The defenses built around New Orleans worked, but also showed their limits.

As people across southeastern Louisiana slowly began to take in the scale of damage from Hurricane Ida on Monday, a task severely hindered by widespread power outages and limited phone service, search-and-rescue teams fanned out to respond to calls for help that had gone unanswered.

In Jefferson Parish, where there have been reports of people climbing into their attics to escape rising waters, the authorities had received at least 200 rescue calls since Sunday and crews were anxious to get to those who may still need their help, said Cynthia Lee Sheng, president of Jefferson Parish. More than 70 people were rescued from the fishing village of Jean Lafitte on Monday, she said, though one woman there was found dead.

New Orleans remained without electricity. All eight transmission lines that deliver power to the city were knocked out of service by Ida, which made landfall late Sunday morning near Port Fourchon with maximum sustained winds of 150 miles an hour. By early Monday morning, the hurricane had weakened to a tropical storm as it moved inland.

Entergy, a major power company in Louisiana, said on Twitter on Monday that it would most “likely take days to determine the extent of damage to our power grid and far longer to restore electrical transmission to the region.”

The New Orleans mayor, LaToya Cantrell, urged residents who had evacuated not to return to the city anytime soon, given the outages and other challenges it is facing in the aftermath of the storm. “Now is not the time for re-entry into the city of New Orleans,” she said at a news conference on Monday afternoon, later adding: “Again, if you evacuated, stay where you are. We will notify you when it is safe to go home.”

Dozens of streets in New Orleans were flooded with runoff from the storm’s heavy rains, according to the National Weather Service, which advised people to remain sheltered in place. But the system of levees, barriers and pumps that protects New Orleans appeared to have held firm against the onslaught of Hurricane Ida, officials said, passing the most dramatic test since being expanded and hardened after Hurricane Katrina.

In a news conference on Monday afternoon, John Bel Edwards, the Louisiana governor, said he was thankful that the flood protection system kept the damage of Hurricane Ida from being far worse than it might have been, but he also prepared residents of south Louisiana for a tough slog ahead with more than a million people without power.

“This was an extremely catastrophic storm,” the governor said. “If there’s a silver lining, and today it’s kind of hard to see that, it is that our levee systems really did perform extremely well.”

Ms. Lee Sheng said in an interview that Jefferson Parish officials had not yet been able to make contact with residents of Grand Isle, a narrow beachy islet of homes on stilts facing the Gulf of Mexico, near where the storm came ashore. Though many residents evacuated before the storm, she estimated that about 40 people had remained behind.

Sheriff Joseph Lopinto of Jefferson Parish said on Monday afternoon that a crew was able to see Grand Isle by helicopter, getting thumbs up from people on the ground.

“Grand Isle got hammered probably harder than they’ve ever been hammered before,” the sheriff said in an interview with WWL radio.

Still, across the parish, including in storm-pummeled areas like Grand Isle and Lafitte, damage varied from house to house, he said. Houses raised 10 feet in the air survived, while those closer to the ground did not, he said.

Several small towns in the southern half of the parish, outside the giant storm protection system encircling New Orleans and some of its suburbs, were inundated, she said. The levees surrounding the towns had overtopped, she said, sending several hundred people who were there riding out the storm into attics and onto roofs.

“The further south you go, you are having very high water,” Ms. Lee Sheng said, adding that search-and-rescue teams went out at first light on Monday morning.

The northern end of the parish fared much better in terms of structural damage, but was facing what she called “a breakdown of systems.” No one in the parish had electricity, she said; phone communication was impossible in many areas; and broken water mains were draining the parish of its usable water. “Other than search and rescue today, that is the critical issue,” she said.

Over 240,000 people in the parish were affected by water outages, according to figures from the state department of health. Officials in Jefferson Parish, as with those in New Orleans and in other parishes across southeastern Louisiana, urged people who had left before the storm not to return immediately.

“We’re asking people to stay away,” the sheriff said.

State officials said that 185 buses were ready to pick up people who stayed behind in parishes, like Jefferson, where there was no electric power and little drinking water, and move them to other parts of the state.

People venturing out on Monday in the hardest-hit parts of the state found smashed buildings in Houma, mangled infrastructure in Bridge City and streets still submerged in LaPlace, the first hints at the regionwide fallout from a night of destruction. LaPlace, a town of quiet subdivisions where many evacuees from New Orleans had decided to settle down after Katrina, was still badly flooded in areas, and desperate calls had gone out over social media all night for boat rescues.

