MIAMI — A blissfully quiet final few weeks in the Atlantic Basin marked the end of another record-breaking storm season that once again left South Florida unscathed.
The rest of the country wasn’t as lucky, as Hurricane Ida rapidly intensified into a major storm faster than Louisiana could call for evacuations. The aftereffects of Ida then swung northeast, bringing extreme rain that drowned dozens of people in their cars and basements.
At least six months stand between South Florida and the next hurricane season when our luck will be tested once again — potentially along with our evacuation planning. A growing body of research shows that storms are growing stronger faster, a trend that will challenge coastal cities’ ability to safely move residents out of danger zones — and climate change may be a factor.
A few other notables about the 2021 hurricane season:
Ida also was one of six storms this year that rapidly intensified, defined by the National Hurricane Center as gaining at least 35 mph of wind speed in 24 hours. Ida nearly doubled its wind speed in the 36 hours before it crashed into the Gulf Coast.
Six of the seven hurricanes this season rapidly intensified, a trend that appears to be more common in recent years, and one that can potentially be linked to climate change.
Climate change has a subtle relationship with hurricanes. It can change things like sea surface temperature, with hot water juicing up a storm. But it also can cause shifts in atmospheric airflow that actually tears storms apart.
Scientists are comfortable saying that climate change will bring more storm surge in the future as seas rise, and studies have found that global warming could actually lead to fewer hurricanes per year in the future. The downside is that more of those storms are more likely to be powerful, major hurricanes.
And strong storms are more likely to rapidly intensify, said Kieran Bhatia, vice president of Climate Change Perils at Guy Carpenter. He said about 80% of major storms rapidly intensify.
“If you’re gonna get to the highest intensity you’re gonna have to get there fast,” he said.
To rapidly intensify, storms need warm ocean water, low wind shear, high relative humidity and high potential intensity, a metric that approximates a speed limit for a hurricane by calculating the difference between nearby ocean temperature and the surrounding atmosphere.
Climate change could turn up the dial on all of those factors, emerging studies suggest, leading to more instances of rapid intensification.
Bhatia was the lead author on a 2019 paper published in the journal Nature Communications that found that rapid intensification is already becoming more common in the Atlantic Ocean, and their climate models suggested it wouldn’t have been likely without human-driven climate change.
“The thermodynamics is the main driver,” he said. “That’s really leading to a more conducive environment for these rapid intensification events.”
Bhatia’s paper also found that the probability of rapid intensification occurring in the Atlantic had risen dramatically. In the 1980s, it was 1 in 100. As of the early 2000s, it’s 1 in 20.
A challenge for evacuations
New Orleans, like most cities, needs 72 hours of notice to call for and complete evacuation procedures. The NHC issued its hurricane watch 54 hours before landfall and only upgraded to a hurricane warning 30 hours in advance. At the 72 hour mark, Ida was only a tropical depression with sustained winds around 35 mph. By the time it hit Louisiana, it had 150 mph sustained winds.
“The rapid intensification took time from us,” Louisiana State Transportation Secretary Shawn Wilson said at a press conference as the storm was approaching.
Experts say rapid intensification of storms can make evacuations more challenging. The goal is to have everyone safely off the road by the time the winds start to pick up, and that gets tough when storms power up seemingly overnight.
Bhatia, who used to work with emergency managers to share hurricane research with community organizations in South Florida when he attended the University of Miami, said this is a situation where the physical science of hurricane prediction needs to balance with the social science of how to best evacuate people.
“Can we work together on making sure that information is available to them right as it’s needed and can they be a little bit more flexible that when the confidence is high enough they can move faster with their evacuations?” Bhatia said.
Hurricane prediction science has come a long, long way in the last few decades. Scientists are able to predict storms more accurately and further in advance than ever before. The NHC makes about half as many errors when it comes to tracking storms now versus the mid-1990s, according to a 2020 NHC paper.
But some parts are still tougher than others. In general, hurricane scientists are far better at predicting where a storm will go than how strong it will be when it gets there. Despite that, the NHC’s error rate for tracking intensity has begun to creep down in the last decade.
Rapid intensification, however, is still one of the most difficult features of a storm to predict. A 2021 research paper on the topic found that storms with rapid intensification have about three times the errors as a storm without it.
“They’re very hard to forecast. We know when they happen, we know the conditions where they tend to happen but it’s very hard to say this storm is going to have rapid intensification and when,” said Suzana Carmago, a research professor in the division of ocean and climate physics at Columbia University. “There’s a lot to be understood.”
Sometimes, that uncertainty results in a false alarm. In 2015, the NHC predicted that Tropical Storm Erika would rapidly intensify as it approached Florida, but instead, the storm fizzled out.
From about that point on, however, the NHC’s skill at predicting rapid intensification has improved slightly. That same 2021 paper, published in the scientific journal Atmosphere, found that from 2016-2020, the hurricane center’s best model had about a 20% chance of correctly detecting rapid intensification within 24 hours of it happening.
“Although the guidance models and NHC forecasts are beginning to show some ability to forecast RI, especially within the past five years, there is still considerable room for improvement,” the authors wrote.