Why Hurricane Florence is unusual and dangerous

September 14, 2018, Columbia University

Hurricane Florence is expected to bring heavy rains and dangerous storm surge to the Carolinas between Thursday and Saturday. Image: NOAA

North and South Carolina could soon encounter the strongest they've seen in decades. Hurricane Florence is heading for the eastern seaboard as a dangerous Category 4 storm, threatening winds of 130 miles per hour, 13-foot storm surges, and torrential rainfall.

Florence is expected to blast coastal areas, and possibly areas further inland, between Thursday and Saturday. Weather models show the storm slowing down or stalling over or near the Carolinas, thus increasing the chances of dangerous rainfall levels.

Virginia, North Carolina, and South Carolina have issued mandatory evacuation orders of coastal communities forecast to be in the storm's path.

Hurricane and disaster experts within the Earth Institute are closely monitoring the storm. Below, they explain what makes Hurricane Florence unusual, and unusually dangerous.

Hurricane and disaster experts within Columbia's Earth Institute are available to answer questions from the media about hurricane physics, emergency response, the role of climate change in creating strong storms, and more.

Why Hurricane Florence is Unusual

It's somewhat rare for hurricanes this intense to make landfall in the Carolinas, said Suzana Camargo, executive director of the Initiative on Extreme Weather and Climateat Columbia's Lamont-Doherty Earth Observatory. "It's a not a typical track," she says, referring to the storm's expected path. Normally hurricane trajectories curve backwards toward Europe, sending the storm back into the ocean. Not so for Florence—simulations show it making a beeline for the Carolinas.

"Hurricanes basically go with the winds," Camargo explains. "The winds take them where the winds are going… When you have areas of high pressure, that's where hurricanes don't go. Right now we have this very strong high in the Atlantic that's not allowing the storm to recurve."

Why Florence is Difficult to Predict

What concerns Camargo most is that the forecasts for what will happen when Hurricane Florence nears the coast have been very uncertain. Different models are showing different predictions for exactly where the storm will go, whether it will make landfall, and how slow it will be moving. That's due to a second high pressure system in the Midwest, similar to the one in the Atlantic, which also wants to push the hurricane away.

"To determine where the hurricane will go, we have to predict which high is going to be stronger," says Camargo. "It's like they're competing, and it depends on the winds and what's happening in the storm."

It is also difficult to predict exactly how strong, or intense, Hurricane Florence will be. Hurricane scientist Chia-Ying Leefrom the International Research Institute for Climate and Society at Columbia's Earth Institute explained that hurricane intensity change depends on large-scale factors like the vertical structure of the winds and ocean temperatures, as well as small-scale processes such as thunderstorms within the hurricane. "These processes are not fully understood," she wrote via email. "The smaller scale features are not well observed, and are chaotic in nature. Thus, it is harder for computer models to have good representation of these processes and accurately predict storm intensity change. So, while we often know whether a storm is going to intensify or weaken (because we know the large-scale conditions), exactly how much a storm intensifies or decays is less predictable.

Why Hurricane Florence is unusual and dangerous
View of Hurricane Florence from the International Space Station on Sept 12, 2018. Credit: NASA

The Potential Impacts

While Hurricane Florence's exact trajectory and intensity are uncertain, the storm poses a significant threat to the Carolinas in all scenarios.

Radley Horton, a climatologist at Lamont-Doherty says that where the storm slows down will be critical. "Unfortunately, it currently appears this will occur close enough to land to inflict serious flooding," he says. "The slow movement, should it indeed happen near shore, would virtually guarantee that the storm surge coincided with at least one, and possibly more than one 'astronomically' high tide, leading to further ocean flooding and accumulated wave damage."

Hurricane Florence is also expected to bring intense rainfall. And if the storm slows or stalls near the coast, it would increase flood hazards "because there's more time for winds to drive surge and more accumulated rain," explains Tim Hall, who models hurricane risk at the Earth Institute-affiliated NASA Goddard Institute for Space Studies.

And the effects likely won't be limited to the coasts. "Significant effects could also be felt as far inland as parts of Tennessee, Kentucky, Alabama and West Virginia," Jeff Schlegelmilch and Irwin Redlener from Columbia's National Center for Disaster Preparedness write in The Hill. "This could lead to flash flooding and damaging winds further inland. Additionally, an influx of people evacuating from the storm or traveling for supplies could lead to temporary shortages of fuel and other necessities, as has been seen in the aftermath of other storms."

What Role Did Climate Change Play?

"The airtight link between Florence and climate change is through sea level rise," says Horton. "Coastal Carolina and Virginia have experienced approximately a foot of over the past century, the majority of which is due human-driven climate change. As a result, when a storm like Florence occurs, additional areas are flooded by the sea that would not have flooded had human activities not melted land-based ice and warmed the ocean, causing it to expand."

In addition, warmer oceans can add more energy into hurricane systems. Meanwhile, a warmer atmosphere is able to take up more water vapor, leading to more intense rainfall later.

Lastly, "there's evidence that a warming climate is slowing the large-scale wind patterns that steer storms," says Hall. He cites a recent paper in Nature that found that hurricanes are traveling around the planet more slowly. "I'd expect this slow-down to be realized as more frequent stalls and abrupt changes in direction," he says. "Harvey last year is an example, and Florence could be, too."

Addressing the Challenges that Lie Ahead

Schlegelmilch and Redlener praised the disaster preparation and evacuations in Virginia and the Carolinas, but warned that recovery from the storm could be long and challenging.

While the nation needs to improve on disaster preparedness as well as disaster response and recovery, Earth Institute director Alex Halliday reminds us that taking action on climate change will also help to spare lives and human misery in the future.

"Climate change is expected to result in increased numbers of very intense storms like Hurricane Florence," he says. "With higher temperatures, it is not surprising that there would be more evaporated water and energy in the atmosphere. To limit the future loss of lives, homes, and billions of dollars in damages, we need to address at the local, state, and national levels. Within Columbia's Earth Institute, researchers are using advanced modeling techniques to understand today's storms and to predict what future ones might look like. We are testing the effectiveness of sea walls, recommending recovery processes after disasters, and finding innovative ways to decarbonize our society. If all parts of society can come together on a united front, we can better deal with the weather challenges that lie ahead."

Explore further: Georgia governor declares emergency ahead of hurricane

More information: James P. Kossin. A global slowdown of tropical-cyclone translation speed, Nature (2018). DOI: 10.1038/s41586-018-0158-3

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1 / 5 (1) Sep 14, 2018
Be afraid! Be very afraid! This storm was "dangerous" and "unusual." Get everybody out now! No, wait! Landfall and now it's a Cat 1. Climate Change killed the hurricane! Hurray.

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