Forecasters said it could make a US landfall as an “extremely dangerous” Category 4 storm on the five-step Saffir-Simpson scale, generating winds of 225 kilometres an hour, heavy downpours and a tidal surge that could plunge much of the Louisiana shoreline under water.
Ida's pace had accelerated overnight and carried top winds approaching 140kph as it headed north-west, the National Hurricane Centre said on Saturday morning.
The storm will continue to intensify rapidly over Gulf waters before coming ashore late on Sunday.
Flooding from Ida's storm surge – high water driven by the hurricane's winds – could reach between three 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, leading petrol stations to run dry and massive queues to form on highways leading from the shore.
Forecasters expect coming winds will cut power to hundreds of thousands of homes in the area.
Lifelong Gulf resident Hailey DeLaune, 29, said she and her partner spent Friday evening boarding up the windows of his house in Gulfport, Mississippi, and gathering provisions to ride out the storm.
“Hurricanes have always been part of my life,” said the high school teacher, who was born during 1992's Category 5 Hurricane Andrew.
“You just run through your list and hope for the best.”
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, urged residents to ready themselves for the hurricane.
“Now is the time to finish your preparations,” he told a Friday news conference. “By nightfall tomorrow night, you need to be where you intend to ride out the storm.”
Mr Edwards declared a state of emergency and US President Joe Biden issued a pre-landfall federal emergency declaration at the Louisiana governor's request.
It authorised the US Department of Homeland Security and the Federal Emergency Management Agency to co-ordinate disaster relief efforts in the state.
Mr Edwards also said he had authorised the activation of all 5,000 troops in the Louisiana National Guard for emergency deployments as needed.
On Friday, Ida smashed into Cuba's small Isle of Youth, off the south-western end of the Caribbean island nation, toppling trees and tearing roofs from dwellings.
Jamaica was flooded by heavy rains, and there were landslides after the passage of the storm.
Many roads were impassable, forcing some residents to abandon their homes.
Ida, the ninth named storm and fourth hurricane of the 2021 Atlantic hurricane season, may well exceed the strength of Hurricane Laura, the last Category 4 storm to strike Louisiana, by the time it makes landfall, forecasters said.
The region was devastated in August 2005 by Hurricane Katrina, which killed more than 1,800 people.