- Video report by ITV News Washington Correspondent Robert Moore
Tropical Storm Barry is expected to make landfall over Louisiana within the next few hours.
Officials have warned there is the possibility of heavy, life-threatening flooding in the area. Around a foot and a half of rain is expected in certain areas.
Around 10,000 people in the low lying areas along the Gulf Coast are expected to be effected.
The slow-moving tropical storm has the potential to strengthen to hurricane status.
National Guard troops and rescue crews are posted around the state with boats, high-water vehicles and helicopters.
Utility repair crews are in position and homeowners have sandbagged their property or packed up and left.
Tourists have crowded New Orleans’ airport in the hope of catching an early flight away from the storm.
- ITV News Correspondent Emma Murphy reports from beside one of the levees authorities are hoping will hold back rising water:
Residents, who are stockpiling supplies and bolstering defences at home, have been warned.
"There are three ways that Louisiana can flood: storm surge, high rivers and rain," Louisiana governor John Bel Edwards said. "We're going to have all three."
He warned of a dangerous combination with the already-high Mississippi River, which has been swelled by heavy rain and snowmelt upriver this spring.
National Hurricane Centre director Ken Graham warned: “This is happening. … Your preparedness window is shrinking.
"It’s powerful. It’s strengthening. And water is going to be a big issue.”
Forecasters said slow-moving Barry could unload 10 to 20 inches of rain through Sunday across a part of Louisiana that includes New Orleans and Baton Rouge, as well as southwestern Mississippi, with pockets in Louisiana getting 25 inches.
Louisiana is no stranger to devastation caused by severe weather: The state was all but destroyed when Hurricane Katrina hit in 2005, killing 1,800 people across America's south.
In New Orleans, where days ago houses were damaged by a smaller storm, Barry's heavy rain and high-speed winds will be a severe test for the city's post-Katrina improved defences.
Despite a multibillion-dollar hurricane-protection system, which includes more than 70 pump stations that remove floodwaters, the city mayor isn't confident the defences will cope.
Mayor LaToya Cantrell said the pumping system that drains the city's streets is working as designed but Barry could dump water faster than the pumps can move it.
"We cannot pump our way out of the water levels and the water falls that are expected to hit the city of New Orleans," she warned, "be prepared to shelter."
President Donald Trump has declared a federal emergency for Louisiana, authorising the Department of Homeland Security and Federal Emergency Management Agency to coordinate all disaster relief efforts.
As a result National Guard troops and rescue crews are stationed around the state with boats and high-water vehicles.
Helicopters are also on standby, and supplies including drinking water and blankets are ready for distribution, the Guard said.
"So here's the takeaway: Dangerous situation," said National Hurricane Center director Ken Graham. "That kind of rainfall in this system could cause flash flooding, cause ponding of water."
Forecasters say while their models don't "explicitly show Barry becoming a hurricane, it is still possible for that to occur before landfall."