Spatiotemporal analyses of Extreme Precipitation and Flooding in Tennessee

Water is a constant companion to life, but when it arrives with too much force or in the wrong place, it becomes a destructive force capable of reshaping landscapes and devastating communities. In the United States, two distinct types of water disasters dominate the headlines: floods, which are the slow, rising inundation of dry land by overflowing rivers and streams, and flash floods, which are sudden, violent surges that can form within minutes of heavy rain. While scientists have long known that extreme weather events are becoming more frequent, understanding exactly where and how these dangers are changing requires looking at the past through a specific lens. Researchers must examine not just how many storms occur, but where they happen, how often they strike the same places, and whether the damage they cause is getting worse over time. This kind of investigation helps communities prepare for a future where the rules of water and weather may be shifting.

A team of researchers at East Tennessee State University recently applied this kind of deep, time-focused investigation to the state of Tennessee. They built a digital model that allowed them to watch thirty-two years of weather history unfold, county by county, to see how heavy rain, river floods, and flash floods have behaved across the state. Their goal was to move beyond simple counts of storms and instead map the evolving patterns of danger. They found that while the number of flood events has gone up in some areas and down in others, the story of the damage tells a different, more urgent tale. In the central part of the state, the frequency of heavy rain is climbing, and the financial toll of flood disasters is rising sharply, even in places where the number of floods has not necessarily increased.

To understand these changes, the researchers treated the state like a living timeline. They gathered records of every heavy rain event, every river flood, and every flash flood recorded between 1991 and 2022. For the rain, they looked at storms that were strong enough to be expected once a year, once every two years, and once every five years. For the floods, they tracked both how often they happened and how much money they cost in property damage. They then fed this massive amount of data into a computer system that organized the information into a three-dimensional grid. In this grid, the horizontal layers represented the geography of Tennessee's ninety-five counties, while the vertical axis represented time, stretching from the early 1990s to the present. This allowed the team to see not just where events happened, but how those patterns moved and intensified over decades.

The results revealed a state in transition. Across Tennessee, the number of counties experiencing an increase in heavy rainfall is far greater than the number seeing a decrease. This trend is particularly strong in the middle of the state, where a cluster of counties has become a consistent hotspot for intense rain. In some of these central counties, the number of heavy rain events has doubled or tripled in recent years compared to the early part of the study period. The researchers identified specific areas, such as the Nashville Basin and the edges of the Cumberland Plateau, where these heavy rains are becoming a regular, intensifying feature of the local climate.

When the team turned their attention to flash floods, the picture became more complex. In the western part of the state, specifically in Shelby and Tipton counties, the number of flash flood events is clearly rising. However, in many other parts of the state, the number of flash floods has actually decreased or stayed the same. This might suggest that the danger is fading in some places, but the data on property damage tells a different story. In the central and eastern parts of the state, the money lost to flash floods is climbing steadily. This means that while there may be fewer flash floods in some areas, the ones that do occur are causing more severe damage. It is a bit like a storm that hits less often but brings a heavier punch when it arrives.

The researchers also looked at traditional river floods, which tend to rise more slowly than flash floods. Here, the trend is clearly upward in the middle and eastern sections of the state. Counties in central Tennessee are seeing more frequent river flooding, and the financial cost of these events is rising in the east. Interestingly, the study found that the number of flood events and the amount of money lost do not always move in the same direction. In some counties, the number of floods is going down, but the damage is going up. This suggests that the floods that are happening are becoming more intense, or that the communities they hit are becoming more vulnerable due to changes in the land, such as more buildings and roads that prevent water from soaking into the ground.

One of the most striking findings came from looking at the state's geography and history. The researchers noted that a record-breaking rain event in 2021 in a central county, which dropped over twenty inches of rain in a single day, caused a flash flood that killed twenty-two people and caused over one hundred million dollars in damage. This single event, which was far more intense than anything seen in the previous decades, helped to shift the long-term trends for that region. The study also highlighted how the shape of the state and its borders can hide these patterns. When the researchers analyzed the state as a whole, some dangerous areas were masked by the sheer volume of events in the center. But when they looked at the eastern, central, and western regions separately, new hotspots emerged, particularly in the east, where previously hidden dangers were revealed.

The study concludes that the tools used to map these changes are valuable for anyone trying to protect communities from water disasters. By visualizing how hazards move through time and space, planners can see which areas are becoming more dangerous and why. The research shows that the threat is not uniform; it is concentrated in specific pockets of the state, with the central region facing the most significant increase in both heavy rain and flood damage. The findings suggest that preparing for the future requires more than just counting storms; it requires understanding how the intensity of those storms and the vulnerability of the land are changing together. As the climate continues to shift, these detailed maps of the past offer a crucial guide for building a safer future.