Dig Baton Rouge

After the Storm: State climatologist explains why south Louisiana sustained disastrous flooding

As the people of Louisiana struggle to recover from the disastrous flooding caused by August’s heavy rainstorms, much of the state is left to wonder how this happened. In an age of highly advanced weather tracking technology and flood-resistant infrastructure, how did this storm sneak under the radar to hit the state so hard? Even the state’s top climate scientists aren’t completely sure.
“We’re going to be studying this storm for some time to come,” said Barry Keim, Louisiana’s state climatologist who works as a geography and disaster science professor at LSU. DIG talked to Keim about some of the conditions that created the storm, why the flooding got so bad and what Louisiana can expect from such storms in the future.

DIG: Was there anything unusual about the conditions that created this storm?
Keim: I can trace this storm all the way back to August 3rd as a very weak tropical disturbance that formed off the Big Bend area of Florida, up by [the Appalachicola river], in that general direction…. We get about a hundred of these easterly waves or tropical waves every year, that form in the Atlantic or Caribbean, and we get a few in the Gulf. Those are the kinds of storm systems that can grow into tropical depressions, storms and eventually hurricanes. So it’s kind of like a baby hurricane, if you will. It’s a hurricane in its infancy before it has any serious wind involved in it. And that’s basically what this storm was, was a little tropical disturbance that on the surface was seemingly innocuous in appearance, in fact if I saw this feature on a weather map wafting into Louisiana, I would have just guessed it would mean increased thunderstorm activity a little bit and not many people would even notice it happened.

DIG: But still, it created this very unusual amount of rain.
Keim: This storm just had this uncanny ability to entrain moisture off the Gulf of Mexico. Apparently sea surface temperatures are running high and humidity levels, what we call dew points, were also running extremely high, so atmospheric moisture as humidity was being entrained into the storm system. Apparently this thing was really able to tap that moisture and pull it into the storm in a very efficient way and then convert that moisture into rainfall and put it onto the ground…We get Gulf tropical disturbances here, but they don’t typically produce this kind of rain, they just enhance thunderstorm activity a little bit. There was something unique about this specific tropical disturbance that made it special, and it was able to do these things with the moisture and produce this much rain.

DIG: It seems like it stayed in one place for a really long time. Is that out of the ordinary for a storm like this?
Keim: The storm started on the 3rd, didn’t move out of here until Saturday or Sunday, the 13th or 14th. To have it move just that short distance, from the Big Bend area of Florida to the panhandle and eventually move on over to us, that’s kind of a long life cycle for a storm like this to not just fizzle out. It was kind of caught between two systems, one to the east and one to the west, and it was kind of sandwiched between them without a steering current to move it away from us. That was one of the main problems. The real prolific rain-generating storms tend to just sit in one area. We’ve had a lot of hurricanes that are very efficient rainfall producers as well, but if they’re whipping around at 15 or 20 miles per hour, they don’t hang around in one place long enough to leave a lot of rain in one spot. This one got up here, parked in our area for a few days and was able to continuously pull moisture from the Gulf and drop it as rain….And this rain was not a terribly intense event. It was kind of like moderate rain for an extensive period.

DIG: What landscape characteristics of the areas that flooded contributed to the flooding?
Keim: We have several river systems that drain this part of Louisiana. And of course you have to realize none of the water that falls here makes it into the Mississippi River—the Mississippi and the levees around it basically are like a giant dam that create some pretty unique hydrological units across Louisiana. All the rain that lands here in Baton Rouge has to find its way out of here via the river systems—including the Comite, the Amite, the Tangipahoa and the Chafunkta—and all that water has to work its way down either to Lake Maurepas or Lake Ponchartrain… But those rivers were just ill-prepared, they just didn’t have the capacity to cope with events this big.
You know, the floods of record were back in 1983. If you didn’t flood in 1983 and haven’t flooded since, you have 33 years of time that’s passed. And if you haven’t flooded in your lifetime and didn’t flood in ‘83, you probably felt you were never likely to flood. But we’ve never seen the likes of a rainstorm like this across such a broad, vast area. It just completely overwhelmed the natural and artificial drainage networks we have in place.

DIG: Do you see the flooding and results of it as more of a failure of infrastructure and planning, or was this storm just such a freak incident that we never reasonably could have predicted it?
Keim: Well, I think it was just a freak [incident]. The [National] Weather Service, I think, did a good job letting people know a big storm was in the making. Now, the problem with this storm, since it was what I would call a tropical wave, it wasn’t a named storm. When you have a pretty innocuous-looking wave coming wafting into your area, it doesn’t command a lot of attention—certainly not the attention a named storm would get. When you have a name for the storm, it really takes on a whole new dimension that perks their ears up when they hear about it. The Weather Service did what they could. They were forecasting heavy rains to the tune of, my recollection is something like five to eight inches. Five to eight inches is already a heck of a lot of rain.

DIG: At what point did you realize this was becoming a disaster?
Keim: I was getting that sense by about Friday afternoon, even Friday morning, with the rains overnight. I went driving around in the morning just to see how much flooding had started. Some of the spots that I [know] historically flood during smaller rain events, those areas were already underwater. This was Friday morning. We still had a long way to go. By Saturday morning, when it finally started to die down, I went out, and really saw some things I’d never seen before… One area I saw was on Dalrymple, with the two lakes on either side of Dalrymple, over by University Lake. The two lakes had actually overrun the highway. I had never seen that before.

DIG: In terms of this hurricane season and those coming in the future, what lessons should we learn?
Keim: We always need to be vigilant in south Louisiana, and I’m always amazed at how many people don’t have flood insurance here. My opinion is that if you live in south Louisiana, with very few exceptions, I think everyone needs flood insurance here.

DIG: On the other hand, if you made it through this storm and didn’t flood, you must be feeling pretty confident nothing will cause you to flood.
Keim: Yeah, well you know, I didn’t flood in this event, and I have no intentions of giving up my flood insurance. If you live here, you just never know what could get dropped on your house.

DIG: In this period while there is still some flooding and water levels in the rivers are still high, what hazards should we expect?
Keim: All soils are completely saturated for starters, so any rain that falls, it’s like landing on a blacktop parking lot. The water has nowhere to go and there’s no infiltration into the soil, so it all just runs off the surface… Even before this storm, we’ve been in kind of an unsettled pattern. We get thunderstorms here in the summer, that’s common. But rain coverage has been a little heavier and rainfall totals have been a little higher. Now we’ve had this big event, but we’re still in this unsettled pattern… The other thing is that the rivers are still draining very slow, because it’s just kind of a logjam. Water levels are still high in the Amite and other channels, and it’s just kind of causing drainage to back up throughout the system.

DIG: Looking past this year and into hurricane seasons in the future, is there any reason we should expect events like this to be more common in the future?
Keim: There is some thinking with climate change, that with warmer sea surface temperatures we could ultimately entrain more moisture in the atmosphere, which could ultimately produce bigger rainstorms. Some think that this storm here may be an artifact of that, and that may well be. I would argue that we’re really not in a strong position to say that with any level of certainty. It certainly fits the larger pattern and way of thinking with climate change, that we could see more and bigger storms, but personally, I would never attribute any single event to climate change. There’s so many variables that have to come together to make a storm like this, I would argue that any impact of climate change is a very small component of that, and all these other components would have come together regardless.

Photo by Sean Gasser.

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