Tremendous downpours have inundated parts of Louisiana over the last few days, resulting in disastrous flooding and forcing thousands of people from their homes. But what’s causing this historic flooding in areas that rarely receive such significant rainfall days?
An “inland sheared tropical depression” is how the National Oceanic and Atmospheric Administration’s National Weather Service (NWS) described the heavy rain event on Friday morning (Aug. 12). The NWS also noted that the moisture content in the atmosphere was close to an all-time record for the area, even higher than observations during some tropical cyclones.
“Like a tropical depression, the low had a warm core, and the counterclockwise flow of air around the storm brought huge amounts of tropical moisture from the near-record-warm waters of the Gulf of Mexico and northwest Atlantic northwards over land,” meteorologists Jeff Masters and Bob Henson wrote on their blog, Weather Underground.
“The amount of moisture in the atmosphere over the Gulf Coast region over the past week has been nothing short of phenomenal,” they wrote.
A combination of tropical moisture and low pressure fueled the immense rainfall in Louisiana and southwest Mississippi, the meteorologists said. When so much moisture is in the atmosphere, storms can produce rainfall of several inches in a single hour, resulting in astronomical totals over time, the meteorologists said.
In his analysis for Pacific Standard, meteorologist Eric Holthaus noted the rarity of such a significant amount of rainfall.
“By midmorning on Friday, more than a foot [more than 30 centimeters] of rain had fallen near Kentwood, Louisiana, in just a 12-hour stretch — a downpour with an estimated likelihood of just once every 500 years, and roughly three months’ worth of rainfall during a typical hurricane season,” Holthaus wrote. Such an event is known as a 500-year rainfall.
While, historically, such rain events are incredibly rare, this is at least the eighth 500-year (or rarer) rainfall event in America since just last May, Holthaus said.