Florida storm season 2020: The cone isn’t shrinking this year; what that means for forecasting
Decades of improvements in tropical cyclone track forecasts have led to a slimmer and more accurate hurricane cone, but this year will see no such fine-tuning.
A handful of bumpy predictions during the 2019 hurricane season threw enough kinks into the five-year average error rate that the size of the conspicuous cone of uncertainty will remain mostly unchanged, and experts warn gains in predicting a storm’s path are slowing.
“The technology has taken us this far and the question is, is this as far as it will take us and are we going to plateau for a while?” said John Cangialosi, a senior hurricane specialist at the National Hurricane Center and co-author of a 2018 paper that examined track forecast limitations. “I don’t think anyone has a good answer to that, but we’ll all agree it’s never going to be perfect.”
While storm season officially begins June 1, the NHC unveiled new forecast products recently in a webinar with Director Ken Graham that included statistics on successes and challenges in both track and intensity forecasting.
“The average three-day NHC track forecast error for Hurricane Floyd in 1999 was 236 nautical miles,” Graham said.
That’s down to 103 nautical miles in the cone the NHC will use this season.
“There has been a lot of gain,” Cangialosi said.
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 forecasts made between 2015 and 2019. The cone, which covers days 1 through 5 of a forecast, is made up of circles sized so that 66 percent of the time the center of the storm has stayed inside that area.
For example, this season’s 120-hour error rate is 196 nautical miles, about 1 percent smaller than 2019 but 31 percent smaller than in 2010. The error rate at 24 hours is 41 nautical miles this season, unchanged from 2019.
A smaller cone means less uncertainty on where a tropical cyclone is headed, but notably does not forecast impacts outside of the cone such as storm surge, flooding rains and damaging winds that can occur in surrounding areas.
Brian McNoldy, a senior research associate at University of Miami’s Rosenstiel School of Marine and Atmospheric Science, said the cone size shouldn’t be judged year to year, but on a longer time scale.
“You can absolutely have a season that has more track errors than the year before,” McNoldy said. “In 2014 to 2015, there was an undetectable change in the cone. It does happen.”
One storm that dinged track forecast accuracy in 2019 was the mostly-unknown last cyclone of the season — Tropical Storm Sebastien. Sebastien flared up Nov. 19 about 275 miles northeast of the Leeward Islands. Its top wind speeds hit a ceiling of 65 mph before it fizzled hundreds of miles north-northwest of the Azores on Nov. 27.
Although Sebastien lasted more than five days as a tropical cyclone, the first six NHC forecasts predicted it would be absorbed by an approaching front within 72 hours, according to the post-mortem report on the system.
The models repeatedly misjudged Sebastien’s forward speed, which ultimately led to track errors that were up to 4.5 times greater than the five-year average for some time periods. Sebastien remained stronger and taller than the models anticipated, resulting in a faster moving storm that outran the front.
“We ended up with some of the biggest errors we had in 15 years and it really hurt our performance overall,” Cangialosi said. “We were constantly picking the wrong solution. Fortunately, Sebastien didn’t affect any land.”
Hurricane Dorian ended with nearly average track errors, but early model confusion about where its center would form and move tripped up intensity forecasts. NHC’s 8 p.m. advisory Aug. 27 had Dorian scratching over Puerto Rico as a tropical storm and remaining near that intensity until it reached Florida on Sept. 1. Instead, Dorian jogged east of Puerto Rico and hit the northern Bahamas as a Category 5.
“If Dorian went over Hispaniola and got shredded, it would mean maybe some rain for Cuba and South Florida,” said NHC Director Ken Graham. “It could have been a different story.”
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, the 2017 forecasts topped the center's running five-year average by as much as 52 miles at the 120-hour mark.
The reasons for the long-term improvement in forecasts are many.
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.
Cangialosi said he wasn’t disappointed by the lack of progress this season in reducing the cone size because the numbers will jump around short term.
“The underlying thing is, it’s a chaotic system so there will always be a tiny bit of uncertainty. That’s the law of nature,” McNoldy said. “The cone will never shrink to be just a line.”
This story originally published to palmbeachpost.com, and was shared to other Florida newspapers in the USA TODAY Network - Florida.