Heavy rain that slammed Baton Rouge and other parts of south Louisiana three years ago this May surpassed the historic and devastating August 2016 floods for short-term and localized intensity, representing another signal of climate change's impacts, a new study by LSU and other researchers has found.
The rains of mid-May 2021 caused more than $1 billion in damage in south Louisiana and Texas, damaged around 2,700 homes and led to five deaths in Louisiana, while triggering tensions in the Baton Rouge area over flood control and new development.
Published recently in the peer-reviewed journal Physical Geography, the new analysis concluded the rainfall totals approached a 750-year storm for a six-hour period roughly near Bayou Manchac and the East Baton Rouge-Iberville Parish line.
South of Lake Charles and north of the city's airport, the rain approached 14.5 inches in six hours, surpassing an 800-year storm, which has a 0.125% chance of happening in any year.
Developed by LSU, Southern Illinois University and other university and private sector researchers, the study used a computer model that combined 482 actual rainfall measurements across southeast Texas and south and central Louisiana, along with radar-based rainfall estimates, to reach its conclusions about the amount rain that fell across thousands of square miles in both states.
While parts of Baton Rouge and eastern Livingston Parish saw more than a 1,000-year rainfall during the August 2016 storm — the Watson area hit 31.4 inches, for example — those measurements were over two days, not the six-hour blast the May 2021 storm brought.
Flood waters raise near Burbank as rain continues in Baton Rouge on Thursday, May 20, 2021, after Monday night's catastrophic flooding event.
The 2016 rains exceeded the 2021 rains after 12 hours and dumped more over a wider area of Louisiana, the researchers found, but, in that six-hour window of time and in more isolated pockets, the May 2021 storm exceeded the short-term punch of August 2016.
The more extended storm eight years ago triggered widespread flooding that damaged tens of thousands of homes in what federal researchers have said was, in several places, greater than a 500-year flood. The event has become the benchmark for the Baton Rouge region and created a string of new high-water marks on the Amite and other rivers.
No name storms
Like in 2016, the May 2021 rains were not connected to a named storm and, the researchers pointed out, hadn't received much-advanced attention, with forecasters predicting a half-inch to 1 inch and 1.5 inches in isolated spots. The researchers suggested the rains are an example of the kind of sudden extreme weather that past research shows can be produced from rising air and water temperatures in the Gulf of Mexico due to climate change.
"It's unfortunate, but I think this is a pretty clear indication that things have been changing in the past 50, 60, 70 years in regards to climate, which we're seeing upward changes in temperature, and precipitation is responding," said Vincent M. "Vinny" Brown, climate research director for LSU's Southern Climate Impacts Planning Program and the study's lead author. "We're seeing more water vapor in the atmosphere leading to more intense short-duration rainfall events."
Every 1.8-degree Fahrenheit increase in air temperature boosts the air's ability to hold water vapor by 7%.
Brown, former State Climatologist Barry Keim and an Illinois researcher, who were part of this study, were also authors of an earlier analysis that concluded rainfall became shorter but more intense in Louisiana between 1960 and 2017 after they examined rainfall measurements across the southeast.
In this photo taken by a drone is an aerial view of the flooded Siegen Calais apartments Tuesday, May 18, 2021, in Baton Rouge, La. Heavy rains have swept across southern Louisiana, flooding homes, swamping cars and closing a major interstate. (John Ballance/The Advocate via AP) ORG XMIT: LABAT404
The August 2016 rains stemmed from a tropical wave that drifted slowly across the Gulf Coast. The May 2021 rains had a different mechanism — broad thunderstorm complexes known as "mesoscale convective vortices" — that arose from storms in eastern New Mexico and western Texas.
At the time of the May 2021 storm, the Gulf's water temperatures along Texas and Louisiana were nearly 2 degrees to more than 5 degrees Fahrenheit higher than averages since 1981, the study found. Moisture in the atmosphere available for rain was near or above the 90th percentile.
Those thunderstorm complexes were able to tap into the moisture in the air efficiently and, the study found, dropped 50% to 70% of the rain that fell between May 16 and May 20 in just six hours in many places, including south of Baton Rouge and Lake Charles.
"Water vapor is the fuel for extreme events," Brown said. "When you get elevated levels of water vapor, that's what these storms are using to dump these hellacious amounts of rain."
Fueling debate over development
In the Bayou Manchac area along the Iberville-East Baton Rouge line, the new study estimated 10.8 inches of rain fell over six hours during the late night and early morning of May 17-18, 2021, the analysis found.
The area, including the Bluff Road area of Ascension Parish and Spanish Lake area in Iberville, was hit hard with flash flooding.
As workers stand on a temporary bridge, high water from the Bluff Swamp flows Friday, May 28, 2021, into Bayou Manchac, bottom, through a new, temporary drainage channel, center, left, cut across Alligator Bayou Road. Ascension Parish officials made the cut to alleviate flooding in the region. A similar effort happened after even more severe flooding in the August 2016 flood.
Iberville Parish officials put up controversial AquaDams on Manchac Road to block floodwater from entering the Spanish Lake swamp basin from creeks draining south Baton Rouge, sparking a lawsuit from the East Baton Rouge Parish government.
Ascension leaders were forced to cut Alligator Bayou Road and install pumps along Bayou Manchac to allow the flood water already in the slow-draining Bluff Swamp to escape into the bayou more quickly.
The flash-flooding also helped fuel the already-simmering political unrest in Ascension Parish over development and helped spark a building moratorium and new tighter building rules that have contributed to slower home development since then.