Hurricane Ida has intensified over warm Gulf of Mexico waters, prompting tens of thousands to flee coastal areas, while US President Joe Biden pledged aid to help states quickly recover once the storm has passed.
Forecasters said Ida could make a US landfall on Sunday as an "extremely dangerous" Category 4 storm, generating winds of 220km/h, heavy downpours and a tidal surge that could plunge much of the Louisiana shoreline under several feet of water.
On Saturday afternoon Ida was about 386km southeast of the mouth of the Mississippi River, packing top winds of 169km/h and aiming for the Louisiana coast, the National Hurricane Center said.
"We're concerned about explosive development shortly before it makes landfall," said Jim Foerster, chief meteorologist at DTN, which provides weather advice to oil and transportation companies.
Flooding from Ida's storm surge - high water driven by the hurricane's winds - could reach between 3 and 4.5 metres around the mouth of the Mississippi River, with lower levels extending east along the adjacent coastlines of Mississippi and Alabama, the NHC said.
Officials ordered widespread evacuations of low-lying and coastal areas, jamming highways and leading some petrol stations to run dry as residents and vacationers fled the seashore.
"I left Fourchon last night at 8 o'clock and it's a ghost town," said Andre LeBlanc, a sportfishing captain speaking from his inland home in Lafayette, Louisiana. "We were some of the last to get out of there."
Utilities were bringing in extra crews and equipment to deal with expected power losses. Hundreds of thousands of homes could fall dark as Ida's strong winds carry well into Louisiana and as far east at Mobile, Alabama, said DTN's Foerster.
Biden on Saturday said 500 federal emergency response workers were in Texas and Louisiana to respond to the storm.
Aid workers have "closely coordinated with the electric utilities to restore power as soon as possible", Biden said at a briefing with Federal Emergency Management Agency officials.
Louisiana Governor John Bel Edwards, whose state is already reeling from a public health crisis stemming from a fourth wave of the COVID-19 pandemic, said Ida's winds will be fierce.
"We have a very serious situation on our hands," Edwards said at a briefing. "This will be one of the strongest hurricanes to hit anywhere in the state of Louisiana since at least the 1850s."
Australian Associated Press