Small communities that had slowly built back from hurricanes past were flooded again as local levees overtopped.

As Ida moved through the state, the storm caused “catastrophic transmission damage” to the electrical system, leaving over a million utility customers without power. Five hospitals have been evacuated or were actively considering evacuation on Monday afternoon, said a spokesman for the state department of health.

The center of the storm crossed into western Mississippi on Monday, slowing and weakening as it swept northward. By late afternoon it had weakened to a tropical depression, with its maximum sustained winds diminished to 35 miles an hour, but was still producing heavy rain. Its path is expected to curve northeastward through the day and evening, and then into the Tennessee Valley on Tuesday, where some areas may get six to eight inches of rainfall.

At least three deaths have been attributed to the storm, though state officials say they are expecting to learn of more. In addition to the woman in Jean Lafitte, a man in Prairieville, La., about 30 miles southeast of Baton Rouge, died after a tree fell on a house, according to the Ascension Parish Sheriff’s Office. And a man died while driving in New Orleans, the mayor’s office there said.

HOUMA, La. — Large oak trees smashed into homes where terrified families took shelter as Hurricane Ida tore through. Windows of quaint local shops shattered. Debris spread across roads, making them impossible to navigate.

Residents of Houma, a small city about 60 miles southwest of New Orleans, stepped out on Monday from the refuges where they rode out the Category 4 storm, and took stock of a region battered.

Jazmine Carter, 20, said she and her parents had watched in horror on Sunday night as a giant tree a few feet away from their house broke and crashed into the one next door, while power lines flew around them. “Trees were falling everywhere —  it was scary,” said Ms. Carter, a cashier at a local Walgreens store.

Her mother, Hannah Carter, 39, said, “I’ve never seen anything like it.”

“The trees and power lines were swaying back and forth, and then they finally snapped,” she added. “It was horrifying.”

She turned to look at her daughter, who was surveying the damage around her. “At least nobody got hurt or died,” she said. “That’s really what matters.” she said.

In Houma and surrounding communities, electric power was expected to be out for days, and perhaps weeks, officials said. On Monday, Shanell Short, 35, watched as two men picked up debris that covered most of her block — mostly tree branches, trash and utility cables. “This is going to take a long time to clean up,” she said.

A woman nearby saluted a parade of ambulances driving down Main Street.

One hair salon in town lost its front wall, leaving the chairs and supplies inside in plain view. Francisco Del Angel Morales, 48, and his 10-year-old son looked on in awe, surprised that the chairs and even candy on the counter seemed untouched — as if the wall had disappeared with a magic trick.

“It’s all very surreal,” Mr. Morales said. “This whole area looks completely devastated.”

In a neighborhood of trailer homes, some were torn in half, and many more had lost walls or roofs. “It’s total destruction everywhere you look,” said Clifford Conerly, 43, a landscaper.

Craig Adams, 53, had planned to spend the night of the storm in his beige-colored trailer, but his daughter had begged him at 9 p.m. to seek shelter somewhere sturdier. On Monday, he was thankful she had. The two-bedroom trailer was wrecked, with only the air-conditioner surviving among piles of mangled furniture, kitchen supplies and personal belongings.

“Every little thing that I owned and had, it’s gone,” Mr. Adams said. “I’m going to have to start all over again. You always see other people going through this on the news. You never think it’s going to be you —  until it is.”

President Biden on Monday promised people in Louisiana and Mississippi that his administration would be there to help them recover from the damage wrought by Hurricane Ida “for as long as it takes.”

“We know Hurricane Ida had the potential to cause massive, massive damage, and that’s exactly what we saw,” he said, speaking during a virtual news conference with state and local officials. “We’re about as prepared as we could be for the early stage of this, so there’s a lot more to do.”

Mr. Biden said his administration had resources in place in the region before the hurricane made landfall, including millions of meals and liters of water and more than 200 generators, with more on the way to help with the vast power failures in Louisiana. More than 5,000 National Guard troops have been deployed to help with the search-and-rescue efforts. He said the storm surge and flash flooding in the region was continuing, with roads blocked from debris and downed power lines. “We need people to continue to shelter in place, if it’s safe for them to do so,” he said.

Jen Psaki, the White House press secretary, said Alejandro N. Mayorkas, the secretary of the Department of Homeland Security, and Deanne Criswell, the FEMA administrator, planned to travel to Baton Rouge on Tuesday morning. Ms. Psaki said Ms. Criswell planned to go on to Jackson, Miss., that evening before meeting with Gov. Tate Reeves of Mississippi on Wednesday to tour the damage from Ida.

