Hurricane Florence Will Likely Bring a Deluge of Rainfall to the Carolinas

A storm with a path unlike any the Carolinas have experienced is expected to make landfall in the coming hours.

Hurricane Florence—a once-Category 4 storm with winds up to 130 miles per hour—has been rapidly moving toward the East Coast this week, and its trajectory is atypical for a storm of its nature. Just yesterday, weather models indicated that Florence would be shifting its course, embarking on a “big, grand tour” of the southeastern United States, U.S. Fish and Wildlife Service (FWS) meteorologist Kevin Scasny said in a statement.

Hurricane Florence as it was building strength in the Atlantic

Atmospheric scientists are not used to seeing a storm like Florence move as far inland as it seems to be heading.

“Many of us right now in the meteorological community are sort of scratching our heads,” Marshall Shepherd, a professor of geography and atmospheric sciences at the University of Georgia and former president of the American Meteorological Society, told NOVA Next on Tuesday.

Most hurricanes originating in the Eastern Atlantic Ocean tend to curve northward before making landfall on the East Coast. Due to a large air mass blocking its path, Florence was unable to do so. This shift in trajectory is the reason Florence’s path surprised weather experts—it’s rare for a hurricane to not curve northward after being so far east in the Atlantic Ocean.

Levi Cowen, a graduate student studying tropical cyclones at Florida State University, produced this video for his YouTube channel explaining the unique dynamics of Florence.

With Florence’s movement toward land, the primary concern is the deluge of rainfall expected to strike the Carolinas. Should the storm stall in a way similar to Hurricane Harvey, life-threatening rain and flooding will occur. As a hurricane loses forward motion, its energy dissipates as a result of friction. The longer it remains in a single place, the more rain will fall there.

The climate of a specific area also plays a role in the intensity of rainfall from a hurricane. Warmer air can hold more water vapor. In the case of Florence, the expected impact area has high levels of water vapor, which contribute to the amount of rain that can fall. At the same time, the hurricane is slowing down because the storm’s steering winds, which are responsible for guiding the system in a particular direction, are impeding its progress.

With storm winds, storm surges become a serious concern; the tides increase in size during a storm surge because the water level rises due to intense wind. In the case of the Carolinas, both the warmer air and tide size will contribute to severe weather conditions.

As of Thursday morning, Florence has lost some power, changing from a Category 4 to a Category 2 storm. As the hurricane winds slow, this shift is indicative of stalling behavior—heavy rain, storm surges, and flooding will likely impact the Carolinas in the next few days.

Severe weather events like Hurricane Florence are evolving, which means risk assessment must evolve, too. Because the climate underlying any specific weather event is constantly changing, atmospheric conditions 20 or 100 years from now are going to be different enough that scientists won’t necessarily be able to rely on previous statistics for information about storm risk.

Kerry Emanuel, professor of atmospheric science at the Massachusetts Institute of Technology, hopes to introduce a new risk assessment paradigm that involves a simple hurricane wind model based on simple physical principles. In a 2017 paper, he used a method that aims to determine the probability of intense rainfall events on the area affected by Hurricane Harvey.

By simulating thousands of storms and modeling how they will behave by taking certain physical principles into account, Emanuel’s goal is to see how an area will be affected by severe weather on a longer timescale. In other words, he’s trying to understand what, for example, a 100-year storm is going to look like in the year 2100.

Due to its unprecedented trajectory, Florence is expected to leave devastating damage in its wake. Being able to analyze the weather and future climate of an area using the physically-based techniques proposed by Emanuel could enable better decision-making with respect to land development.