Improvements in tropical cyclone track forecasts the past five years means the 2019 hurricane season will open with a smaller cone of uncertainty, but experts warn gains in accuracy are slowing and may have already hit a wall.

Modest upgrades will be reflected this year in the one to three-day predictions, while no changes were made to forecasts at the 3, 12, 96, and 120-hour periods.

The largest improvement was in the 48-hour forecast where the error rate was reduced by 8 percent to 68 nautical miles.

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“In general, the year-to-year reductions are not huge, but over time they accumulate to be noticeable and important,” said Brian McNoldy, a senior research associate at the Rosenstiel School of Marine and Atmospheric Science. “The only reduction that is noteworthy (this year) is the 48 hour one at 8 percent.”

While storm season officially begins June 1, the National Hurricane Center is already watching Subtropical Storm Andrea, which formed Monday afternoon southwest of Bermuda. It is no threat to the U.S.

The size of the hurricane forecast cone is adjusted each year before June 1 based on the error rates of the previous five seasons, so this year’s cone builds on improvements made between 2014 and 2018. A smaller cone means less uncertainty on where a tropical cyclone is headed.

But an essay published by the American Meteorological Society and written by National Hurricane Center meteorologists Chris Landsea and John Cangialosi says recent hurricane seasons suggest improvements in track forecasts have slowed or “perhaps even come to a halt.”

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For example, while error rates at the 24-hour and 72-hour forecasts are still showing some improvements, the 120-hour forecast error trend is flat, according to Landsea, the NHC’s chief for the Tropical Analysis and Forecast Branch, and Cagialosi, a senior hurricane specialist.

This season’s 120-hour error rate is 198 nautical miles, unchanged from 2018 but 31 percent smaller than in 2010.

“Despite incredible improvements in tropical cyclone track forecast errors and skill, it is well accepted that making perfect forecasts will never happen,” the essay notes. “If indeed track prediction will cease to improve, this conclusion is of critical importance for planning by all users of tropical cyclone track forecast information.”

In 2017, the hurricane center celebrated its most accurate track forecast season since modern mapping of tropical cyclones began nearly 50 years ago. For every measurement, which includes days 1 through 5, the 2017 forecasts topped the hurricane center's running five -year average by as much as 52 miles at the 120-hour mark.

The reasons for the improvement in forecasts are many.

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Pieces of code that go into computer models representing processes in the atmosphere, such as radiation and clouds, have gotten better. Computers are running at higher resolutions. More sophisticated satellites circle the Earth beaming down images so clear, forecasters see towering cloud tops and fields of Saharan dust like never before.

Dan Brown, a senior hurricane specialist at the NHC, said it’s not necessarily technology that is limiting track forecast gains as much as Mother Nature.

“The atmosphere is chaotic,” Brown said. “With the short-term errors getting quite small, it wouldn’t be surprising if we start seeing the improvements slow down.”

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Three-day tropical cyclone forecasts were introduced in 1964, but it took nearly 40 more years of technological advancements to increase it to five days.

Behind the scenes, the National Hurricane Center is working on a 7-day forecast that may be ready for a public debut in two to three years.

“We’re getting pretty good with the track forecasts,” said NHC Director Ken Graham at the Governor’s Hurricane Conference last week. “We still have work to do with intensity.”

Kmiller@pbpost.com

@KmillerWeather