Ms. Psaki said there were no immediate plans for Mr. Biden to travel to the region because the White House does not want to affect the response efforts. A presidential visit requires more local resources for logistics and security than do visits by other administration officials.

“People in Louisiana and Mississippi are resilient,” Mr. Biden said. “But it’s in moments like these, we can certainly see the power of government responding to the needs of the people, if government’s prepared, and if they respond.”

He said, “We’re going to stand with you and the people of the Gulf for as long as it takes for you to recover.”

In addition to the meals, water and generators sent to Louisiana before the storm, Ms. Psaki said more than 3,600 FEMA employees have been deployed to Alabama, Florida, Georgia, Louisiana, Mississippi and Texas.

The Department of Health and Human Services also provided a 250-bed federal medical shelter to Alexandria, La., which is about two hours from Baton Rouge. More than 300 federally deployed health care workers are on the ground to help stem the spread of Covid-19, she said. The state has one of the lowest vaccination rates in the country and has been overwhelmed in recent weeks with new cases.

The $14.5 billion flood-protection system built around New Orleans after Hurricane Katrina seems to have succeeded at keeping the city from going underwater again.

As of Monday morning, water from Hurricane Ida had not pushed past, or “overtopped,” any of the 192 miles of flood barriers that make up that system, according to the Flood Protection Authority, the local agency that runs the Hurricane Storm Damage Risk Reduction System. Nor have any of those barriers suffered a structural failure, called a breach.

And while most of New Orleans is without power, the pumps that are designed to move flood water out of the city still work, because those pumps run on generators, according to the flood authority.

In short, the system worked, according to Elizabeth Zimmerman, who ran disaster operations for the Federal Emergency Management Agency during the Obama administration.

“It’s a major accomplishment,” Ms. Zimmerman said. “The things that were built were a major step forward.”

In a news conference on Monday afternoon, John Bel Edwards, the Louisiana governor, said he was thankful that the flood-protection system kept the hurricane damage from being far worse than it might have been.

“If there’s a silver lining, and today it’s kind of hard to see that, it is that our levee systems really did perform extremely well,” he said.

But that success doesn’t mean residents are safe. “It’s a good time to remind people that just because the storm has passed, it doesn’t mean that dangers have not,” Mr. Edwards said, referring to the deaths caused by accidents with generators that have followed past storms.

“There are an awful lot of unknowns right now,” he added. “There are certainly more questions than answers. I can’t tell you when the power is going to be restored. I can’t tell you when all the debris is going to be cleaned up and repairs made and so forth.”

All eight transmissions lines that bring electricity into the city are out of service, according to a statement Sunday by Entergy, the power utility. On Monday, the company said 216 substations and more than 2,000 miles of transmission lines were out of service.

“Those in the hardest-hit areas could experience power outages for weeks,” the company said in a statement.

Four hospitals were damaged in Louisiana, according to the Federal Emergency Management Agency. New Orleans’s 911 call system was down, Mayor LaToya Cantrell wrote on Twitter.

City officials pleaded with residents to stay off the roads. “Now is not the time to leave your home,” the New Orleans Police Department wrote on Twitter. “There is no power. Trees, limbs and lines are down everywhere.”

The fact that New Orleans has no electricity, despite huge investments in storm protection over the past 16 years, demonstrates the challenge of adapting to climate change, according to Daniel Kaniewski, who was in charge of resilience at FEMA until 2020.

The work that followed Katrina focused on preventing a repeat of catastrophic flooding, said Mr. Kaniewski, now a managing director at the professional services company Marsh McLennan. But that work focused less on other types of infrastructure, like the power grid.

“If we’re only preparing for the last disaster, we’ll never be prepared for the next one,” he said.

Officials and those who chose to ride out the storm in New Orleans assessed destruction from Hurricane Ida on Monday.

NEW ORLEANS — A drive around some New Orleans neighborhoods Monday morning revealed a city bruised but not beaten.

Uprooted trees and broken branches were everywhere, from the Bywater neighborhood to Uptown. St. Charles Avenue, a grand uptown boulevard, was clogged with tree limbs and littered with green. In the French Quarter, the streets seemed to have been washed almost clean.

A roof had come down in a twisted mess of tar from a pink four-story building at Toulouse and Decatur Streets, attracting TV news crews looking for signs of damage. An old brick building near City Hall had been dramatically blown to bits by the wind. Bricks were littered in heaps, and had crushed a nearby car.

