As I write this on Sunday evening, Tropical Storm Debby is slowly inching towards the northern Florida coast. It is expected to become a hurricane before making landfall. The same region was hit by Hurricane Idalia (2023) last year. As concerning as a landfalling storm is, I am equally concerned that people in parts of Georgia and the Carolinas may not fully appreciate what they are about to experience after landfall. Irrespective of its category, Debby is likely to be a catastrophic rainfall event in the coastal southeastern United States.
Why do I say that? The storm is predicted to drop between 1 to 2 feet in southeastern coastal areas and linger for days. Before delving into the meteorology of the potentially historic rainfall event that looms, let’s turn to the National Hurricane Center for an update as of 5 pm EDT on Sunday. NOAA forecasters wrote, “Debby will continue to move over waters of high heat content and remain in an environment of low vertical wind shear into Monday morning…. given the favorable oceanic and shear conditions, significant strengthening is expected before landfall.”
The Florida peninsula has dealt with storm surge, wind gusts, and tornadoes most of Sunday and now the easternmost Florida panhandle braces for impacts. Life-threatening storm surge, hurricane winds, excessive rainfall, and tornadoes are likely. Given recent trends in the Gulf of Mexico, I would not be surprised if some rapid intensification occurs before landfall too. After a landfall in northern Florida some weakening is likely, but here is where it gets particularly deceptive and worrisome.
NOAA forecasters went on to say, “Potentially historic heavy rainfall across southeast Georgia and South Carolina through Friday morning will likely result in areas of catastrophic flooding…. Heavy rainfall will likely result in considerable flooding impacts from the Florida Big Bend region through southeast GA and the Coastal Plain of the Carolinas through Friday.” Even before seeing this language, I pondered the right word to use for several minutes before landing on “catastrophic.” In the end, I went with it too.
As a meteorologist, the analogue that comes to mind is Hurricane Harvey (2017) and messaging challenges associated with it. We warned that Harvey would dump several feet of rainfall over southeastern Texas once it was downgraded to a tropical storm. Yet, the magnitude still seemed to “surprise” some people. The system stalled and produced historic, life-altering flooding in Houston and surrounding areas. While Harvey is not the perfect analogue, Debby will make landfall and then drift to near the Georgia and South Carolina border.
At that point, it is projected to stall or meander. NOAA’s Weather Prediction Center discussion explains, “With a col in place across the Southeast U.S. in the wake of the departing trough across the Northeast, steering flow aloft will be rather weak and disorganized, resulting in a severe slow down for the tropical cyclone and model guidance typically struggles in these types of situations.” If it stalls over water, it could likely re-intensify. Either way, it is a dreadful scenario.
Much of the Southeast has experienced sustained rainfall for days so the soil is already saturated in some places. When you couple that with heavy, sustained rainfall, flooding is going to happen along with falling trees, power outages, and even potential water supply disruptions. In case it slipped by you earlier, the region will likely be dealing with Debby this entire work week. Large swaths of rainfall in the 1 to 2 feet range are expected across much of coastal Georgia and the Carolinas. Such amounts will lead to compound flooding when you factor in flash flooding, riverine flooding, and storm surge inundation. If you live in the moderate or high-risk areas depicted in the graphic above, make sure you understand your flood risks and plan accordingly.
Before you say, “But Dr. Shepherd it will only be a tropical storm at that point.” Some of the most catastrophic rain events in history happened when the storms were not really “strong” from a Saffir-Simpson scale perspective. Such storms include Allison (2001), Harvey (2017), and Florence (2018). In these situations, try to focus on potential impacts not category.
My fear is that many people residing in affected regions or vacationing in there have no benchmark experience for this type of event or may underestimate the risks associated with a stalled tropical system. There are also dangerous cognitive or other biases that may appear. The Decision Lab website defines optimism bias as, “Our tendency to overestimate our likelihood of experiencing positive events and underestimate our likelihood of experiencing negative events.”
A recent Stanford study also finds that over time people’s perception of hurricane risk drops after they have experienced previous storms. Motivated reasoning is defined by a University of Notre Dame website as, “Our tendency to use reasoning not to discover what’s really true but to justify—both to ourselves and to others—the conclusion we prefer, or have already made.” All of these are common. I remember numerous conversations with my dad, a long-time resident of coastal Florida.
Inland freshwater flooding is the deadliest aspect of a hurricane, but there is a perception that it is “just” rain. Images of violent winds speeds or turbulent seas can shape the risk perception for many people when it comes to storms like Debby. Rainfall can often be “slept on.” That’s why I write these articles.
