Total three-day precipitation for 12-14 August 2016. Black box shows the geographic region, described as the Central U.S. Gulf Coast region, used to calculate extremes (29º-31ºN, 85º-95ºW). Data was taken from the 'real time' version of the NOAA Climate Prediction Center (CPC) unified gauge-based analysis of daily precipitation over the contiguous United States. Graphic: Climate Central

August 2016 (Climate Central) - Torrential rains drenched south Louisiana in mid August, with parts of the state receiving nearly 30 inches of rain from August 10 to the 17. The state capital, Baton Rouge, suffered through nearly a foot of rain on a single day, August 12, and nearly as much the day after. Historic flooding of the state claimed at least 13 lives and damaged more than 60,000 homes. Rescue teams saved  30,000 people from the floodwaters and relocated displaced residents to drier grounds. Among the evacuees were Louisiana governor John Bel Edwards and his family, who sought shelter when rainwater flooded the governor’s mansion in Baton Rouge. The federal government declared a dozen parishes major disaster areas. The Red Cross deemed the flooding in Louisiana the worst natural disaster in the United States since Superstorm Sandy slammed into the U.S. East Coast in 2012. The extreme nature of this event left many asking whether climate change may have played a role.

On August 12, NOAA’s Weather Prediction Center described the heavy rain event as “an inland sheared tropical depression.” The event bore some of the same characteristics of a tropical depression: very slow-moving low pressure system combined with extreme tropical moisture.  The subsequent historic rains and floods that paralyzed Louisiana and other parts of the Gulf Coast resulted largely from a combination of these two conditions.

Scattered thunderstorms began popping up in parts of Louisiana on August 9 with the storm system forming to the east, off the Florida Panhandle. As the slow-moving storm crept west, it sucked up tremendous amounts of tropical moisture from the Gulf of Mexico. Downpours increased as the storm moved closer to Louisiana. Regions in Louisiana and nearby Mississippi received more than six inches of rain on Thursday, August 11. The next day a continuous downpour soaked Baton Rouge, submerging the state capital in 11.24 inches of rain.  Even greater amounts came down in other parts of the state.  On August 13, the nearly stationary area of low pressure continued to wring moisture from the Gulf, dumping more than 10 inches on sodden areas of Louisiana. At the close of the weekend, August 14, scores of cities in the region could measure total rainfall in feet. The unnamed storm would eventually shower Louisiana with an estimated 7.1 trillion gallons of water —  more rain than Hurricane Isaac in 2012, and three times as much rain as Hurricane Katrina in 2005.

With the deluge came catastrophic inland flash flooding and river flooding.  More than a half-dozen rivers in southeast Louisiana broke records, not just by inches but by feet.  The Amite River in Magnolia, Louisiana crested at 58.56 feet, more than six feet above the old mark established on 27 April 1977.

Figure 1 shows the total three-day rainfall which occurred from August 12 to the 14. This quantity represents the meteorological variable that best captures the event. [more]

Louisiana Downpours, August 2016

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