Rainfall outlook for this afternoon in southwest and south-central Louisiana. (National Weather Service)

Tropical depression Barry is finally unloading a dangerous torrent of flooding rain in parts of Louisiana on Monday, arriving later than expected.

Rainfall totals have topped 15 inches in parts of southwest and south-central Louisiana, a lot of it falling since early Monday morning, and flash flooding warnings are flying. This area is in a high-risk zone for flash flooding through Monday night that extends into Mississippi.

“The event is still unfolding,” said David R. Novak, director of the National Weather Service’s Weather Prediction Center. “Radar estimates of Barry’s … total rainfall are approaching 20 inches in parts of Louisiana.”

Since late last week, forecasters said Barry would be a drawn-out event, with heavy rain a threat lasting two to three days. But the worst was expected over the weekend, when meteorologists predicted tremendous amounts of rain in Louisiana. Rivers were forecast to surge to historic heights.

In New Orleans, the combination of excessive rainfall flowing into the Mississippi River and a surge of ocean water from the Gulf of Mexico was predicted to raise the river level to the highest crest since at least 1950. Forecasters feared the predicted crest of 20 feet would test protective levees along the river near that height.

But the worst of the rain and surge eluded the Big Easy as the storm was slow to develop, and the river crested at just 17 feet.

Why the weekend rain was delayed

In much of Louisiana, the deluge from the former hurricane didn’t happen in the anticipated time frame.

Instead of widespread rainfall totals of 10 to 15 inches in southern and eastern Louisiana, with localized totals up to 25 inches, amounts were mostly in the two- to seven-inch range through Sunday night, with some isolated amounts (estimated by radar) around 10 to 12 inches.

It turns out that dry air perched over the Pelican State ate away at the plume of moisture attempting to barge inland.

“One piece of the puzzle is the strong northerly winds that have impeded Barry from the outset,” wrote Bob Henson and Jeff Masters at Weather Underground.

These winds cast dry air into the thunderstorms feeding onshore, decreasing the intensity of their rainfall. The moisture-starved continental air flowing into southern Louisiana could be seen on water vapor satellite imagery in shades of bright yellow.

The northerly winds also caused Barry to become lopsided, with its storm center divorced from the deeper moisture over the Gulf of Mexico. This slowed Barry’s intensification and meant that when its storm center came onshore Saturday, most of the heavy rain was still over the gulf.

The lack of rain over the weekend was a relief in cities such as New Orleans and Baton Rouge, where forecasts Thursday and Friday called for double-digit totals.

“We absolutely made it through the storm. Beyond lucky, we were spared,” New Orleans Mayor LaToya Cantrell (D) said in a public briefing. “As those [rain] bands moved closer to New Orleans, it just seemed to go around us.”

New Orleans received just one to three inches of rain Friday through Sunday, while Baton Rouge was somewhat soggier, registering over four inches.

Barry is finally delivering

Despite the underwhelming weekend totals, extremely moist tropical air has slowly but surely displaced the dry air lodged over Louisiana, and the intensity of rainfall has crept upward.

A zone of excessive rainfall developed between southwest and east-central Louisiana during the predawn hours Monday, dumping 10 to 15 inches of rain across Avoyelles, Evangeline, Allen, Beauregard and Calcasieu parishes (this is approximately the zone from Lake Charles to Natchez). Fourteen inches were reported near Ragley, which is about 30 miles north of Lake Charles.

“Dangerous, significant flash flooding is likely, especially across south-central Louisiana,” the Weather Service wrote in a special statement Monday morning. It is predicting up to five to eight inches more of rain in this area through the afternoon and rainfall rates as high as two to four inches per hour.

Ultimately, the National Weather Service’s forecast for up to 25 inches of rain from Barry could end up materializing, just on a slightly different schedule than initially predicted and farther to the west.

“While rainfall totals were less than forecast in some areas, other locations are on track to meet forecast expectations with ongoing heavy rain,” Novak said.

The challenging forecast for Barry shines a light on how difficult it can be for forecast models to nail down the specifics of the placement and timing of extreme rainfall days in advance.

“We … need to acknowledge abysmal rain forecasts for Saturday,” tweeted Steve Caparotta, a meteorologist in Baton Rouge. “Even as forecast accuracy has grown by leaps & bounds in recent years, still much room for improvement.”

While the precipitation forecasts can struggle, forecast models have proved to be quite skilled at identifying where tropical weather systems will develop and in predictions about their tracks.

The models accurately simulated that an area of disturbed weather over the Midwest would drop into the Gulf of Mexico and develop into a tropical cyclone up to eight days ahead of time.

The worst of Barry’s rain should finally taper off in Louisiana late Monday, but flooding rain will remain a risk farther north and northeast through Tuesday.

“It’s important to highlight portions of Louisiana, Mississippi, Arkansas and western Tennessee remain at risk for flash flooding,” Novak said.