July 11, 2019

Life-Threatening Flooding Likely As Tropical Storm Barry Crawls Toward Louisiana

Extremely heavy rain associated with Tropical Storm Barry will lead to life-threatening flooding across southern Louisiana and the lower Mississippi River Valley over the next couple of days. The slow-moving storm could reach hurricane strength before it makes landfall on the Louisiana coast on Saturday. Latest forecasts show double-digit rainfall totals across the eastern part of the state, compounding ongoing flooding issues across the region.

NHC Forecast

The disturbance we've been watching for the last couple of days developed a closed surface circulation on Thursday morning, allowing the National Hurricane Center to upgrade the system to Tropical Storm Barry. The NHC's 7:00 PM CDT advisory shows the storm slowly organizing over the next two days, possibly strengthening into a hurricane before making landfall in southern Louisiana on Saturday morning.

Conditions across the northern Gulf of Mexico are conducive to strengthening if the storm can get its act together fast enough to take advantage of its environment. Barry's structure on Thursday afternoon was...lacking...with an exposed low-level circulation south of the mouth of the Mississippi River, and almost all clouds and convection fanned-out on the southern side of the system.

The window for strengthening will begin to close the longer the storm remains disorganized. That's great news in terms of wind and storm surge, but Barry is set to bring flooding rains to the northern Gulf Coast regardless of its strength at landfall.

Rainfall Forecast



While wind speeds and the phrase "hurricane warning" will get the most attention, the real story of this storm is the water. Barry threatens to wring out a tremendous amount of tropical moisture over the northern Gulf Coast through this weekend.

The latest rainfall prediction from NOAA's Weather Prediction Center shows the potential for 20" or more—yes, that's twenty inches or more—of rain across southern Louisiana. A wider swath of half a foot or more of rain spreads up the Mississippi River toward the Memphis area.

Not everyone will see all of the rain in the forecast. But the storm will move slow enough that rain bands and thunderstorms will be able to tap into a deep reserve of tropical moisture and produce copious amounts of rain in a short period of time.

It's important to note that these predicted rainfall amounts will change as the storm gets closer to land and forecasters get a better handle on the structure and future track of the storm. Small changes in intensity, organization, and track will shift the bullseye for heavy rain with time. The overall point is that everyone in Louisiana should prepare for a potentially significant flooding event.

Flooding Potential

NOAA/NWS
The combination of accumulated water from heavy rain, the Mississippi River rising from excess upstream runoff, and a potential storm surge could severely strain the ability of New Orleans and surrounding areas to stave off floodwaters.

The Mississippi River is already in flood from months of heavy rain across the central United States. Add that on to the flash flood emergency that played out across New Orleans on Wednesday—dropping more than half a foot of rain in a couple of hours—and it won't take much heavy rain to cause major flooding along the area's already-strained waterways.

River flooding forecasts from the National Weather Service show the Mississippi River in New Orleans cresting at 19 feet if current precipitation forecasts hold up, which would be the highest crest recorded there since February 1950. The levees along the Mississippi in New Orleans are only 20 feet tall, so the water would only be about a foot away from the top.

The entire city of New Orleans sits below sea level. Not only does it face a threat from the bodies of water that surround the city, but rainwater has to be pumped out of the city because it can't seep into the ground. The pumps can handle rainfall rates of 1.00" in the first hour and 0.50" in every subsequent hour after the rain stops. Rain that falls faster than that will cause flooding in spots around the city until the pumps can catch up with the excess water.

It's not just New Orleans, either. The excessive rainfall amounts forecast across Louisiana and throughout the lower Mississippi River Valley will cause extensive and life-threatening flash flooding across the region.

A scenario like this played out not too long ago. A disturbance over the northern Gulf of Mexico brought a widespread swath of 10"-20" of rain to central and eastern Louisiana back in the summer of 2016. The resulting flooding was some of the worst in Louisiana's modern history, killing more than a dozen people and causing more than $10 billion in damage. Many homes destroyed by the floods in 2016 didn't have flood insurance, as the owners thought they were safe from flooding.

Winds

Flooding is the greatest threat, but we can't ignore the winds. Hurricane warnings are in effect for the central Louisiana coast for the potential that Barry could strengthen into a hurricane before reaching land. In practical terms, though, the difference between a 70 MPH tropical storm and a 75 MPH hurricane is negligible.

Wind gusts above 70 MPH and soggy ground will allow trees and power lines to fall with relative ease. Widespread power outages are likely where the strongest part of the storm makes landfall. Strong winds will easily snap tree limbs and blow around small debris—stuff like trash cans and yard decorations.

Tornadoes

Tornadoes are always a threat in the right-front quadrant of any landfalling tropical system. Folks along and to the east of Tropical Storm Barry's track will stand the greatest threat for tornadoes. Tropical tornadoes are different from tornadoes you'd see in a "regular" thunderstorm. They can happen so quickly that forecasters can miss them between radar sweeps. Eastern Louisiana, Mississippi, and Alabama will have to be on the lookout for tornadoes as the storm makes its way inland.

[Satellite Image: NOAA]


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I graduated from the University of South Alabama in 2014 with a degree in political science and a minor in meteorology. I ran Gawker's The Vane for two years and I've contributed to Mental Floss, Forbes, Popular Science, and the Washington Post's Capital Weather Gang. I also teamed up with Outdoor Life to write a book called The Extreme Weather Survival Manual, which came out in October 2015.

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