- The Tennessee flash flood evolved into a destructive, deadly event within hours.
- Some areas picked up 17 inches of rain.
- This was another example why you should take flash flood warnings seriously.
While the search is over for survivors in the deadly flooding that ripped through parts of Middle Tennessee Saturday, the long road to recovery has only just begun.
After 20 people lost their lives and dozens more lost their livelihoods, many are left wondering why this happened and whether the tragedy could have been prevented.
People caught up in the flooding described it as being like a “mini tsunami.” The horror was captured in one 911 call, reported by The Tennessean.
The caller says she is trapped in her home and can’t swim.
“How did the water get so high … Please help! … For the love of God!” the desperate caller tells the dispatcher.
This is the grim reality of extreme rainfall. Its enormity was difficult to imagine, let alone predict.
And in Tennessee, it combined with topography and, for the victims, simply being in the wrong place at the wrong time, to create a disaster.
A Recipe For Disaster
The setup for this event is common in summer and was much more subtle than, say, a slow-moving landfalling tropical storm or hurricane.
An old frontal boundary was parked over Tennessee last Friday night.
A weak upper-air disturbance arriving from the mid-Mississippi Valley at about 15,000 to 20,000 feet above the ground combined with a flow of warmer, more humid air from the south and southwest and caused air to rise and create instability for thunderstorms.
Since the wind flow steering the thunderstorms was both weak and oriented perpendicular to the flow of warm, moist air, these thunderstorms repeatedly formed and tracked over the same areas for hours like boxcars of a train over the same section of a train track. This is a process meteorologists call training, as the radar loop tweeted below shows.
These training thunderstorms took advantage of exceptionally moist air, even by Tennessee standards.
According to the National Weather Service, the total moisture in the atmosphere — known as precipitable water — over Nashville Saturday morning was the highest of any previous Aug. 21 in 73 years.
The NWS-Nashville issued a flood watch late Friday afternoon due to these ingredients. It included Humphreys, Dickson, Houston and Hickman counties and the town of Waverly.
In a more typical flash flood, a few of the most flood-prone city streets or low-water crossings may take on a foot of water.
That’s still dangerous. The majority of flood deaths happen in vehicles when motorists try to drive through flooded roads.
But in this case, it became much worse.
The NWS issued its first flash flood warning just before 3:30 a.m. Saturday. The first report of “multiple roads closed due to high water” in Dickson County came in just over 90 minutes later.
NWS then upgraded the warning to a flash flood emergency at 7:47 a.m. This high-end warning is used only in rare situations when a severe threat to life and catastrophic damage is happening or will soon.
In just a few hours, at least 20 people were killed and homes were washed off their foundations. People were forced into their attics and onto roofs, where some were plucked to safety by boat and helicopter.
The victims that have been identified include 7-month-old twins swept from their father’s arms as water filled their home; a teenager who ran outside with friends to take photos of the rising water only to be washed away by a rushing wall of it; a 35-year-old man who was driving when his car was swept away; a 61-year-old grandmother who just moments before the water took her had been sitting on her front porch.
More than 270 homes were destroyed and 160 took major damage, The Associated Press reported.
As the map below shows, the most prolific rain fell over parts of four Tennessee counties.
Parts of Humphreys and Dickson Counties picked up over 12 inches of rain. A cooperative observer in McEwen, about 45 miles west of downtown Nashville, measured just over 17 inches of rain, a potential all-time state 24-hour rainfall record.
The Humphreys County town of Waverly, where the worst of the flooding hit, sits on Trace Creek, 10 miles downstream from McEwen. The creek is one of several streams that crisscross Humphreys County, which is bordered on its west by Kentucky Lake.
Trace Creek overflowed its banks in a way long-time residents have never seen before as the water rushed toward Waverly, picking up debris including trees and cars in its path.
Much of Waverly is considered a special flood hazard area by the Federal Emergency Management Agency. Many individual properties in the area have a higher than average flood risk in ratings by Flood Factor, an online tool from the nonprofit First Street Foundation that analyzes flood hazards by address.
Was This Catastrophe Predictable?
The NWS was ahead of the situation with the flood watch, warning and emergency.
But imagine you’re a resident of one of these four counties affected by the flood.
You may have heard about the flash flood watch Friday evening and made sure to stay off the roads overnight and Saturday morning.
But you probably had no expectation a catastrophic flood would threaten your life and flood your home that morning.
Are these small-scale deadly and destructive events predictable beyond, say, a few hours?
Greg Carbin, Forecast Operations Branch Chief at NOAA’s Weather Prediction Center offered the analogy of one intense tornado on a day with a threat of severe weather.
“In some respects, this flash flood event is in the same vein as, say, the Joplin tornado of a decade ago,” Carbin told weather.com.
“Prior to formation, meteorology cannot currently forecast a violent tornado with high precision, and neither can it predict with a high degree of lead time and accuracy an incredibly unusual extreme rainfall event such as what occurred in Waverly.”
Despite that, Carbin also said that at relatively shorter lead times, meteorologists can see ingredients coming together to support a more extreme event, such as NWS-Nashville detected early Saturday morning.
The increasing use of forecast model ensembles and other techniques offer potential forecasts with greater lead time in the future.
“Things like convection-allowing ensembles help greatly with this since they attempt to capture the possible breadth of event possibilities,” said Erik Nielsen, assistant professor at Texas A&M University and an expert on heavy rainfall.
Nielsen told weather.com situations like the Tennessee flood are “such a challenge that a lot of people are currently intensely working on it.”
That includes scientists in NOAA and the National Weather Service.
“The forecasting of extreme precipitation is recognized by NOAA as a ‘grand challenge’ in need of further funding and research,” Carbin said.
How to Stay Safe From America’s Worst Storm-Caused Killer
Over the past 30 years, flooding has been America’s worst storm-related killer, claiming an average of 85 lives a year.
Scientists say extreme rainfall like what happened in Tennessee is happening more often because of climate change, which means more dangerous situations like last weekend’s will play out more often in the future.
Given all this, what steps can you actually take in a high-impact flash flood on short notice?
First, if you live near a river, creek or stream, know that this type of destructive flood, while unlikely in any year, is possible if such a prolific rainfall event occurs.
The creek near your home with only a few inches of water running through it could become a destructive, life-threatening torrent if 12 inches of rain falls in the span of just a few hours.
“Water is so much stronger and more powerful than we give it credit for,” Shannon Bowen, a professor at the University of South Carolina who researches disaster management and emergency response, told weather.com. “It is very strong when it’s moving rapidly, and in these storms, it’s often moving quite rapidly.”
Bowen advises everyone to know where flooding is likely to happen in your area, and which routes to take to get to safety.
“Know the topography of your land, make sure that anywhere you travel you know where to avoid low-lying areas,” she said.
Secondly, be weather aware.
Have multiple ways of receiving NWS warnings, including via smartphone, NOAA weather radio or local media. Make sure your smartphone or NOAA weather radio is able to wake you up if a warning is issued while you’re asleep.
Give as much attention to flood watches and warnings as you would with tornadoes and severe thunderstorms.
And in the rare case of a flash flood emergency, immediately get to higher ground or as far away from the creek, stream or river as possible.
“If it is a major event that’s unfolding, take action before it’s too late. If you know your home is in a low-lying area that’s prone to flooding, go stay with a relative or a friend for the night,” Bowen said. “Make sure that you have a safe route to get there before you get on the road and just go somewhere that you feel a little bit safer before the floodwaters start to rise,” Bowen said.
She adds that to be safe from flash flooding: “The best thing to do is to avoid being in that situation.”
Being prepared and weather-aware may not save your home or property, but it could save your life.
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