Mayor LaToya Cantrell told New Orleanians to remain indoors, but a few had begun venturing out to walk their dogs, ride bikes and assess the state of things. Though the city looked sturdy and dry on the outside, they knew the drama would now unfold indoors, where the lights might not be coming on for days.

In the Algiers Point neighborhood, Melissa DeRussy, her husband, Husted, and their two teenage children were already out by 9 a.m., raking up leaves and small branches torn from the oak trees on their block. All over the neighborhood, the steady hum of generators blended with the sounds of neighbors checking in on one another and looking things over.

Roof shingles were sprinkled across lawns. A palm tree on one block was ripped in half about six feet from the ground, and a nearby magnolia looked as through it had been dropped into a blender.

Overnight, “it was a little exciting,” Ms. DeRussy said. “Every bump — from possibly the house next door — we had to investigate until it got dark. Then we just couldn’t investigate any more.”

With power knocked out across the city, Ms. DeRussy, who works for a local school, said the family’s next steps were up in the air.

“My colleagues are scattered across the Gulf Coast,” she said “There are just a lot of unknowns this morning.”

At the New Orleans Fire Department station on Poland Avenue, a generator powered the lights and kitchen, but its firefighters were relying on hand-held radios for communication with the outside world.

“We’re all in the dark right now,” said a firefighter who sat near the station’s open garage doors on Monday morning, ready to help anyone walking up for help. “For the most part, we’re getting messages by ear.”

Residents who have lived through other storms said they were not phased — yet — by the power outages and boil-water advisories.

“Guess what? This is part of life in New Orleans,” said Antoine Davis, 58, as he stopped at Duplantier Ice at the edge of the French Quarter to get some bags of ice to keep his refrigerator cold. “This is something I have been dealing with all of my life, because I live here. If we lived in California, there would be fires and earthquakes. If we lived in Tennessee right now, we’d have floods.”

Oxygen supplies are running critically low in hospitals across Louisiana — with some only having one or two days of supply left — and any interruption brought by Hurricane Ida’s destruction could be serious, according to Premier Inc., one of the largest hospital supply purchasing groups in the country.

Ida pummeled much of the state on Sunday evening, leaving hundreds of thousands without power at a moment when hospitals across the Southeast had already been struggling with oxygen shortages for weeks. Driven by a surge in Covid-19 cases, some hospitals are relying on reserve tanks with no other backup options.

“This is a rapidly evolving situation with access and roads — it remains to be seen what might happen in the days ahead,” said Premier’s chief customer officer, Andy Brailo. “What we all want to avoid, obviously, is hospitals not being able to have the adequate oxygen supply for their patients or putting their patients at risk.”

He said delivery trucks have been giving hospitals partial refills because demand had been so high. Supply is further limited because oxygen needs to be delivered within hours, meaning that supplies must come from within a 250-mile radius of a hospital, he added. Premier is coordinating with the Federal Emergency Management Agency about the scarcity of oxygen in the region.

The shortage goes beyond hospital supply. Mr. Brailo said individual canisters and tanks used by discharged Covid patients and those with disabilities were also in high demand. CrowdSource Rescue, a volunteer emergency response group, performed about a dozen oxygen-related rescues on Monday, including one of a woman who was dependent on oxygen after a Covid-19 infection, according to Loren Dykes, the group’s director of operations.

In the days ahead, Ms. Dykes said she expected to receive more oxygen-related distress calls, especially for Covid patients, who she said were not going to be as prepared as people with disabilities, who have more experience and tend to stockpile supplies.

New Orleans has opened oxygen exchange sites for residents to get a full free tank of oxygen. Mike Hulefeld, chief operating officer for Ochsner Health, one of the largest hospital systems in Louisiana, said on Monday that thanks to generators, hospitals were faring well. The hospital network had 10 days’ worth of supplies for the hospitals it anticipated would be hardest hit, and each of its locations had backup power and fuel.

But those who rely on ventilators or oxygen concentrators to help them breathe, including recently discharged Covid-19 patients, are also going to be at increased risk because of the power outages. According to the U.S. Department of Health & Human Services data on Medicare beneficiaries, there are 3,706 Medicare beneficiaries in Jefferson Parish who are dependent on power for their medical devices; in Orleans Parish, 2,215 Medicare beneficiaries are medically dependent on power.

Tariro Mzezewa contributed reporting.

When Hurricane Ida made landfall on Louisiana’s southeastern coast just before noon on Sunday, its maximum sustained winds were roaring at 150 miles an hour.

