Hurricane Barry has made landfall in Louisiana and weakened to a tropical storm.

The storm has moved westward shift and appears to have spare low-lying New Orleans from the massive flooding feared earlier this week.

The National Hurricane Center, which hours earlier said Barry had become the first Atlantic hurricane of 2019, said the storm slowed as it came ashore near Intracoastal City, Louisiana, with maximum sustained winds of 115 kilometres per hour.

"Weakening is expected as Barry moves farther inland, and it is forecast to weaken to a tropical depression on Sunday," the NHC said in its latest advisory.

The storm could still bring dangerous rainfall flooding and storm surge to coastal regions southwest of New Orleans and to Baton Rouge and Lafayette.

But the threat of major flooding from the historically high Mississippi River overtopping levees appeared to have passed.

The river crested last night at just under 5.18 metres in New Orleans, the National Weather Service said, much lower than a prediction of 6.1m earlier this week, which would have approached the height of the levees.

The river was expected to surge again on Monday, the weather service said.

The "lopsided" nature of the storm meant most of the rain was expected after landfall and its slow speed increased the risk of flooding, the weather service said.

Life-threatening, significant flash flooding and river flooding will become increasingly likely later today and overnight, especially across portions of south-central and southeast Louisiana into Mississippi, forecasters said.

The Mayor of New Orleans, LaToya Cantrell, had warned 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.

New Orleans was already saturated after thunderstorms drenched it with a foot of rain on Wednesday.

While street flooding seemed probable, the US Army Corps of Engineers insisted that significant breaching of the levees in New Orleans was unlikely.

Levee floodgates normally left open to allow passage of traffic were being closed, along with a giant ocean surge barrier erected after Katrina.

The level of Lake Pontchartrain, an estuary on the city's northern flank, rose by almost a metre yesterday, triggering closure of a floodgate on a drainage canal that breached during Katrina, officials said.