it helped better communicate the potential danger of the immense storm as it continued to do damage, Brown explained.

The changes come following a record-breaking year for Atlantic hurricanes — 10 formed in a row including Irma, the most powerful ever recorded outside the Caribbean and Gulf of Mexico. Last year put the warnings tools to the test, Brown said, but does not necessarily drive this year's changes, which were already being planned.

"Improving products is always on our minds," he said.

However, the storms did confirm some efforts, particularly a big push to improve warnings about storm surge.

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"Despite the fact three Cat 4 hurricane landfalls occurred in the U.S., there were no known storm surge-related deaths. There may have been other factors, but [storm surge warnings] certainly played into it," he said. "The mapping is helping with better awareness."

The center is also in the midst of a longer-term look at how it issues warnings as part of a 10-year effort to improve intensity forecasts. While forecasters have improved track projections dramatically, rapidly intensifying storms like Irma and Maria remain difficult to forecast.