New Orleans residents stocked up on supplies and prepared their homes as a strengthening Tropical Storm Barry threatened severe rain and flooding when it comes ashore as possibly the first Atlantic hurricane of 2019.
The storm was forecast to bring torrential rains of up to 64cm in certain places.
This could cause life-threatening flooding along the Mississippi River, which has been running at flood stage for months, officials warned.
Residents were ordered to evacuate some nearby areas, but the New Orleans mayor said no evacuations were ordered from the low-lying city.
US President Donald Trump declared a state of emergency for Louisiana and the region's oil production was cut in half as energy companies evacuated offshore drilling facilities.
Barry packed maximum sustained winds of 100km/h this morning and was centred 160km southwest of the mouth of the Mississippi River.
Barry will likely strengthen into a hurricane, the National Hurricane Center said, with winds of at least 119km/h by the time it reaches the central Louisiana coast tomorrow.
It was forecast to bring a coastal storm surge into the mouth of the lower Mississippi River that winds through the heart of New Orleans, pushing its crest to 5.79m tomorrow.
That would be the highest since 1950 and dangerously close to the top of the city's levees.
New Orleans is already saturated after torrential rains flooded streets on Wednesday.
Memories of Hurricane Katrina in 2005, which flooded much of the city and killed 1,800 people, are deeply embedded in New Orleans.
The brunt of Barry is expected to skirt the western edge of New Orleans, avoiding a direct hit.
Mayor LaToya Cantrell said the city has not ordered any voluntary or mandatory evacuations.
But she added that 48 hours of heavy downpours could overwhelm pumps designed to purge streets and storm drains of excess water.
"There is no system in the world that can handle that amount of rainfall in such a short period," Ms Cantrell said on Twitter.
As hazardous conditions arrive along the Gulf Coast today with #Barry, get information on local weather and impacts from @NWSNewOrleans @NWSLakeCharles @NWSMobile @NWSJacksonMS pic.twitter.com/XRl4EiBHoN— National Hurricane Center (@NHC_Atlantic) July 12, 2019
Louisiana Governor John Bel Edwards warned: "The more information we get, the more concerned we are that this is going to be an extreme rain event."
Officials from the US Army Corps of Engineers, which maintains the levees, insisted that no significant breaching of the levees in New Orleans was likely.
Mandatory evacuation orders were issued for areas of Plaquemines Parish beyond the levees southeast of the city, and for low-lying communities in Jefferson Parish, to the southwest.
Barry has shut more than one million barrels of offshore oil production and the coastal evacuation orders forced one refinery to halt operations.