South of Tallahassee over the Gulf of Mexico, a storm is forming. On Wednesday, the system (known now as Invest 92L) is projected to evolve into a depression and then a tropical storm or hurricane (named Barry) before heading west toward Louisiana and Texas.

If it proceeds according to the models, Barry will create an unprecedented situation in New Orleans—a cyclone-driven storm surge of two to three feet up a river running very high, lifting the water in the Mississippi River nearly to the top of the levees. At 19 feet above sea level, it would be the highest crest for the river in New Orleans since 1950.

“Right now 19 feet is the official forecast, and we can manage that,” said David Ramirez, the chief of water management for the Army Corps of Engineers’ New Orleans District. His team will be looking out for low points in the levees, junctures with navigation structures, and other fragile points. “The levees protect the city up to 20 feet, but 19 is close and doesn’t include waves splashing up and so on. It’s too close for comfort for us. And that surge could be more or could be less. If things change and it gets higher, at some point, there’s only so much we can do.”

“We have had high-water events in hurricane season but we’ve never had an elevation forecast like this,” Corps spokesman Ricky Boyett told Nola.com on Tuesday.

NEW: A new forecast calls for the MS River at New Orleans to crest at 19 feet, or within a foot of the top of the levees. This due to storm surge. Coordinated by @NWSNewOrleans, NHC and the Corps, but this is still highly uncertain & dependent on track/intensity. #LAwx pic.twitter.com/ThJCW1v7bm — Steve Caparotta, Ph.D. (@SteveWAFB) July 9, 2019

This has not typically been a problem for New Orleans during hurricanes, which tend to arrive late in the summer when the water in the river is low. Over the past 50 years, five storms have sent seven-foot-plus surges up the Mississippi. Most came at low water. If a surge that big hit today, the river would overspill its banks and flood the city.

Hurricane Katrina, for example, sent 13 feet of surge up the Mississippi, but the river was at just 3 feet above sea level before the storm. Today it is at 16 feet. Katrina’s damage came from the north, and that’s where the bulk of the reinforcements have been made since the catastrophic levee failures of September 2005 killed more than 1,400 people.

Fortunately, Barry will not be a Katrina-sized storm. Still, it poses a test. The river levees are built to withstand storm surge, but on the assumption that tropical storms would only coincide with a river eight feet high, half its current height. With the river above 16 feet, even a small surge is a frightening possibility.

As I wrote last month, the Mississippi River in New Orleans has never been so high for so long, which poses risks to infrastructure and navigation. At the time, I thought, the odds of a hurricane storm surge coinciding with the lingering high water were slim. The subhead of my story asked a provocative question: “When will the Mississippi River come for New Orleans?” The answer, it turns out, might be this weekend.

Why is the river so high? In part because of years of intense civil engineering work that has penned in its course, slowing its path to the Gulf, stacking up sediment, and eliminating natural floodplains. Climate change is also likely a factor in the exceptionally rainy year further north that has engorged the river.

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Already this year, the elaborate system of water control has been tested: The Bonnet Carré Spillway, the last chance to relieve pressure before the Mississippi hits New Orleans, has been opened twice this year (for the first time) and in consecutive years (for the first time).

That the river is only at 16 feet in New Orleans now is thanks to Bonnet Carré, which is dumping several rivers worth of fresh water (at great environmental cost) into the Gulf of Mississippi every day.

It’s already too late to try to use the spillway to lower the river levels downstream ahead of the storm. In April, Army Corps spokesman Rene Poche told Nola.com that the engineers would be unlikely to be able to use Bonnet Carré to drop the river levels fast enough, given the length of a typical forecast and the speed of the river. Also, doing so could have unintended effects, like raising the water levels in Lake Pontchartrain, which borders New Orleans on the north, creating a whole new problem.

For New Orleans, the hope is that whatever’s brewing in the Gulf stays far away from the mouth of the Mississippi River—and that any storms after this one kindly wait until the river level starts to recede in August.

This article has been updated with new information.