Hurricane Barry is poised to make landfall in Louisiana as the first Atlantic hurricane of 2019 in a westward shift that appeared to spare low-lying New Orleans from massive flooding.
The National Hurricane Center says Barry became a hurricane with maximum sustained winds of 120 kilometres per hour on Saturday morning, adding that the storm is likely to weaken.
"Barry should quickly weaken below hurricane strength as it moves onshore, and subsequently it is forecast to weaken below tropical storm strength between 24-36 hours," the centre said.
The storm could still bring dangerous rainfall flooding and storm surges 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 on Friday night at just under 5.2 metres in New Orleans, the National Weather Service said.
It was much lower than a prediction of six metres earlier this week, which would have approached the height of the levees.
The river was expected to surge again on Monday at about 5.2m, up slightly because of the expected rains, the weather service said.
Predictions for when the centre of the storm would make landfall were pushed back from sunrise to late morning or early afternoon as Barry crawled across the Gulf Coast at about 5km/h, National Hurricane Center forecasters 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.
Forecasters warned that life-threatening, significant flash flooding and river flooding will become increasingly likely later on Saturday and overnight, especially across portions of south-central and southeast Louisiana into Mississippi.
Authorities had urged New Orleans residents to secure property, stock up on provisions and take shelter.
Some nervous residents opted to flee the city and tourism officials reported an abrupt exodus of out-of-town visitors on Friday.
Mandatory evacuations were ordered in outlying coastal areas beyond the protection of levees in neighbouring Plaquemines and Jefferson parishes, south of the city.
Rain bands were already hitting the coast before sunrise, and more than 62,000 homes and businesses in Louisiana were without power at 7am local time on Saturday, according to tracking site PowerOutage.us.
US President Donald Trump declared a state of emergency for Louisiana on Friday, freeing up federal disaster assistance if needed.
Barry was seen as a test of beefed-up flood defences put in place since Hurricane Katrina, which left much of New Orleans underwater and killed about 1800 people in 2005.