Review of Historic Flooding in Louisiana

Posted: August 24, 2016, 12:21 am by abarney
Courtesy of U.S Department of Agriculture

A multi-billion dollar natural disaster unfolded last week in Louisiana, but it wasn’t because of a tropical storm or hurricane but rather an unnamed, stationary system that poured prolific amounts of rainfall upon southern parts of the state. It’s being called the worst U.S. flooding disaster since Hurricane Sandy, and put more rain down than did either Hurricane Katrina or Rita (keep in mind, it was the storm surge that made Katrina and Rita so devastating).

The radar loop above shows the relentless heavy rain and thunderstorms that pounded southern Louisana from August 11 to 13, 2016. During the three day stretch, rainfall totaled up to between 10  - 20 inches. The map below depicts observed precipitation – with areas in grey ending up over 20 inches! Astoundingly, Watson, LA recorded 31.39" of rain between August 10 - 13, 2016. To put this into perspective, those affect by Katrina and Rita in 2005 received between 10 and 15 inches of rain.


A map showing 7-day precipitation totals ending August 16, 2016. This map encompasses much of the 3-day event that produced floods in Louisiana.


The widespread heavy rainfall led to major river flooding submerging many towns (you can use this interactive map created by NOAA to get a visualization of the flooding). In fact, eight rivers rose into record territory, including the Amite and Comite rivers with the Amite River cresting nearly 5 feet above its previous record. An estimated 110,000 homes have been damaged or destroyed from the flooding, and thousands have been displaced. Many were also stranded, both in their homes and in vehicles. Additionally, 13 people lost their lives as a result of the incredible floods. 


An aerial image taken aboard a Coast Guard helicopter showing the extent of flooding in an area near Baton Rouge, LA. (Credit: U.S. Department of Agriculture)


This event was a rarity on its own – it’s not too often Louisana sees flooding of this caliber from a system that isn’t a tropical cyclone. Such extreme flooding events typically are more isolated, just like the ones seen in the Northeast U.S. last month. However, the areal coverage of the flooding in Louisiana was large, covering over 90 miles of real estate. In fact, the National Weather Service’s Hydrometeorological Design Studies Center provided an analysis of the flooding event to illustrate its return frequency which is typically used for planning purposes. As it turns out, the event ranks as a 1 in 1000 year event for those around the Baton Rouge area meaning the area has a probability of less than 0.1% to see an event that produces rainfall in excess of 20 inches in any 48 hour period! 

NOAA/NWS map depicting the Annual Exceedence Probabiliities for the LA floods. Areas in dark blue were determined to have seen rain with a chance of occurrence less than 0.1% (or less than once in every 1,000 years).


After enduring such historic rainfall, we will have to keep a close eye on the tropics as we head into September and the peak of hurricane season. It will take weeks, if not months, for these areas to recover and any additional tropical impacts will likely only lead to further devastation.

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