Southern Louisiana is under water after three days of record-breaking rain. The historic and deadly flooding began to take shape Friday and continued through the weekend as wave after wave of moisture pummeled the Baton Rouge region.
The disaster was caused by two weather-related features — extreme humidity and near-stationary low pressure that hovered over the Gulf Coast for days.
Precipitable water — a measure of how much moisture is in the air over a certain location — was off the charts. Day after day, weather balloons relayed precipitable-water data that came close to or exceeded any other weather event on record in the region. On Friday morning, the precipitable-water reading was 2.8 inches.
“Obviously we are in record territory,” the National Weather Service wrote.
Moisture values this large increase the likelihood of major flooding when there’s a sufficient trigger mechanism, like a low pressure cyclone. However, much more water vapor can be injected into the air when the flow converges. This is why rainfall totals can be far greater than the total precipitable water measured by weather balloons.
The low pressure system that triggered the storms was not particularly strong, but it was more than enough to scour the moisture from the overly saturated atmosphere. The system had many of the same atmospheric features seen in tropical storms, and it sprawled across the Gulf Coast from Louisiana to the Florida Panhandle.
Satellite imagery depicts how thunderstorms blossomed day after day, pulsing as the sun came up and fading as it sank below the horizon. The massive area of low pressure crawled west and then seemed to stop and linger over Louisiana for three days.
Nine rivers crested at record levels as the water drained into the Louisiana Delta, but river height readings will be difficult to carry out amid the flooding. Many locations along the Amite River are measured manually, and the observers are likely to have evacuated. The National Weather Service is warning that rivers will experience a “long crest,” meaning they will stay in flood stage for days, not hours.
The Amite River levee system was built after the historic floods of April 1983. Now that system is being inundated with much more water than it was designed to withstand.
“The flood control system was designed to handle a recurrence of the 14.6-foot crest observed in that record  event,” Weather Underground’s Jeff Masters wrote. As of Monday morning, the Amite River at Port Vincent was at 17.5 feet.
East of Baton Rouge, the Amite River at Denham Springs crested at 46.2 feet on Sunday, which breaks the record of 41.5 feet on March 8, 1983. Records at this location date back to 1921.
The rain has tapered off since the weekend in southern Louisiana, but the flooding will continue for days. Backwater flooding, which occurs when water backs upstream because of blockage downstream, could occur well away fay from the main rivers.
“This event remains in full swing,” the National Weather Service wrote on Monday morning.