The flooding that has devastated Baton Rouge and the surrounding parishes was caused by a rare weather phenomenon that could become increasingly common, analysts say.
It was largely the product of extremely warm, moist air in the Gulf of Mexico colliding with a slow-moving storm system, which drenched a swath of Louisiana over a several-day stretch.
In a way, it resembled a hurricane, even if it didn't have a name or the battering winds usually associated with one, said Barry Keim, the state climatologist. "This is basically like a hurricane in its infancy," Keim said.
While there wasn't the dramatic build-up that comes with watching a named storm cross the Gulf and there was no one-off phenomenon like El Niño to point to as the perpetrator, the weather system caused havoc nonetheless.
In some areas the rainfall was enough to qualify as a 500-year or 1,000-year event, something that has only a 0.2 percent or 0.1 percent chance of happening in a given year.
Experts caution that it may be too early to directly link the storms with climate change. But rising temperatures and a warmer Gulf of Mexico could mean more moisture in the air and more weather systems like the one that ravaged the state.
"When it comes to the climate change question, incidents of extreme weather such as floods are projected to increase. However, no single event can be directly connected to climate change," said Alek Krautmann, a meteorologist with the National Weather Service office in Slidell.
Heavy storms have become increasingly common across the country over the past half-century due to similar changes in conditions, said David Easterling, with the National Oceanic and Atmospheric Administration's National Centers for Environmental Information.
"We've seen increases in water vapor over the past 35 or 40 years and seen an increase in the rainfall amounts since the middle of the 20th century," Easterling said. "It's completely consistent with what we expect to see with a warming climate."
Last week, a very large and slow-moving low-pressure system began drifting westward across the northern Gulf of Mexico, where the air was exceptionally saturated with moisture, Krautmann said.
Warm, wet air is not uncommon along the Gulf Coast in August, but last week it was primed for serious storms. The amount of moisture in the air was almost twice the amount that is typically seen in August, Krautmann said.
That meant there was an incredible potential for rain, which was unleashed when the system moved through.
"There was a huge amount of water vapor. A lot of that was coming off the Gulf -- a lot of evaporation, a lot of that is warm water -- and that feeds right into that very slow-moving storm that you guys had there," Easterling said. "The storm was unusual, a very slow-moving storm, and it had the ability to dump a heck of a lot of rain."
Because of circulation in the storm, it was able to keep drawing on the moisture from the Gulf, replenishing itself to dump more rain on the state, Keim said.
The totals are staggering. Watson saw about 31.4 inches of rain in three days, Livingston took on nearly 22 inches of water, and Baker recorded more than 21 inches.
Louisiana's previous record rain total over a 48-hour period was a storm in Abita Springs in 1995, when almost 25 inches of rain fell.
With two weeks still to go in August, the month already has been the third wettest for Baton Rouge since officials began keeping records in 1893; almost all of that was due to the weekend's storms.
Of the 22.1 inches of rain recorded at the Baton Rouge airport this month, all but about half an inch fell between Thursday and Sunday. That amounts to about four times the average amount the parish usually sees in August over a period of several days.
Even though rivers and other waterways in the area were at some of their lowest points of the year before the storm, the rainfall was enough to overwhelm them.
Including last week's storms, Baton Rouge has seen about 70 inches of rain so far this year; it typically gets about 60 inches over the course of an entire year, Keim said. Particularly because there were smaller storms just before the flooding, that meant much of the ground was saturated and unable to absorb more water, he said.
Before and after: Photos show how water has receded in many flooded south Louisiana areas
Aerial photos taken over the past few day have shown devastating flooding across areas of south Louisiana. Photographer Bill Feig returned to many flooded locations Tuesday to show how the water has receded.
The flooding has more in common with other coastal floods, such as the ones in South Carolina last year, than with the March deluge that inundated much of the state.
"If you take the event in Louisiana, and the event in South Carolina last year, those events stand out because you had huge amounts of water vapor coming off the ocean," Easterling said.
Keim said it might be impossible to determine exactly why the nation has seen extreme weather this year, including the Louisiana floods earlier this year and major flooding in Texas and West Virginia. Tying those storms directly to a changing climate is even more difficult. He said there were a lot of major rainfall events in the 1930s and 1950s, and this may just be a similar pattern.
"These are huge, catastrophic events, most of them," Keim said. "The big question is what's going on."