Ida lurked off the coast of southeastern Louisiana late Saturday, forecast to intensify into a devastating Category 4 hurricane with 140 mph winds, packing massive storm surge and up to 20 inches of rain after it makes landfall Sunday night.
The National Hurricane Center issued a warning Saturday that "preparations to protect life and property should be rushed to completion today" for areas under a hurricane warning, which includes all of southeastern Louisiana.
"Ida is expected to be an extremely dangerous major hurricane when it reaches the coast of Louisiana. Hurricane-force winds are expected Sunday in portions of the hurricane warning area along the Louisiana coast, including metropolitan New Orleans," the NHC said.
Storm surge could reach 15 feet along parts of the Louisiana/Mississippi coastline and 6 feet on Lake Pontchartrain in New Orleans.
The dangerous storm surge will be exacerbated by extreme rainfall. Much of Louisiana and Mississippi could get 6 inches of rain, while parts of southeast Louisiana and the Mississippi could get 10 to 20 inches. Flash flooding is also possible through early next week.
New Orleans Mayor LaToya Cantrell warned in a press conference Saturday, "If you're going to leave, you need to do that now" or shelter in place.
She told locals to brace for damaging wind, heavy rain and tornadoes, saying, "I'm told that the storm in no way will be weakening."
Collin Arnold, the city's emergency preparedness director, urged people to be ready to shelter in place by midnight tonight if they intend to stay and ride out the storm and that extended power loss "is almost certain."
At least 260 National Guard members will be on the ground in New Orleans by Saturday evening, Lt. Col. Kenneth Baillie said.
Officials warned that Louisiana is much more prepared for Ida than it was for Katrina in 2005. Cantrell said she is "very confident" in a levee protection system that wasn't in place back then.
Mandatory evacuations, for areas outside the levees, were ordered south of New Orleans, effective 3 p.m. local time Friday. In the rest of the parish, evacuations are voluntary.
"We're not calling for a mandatory evacuation, because the time simply is not on our side," Cantrell said Friday. "We do not want to have people on the road, and therefore in greater danger, because of the lack of time."
All departing and arriving flights were canceled at New Orleans Airport on Sunday.
Ahead of Hurricane Ida's anticipated landfall, Alabama Gov. Kay Ivey issued a state of emergency for the state's coastal and western counties on Saturday, effective 2 p.m. local time.
"We still expect the possibility of flooding and even spin-off tornadoes in portions of Alabama. With a storm like this, we always want to hope for the best, but prepare for the worst, which is why I have preemptively declared a state of emergency for our coastal and western counties," Ivey said in a statement.
Ida is forecast to reach major hurricane status and close in on the Louisiana coastline Sunday morning, before making landfall between 5 p.m. and 11 p.m. By early Monday morning, Ida will rapidly weaken and move inland, however, much of Louisiana will still experience strong winds.
Louisiana Gov. John Bel Edwards said people who are evacuating should get to their destinations by Saturday evening, adding, "This could be a life-altering storm if you're not prepared," on Friday.
Edwards said "this will be one of the strongest hurricanes to hit anywhere in Louisiana since at least the 1850s" in a press conference Saturday.
He said that eight to 16 inches of rain is expected but some areas could see as much as 20 inches Sunday through Tuesday from Lafayette through Baton Rouge to New Orleans -- which could see heaviest rainfall.
All hands are on deck in Louisiana: 137 coach buses across the state are assisting in evacuations, 4,013 National Guard soldiers and airmen have been activated equipped with 164 high water vehicles and 34 helicopters to aid in search and rescue operations, and 10,000 linemen have been set up to respond to power outages.
Ida is forecast to make landfall on the very difficult 16th anniversary of Hurricane Katrina.
"I'm also aware that it's very painful to think about another powerful storm like hurricane either making landfall on that anniversary," he said. "But also want you to know that we're not the same state that we were 16 years ago, we have a hurricane risk reduction system in place because of the generosity of the people of United States of America that we didn't have before."
Officials in Jefferson Parish, which includes parts of the New Orleans metropolitan area, urged residents outside of the levee protection system to evacuate.
"It is an unsurvivable storm surge that we are expecting and this storm is not supposed to pass through quickly," Jefferson Parish President Cynthia Lee Sheng said during a press conference Saturday night. "We need you to leave immediately."
A storm surge warning is in effect along parts of the northern Gulf Coast, from east of Rockefeller Wildlife Refuge in Louisiana through New Orleans to the Alabama-Florida border.
Some COVID-19 testing and vaccine sites closed Friday due to the storm, the Louisiana Department of Health said.
The high number of COVID-19 patients in Louisiana also means the state will not be able to evacuate hospitals, the governor said. Cantrell also said New Orleans hospitals were not being evacuated at this time.
"We've been talking to hospitals about making all the preparations possible to make sure that their generators are working, that they have way more water on hand than normal," Edwards said.
Health officials are asking people to avoid emergency departments throughout the storm if possible.
Hurricane warnings and states of emergency have been declared in New Orleans and Louisiana.
Ida should weaken to a tropical storm by Monday, but it's forecast to move inland across Louisiana, with more potentially devastating downpours. About 20 inches of rain are expected, which means extreme floods are possible.
President Joe Biden has approved emergency declarations for Louisiana and Mississippi.
Biden is "closely tracking" the developments and held a call with the governors of Louisiana, Alabama and Mississippi Friday to discuss preparations, the White House said. The president also spoke with the Federal Emergency Management Agency administrator Saturday about ongoing evacuation efforts.
The administration is working to free up hospital beds and pre-position resources, in addition to sending a search-response team, 50 FEMA paramedics and 47 FEMA ambulances to assist with care, White House press secretary Jen Psaki said Friday.
ABC News' Will McDuffie contributed to this report.