Models missed clues that led to Hurricane Dorian’s rapid intensification and stall near the northeastern Bahamas, according to a post-mortem report of the deadly storm by the National Hurricane Center.
Hurricane Dorian’s detonation to a 185-mph storm and calamitous idle over the northeastern Bahamas were missed by forecast models that underestimated the synergy building between the ocean and atmosphere.
A 75-page report released Monday by the National Hurricane Center chronicles the evolution of the deadly Cat 5 cyclone and critiques its forecast, noting that Dorian’s sidestep of rugged Hispaniola accounted for larger than normal errors in predicting the storm’s ultimate power.
Also, while models and forecasts called for a slowdown in forward speed near the northern Bahamas, "none of them indicated that Dorian was going to stall there."
"The longer range forecasts had the storm moving across the Bahamas at a steadier pace, and then, even as the forecast shifted, the full degree of the slowdown wasn’t anticipated," said Michael Brennan, the National Hurricane Center’s hurricane specialist branch chief. "A lot of it was the evolution of the steering flow."
Dorian was the strongest hurricane in modern records to make landfall in the Bahamas when it bullied ashore at Elbow Cay, Great Abaco on Sept. 1, 2019 with 185-mph winds. The island was raked by at least tropical storm force winds for three days, according to the report.
The NHC performs a post-mortem analysis on each storm it forecasts. Among the challenges in Dorian’s forecast were also triumphs. Hurricane forecasters correctly predicted Dorian’s northerly turn that caused it to miss South Florida — a forecast that many models contradicted but one that kept Broward and Miami-Dade counties from having to issue hurricane watches and warnings.
"The models are free to go where they want," Brennan said. "Part of the role of the human is to maintain continuity so the official forecast doesn’t just jump around."
Dorian was the fourth named storm of the busy 2019 season, which had 18 named systems, including six hurricanes. Three of the hurricanes — Dorian, Humberto and Lorenzo — were Category 3 or higher.
Dorian was born out of a tropical wave that moved off the west coast of Africa on Aug. 19. It grew to a tropical storm five days later. Dorian reached the Windward Islands on Aug. 27 as a raggedy system choking on dry air before it would make a move that upended forecasts.
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The storm’s center passed over the high mountains of St. Lucia, which jostled its circulation and jogged the system north where it strengthened to a hurricane.
Instead of going over Hispaniola, it threaded east of Puerto Rico.
Brennan said he was in the forecast office when Dorian underwent the rapid intensification that took it to 185 mph before landfall in the northern Bahamas. He called the moment "sobering."
"It’s scary, because you know what is going to happen on the ground," he said. "Dorian just took advantage of the environment — low shear, very warm water, very high moisture."
When the high pressure system north of Dorian weakened, the steering winds collapsed and the storm slowed "pounding Great Abaco for several hours with its greatest fury," the report says. A second landfall happened near South Riding Point on Grand Bahama. It wasn’t until a trough over the U.S. swung east that Dorian began shifting north-northwest on Sept. 3.
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Although six Category 5 hurricanes have formed since 2016, reaching that echelon is historically rare, which is one reason why forecast models can stumble in their predictions of the strongest storms.
John Cangialosi, a NHC senior hurricane specialist, said the error rate on Dorian’s track was about average.
"The problem is, those numbers make a big difference sometimes," said Cangialosi, noting Dorian’s avoidance of island speed bumps that could have changed its future. "A little miss can have everything to do with what happens later."
Dorian also made landfall in North Carolina near Cape Hatteras as a Category 1 hurricane on Sept. 6. It reached Nova Scotia a day later as a strong post-tropical cyclone.
The NHC’s report estimates Dorian’s damage in the northeastern Bahamas at $3.4 billion. It quotes the Health Minister in the Bahamas saying more than 200 people died in Dorian.
While Dorian left Florida mostly unscathed, Brennan said a small shift in the storm’s forward speed could have meant a Sunshine State landfall.
"Subtle track changes and subtle differences in center and motion in the short term can have big impacts on where the storm ends up," Brennan said. "You don’t want to pay attention to every wiggle and wobble, but there’s a reason we make a new forecast every six hours."