Nearly 10 hours later and 80 miles inland, its maximum wind speeds were still clocking in at a dangerous 105 m.p.h.

Hurricanes typically decay quickly once they make landfall. But experts who study the storms say there are several reasons that Ida remained so intense even as it plowed northward into Louisiana.

Over dry land, and particularly over rougher terrain, wind speeds generally decrease rapidly. Hurricanes require thermal energy to fuel themselves, and the water in the ocean — or in this case, bayous and wetlands — can yield a lot of energy. But heat flows through land slowly, starving hurricanes of one of their primary energy sources.

Southeastern Louisiana is flat, wet and swampy for many miles inland from the Gulf shore where Ida first hit.

“It doesn’t take a lot of water to keep a hurricane going,” said Kerry Emanuel, an atmospheric scientist at M.I.T. A swamp “won’t sustain a 150-mile-per-hour hurricane, but it will make sure it doesn’t decay as fast as it would over dry land.”

The terrain was not all that contributed to Ida’s continuing intensity. Most often when storms hit land, they are already in the process of leveling off or decaying. The unusual case of a hurricane making landfall while still intensifying rapidly is “a forecaster’s nightmare,” Dr. Emanuel said.

Ida was in that category, and after passing the Louisiana shoreline, it took a number of hours to finally adjust to its transition from sea to land.

The widespread loss of power in New Orleans wasn’t supposed to happen again.

Entergy, the power company serving the city, campaigned to build a new natural gas-fueled power plant in the city, arguing that it was needed for just this kind of situation, when the transmission system that normally supplies the city with power generated elsewhere can’t do the job.

Over protests from numerous community groups and city leaders, Entergy got its way, and the plant was built just south of Interstate 10 and Lake Pontchartrain, bordering predominantly African American and Vietnamese American neighborhoods. It went into operation last year, running mainly at times of peak demand.

But when Hurricane Ida knocked out the transmission lines on Sunday, the plant did not save the day for the city. Power was out almost everywhere on Monday, with little prospect of a swift return. And many residents are unhappy.

“The gas plant was built over our objections,” said Monique Harden, assistant director for public policy at the Deep South Center for Environmental Justice, one of the leading organizations fighting the gas plant. “No resident was in support of it. Nonetheless, Entergy with the City Council teamed together and got the gas plant.”

Susan Guidry, a former council member, argued at the time that Entergy should have focused instead on renewable energy technologies like solar power and battery storage to help keep the lights on in New Orleans after a hurricane. But while the utility did build some of that, the gas plant became the focus of its plans.

“If anything happened to the transmission, this gas plant was supposed to supply power to the City of New Orleans,” Ms. Harden said. “This is going to require some investigation.”

Ms. Harden’s organization and others argued for microgrids and other resources that could operate even if the traditional electric grid was knocked out of service. Some residents and businesses have their own solar installations and batteries, or are connected to such sources through microgrids, but customers who are connected only to the traditional power grid do not.

Entergy has warned that it may take its crews days just to assess the damage to its system, and much longer than that to complete repairs.

“It’s getting more and more desperate,” Ms. Harden said. “Our lives are now in the hands of this company.”

Entergy did not immediately respond to a request for comment.

It will take days just for utility crews to determine the extent of the storm damage to the New Orleans power grid, and far longer to restore power to the region, officials of Entergy Louisiana said on Monday.

“We have a lot of rebuilding ahead of us,’’ the company said on Twitter. “We’ll be better prepared to give restoration estimates once assessments are done.”

As of 7 a.m. on Monday, Entergy said there were more than 888,000 power outages in Louisiana after Hurricane Ida thrashed much of the state Sunday evening, snapping cables, damaging buildings, uprooting trees and spreading debris along roads.

On Monday morning, 216 substations, 207 transmission lines, and more than 2,000 miles of transmission lines were out of service, and the company also reported more than 45,000 outages in Mississippi.

Because of Ida’s “catastrophic intensity,” all eight transmission lines that deliver power to New Orleans were out of service, Entergy officials said. The situation caused a load imbalance and resulted in a failure of all power generation in the region.

The city’s Office of Homeland Security and Emergency Preparedness said that the only power in the city was coming from customers’ own generators.

There were reports of communications disruptions as well. Telephone service appeared to be out in some of the hardest-hit areas of southeast Louisiana. And there were problems with mobile phone service.

AT&T said that because of wind damage, flooding and power loss, “we have significant outages in New Orleans and Baton Rouge,” and that its wireless network in Louisiana as a whole was operating at 60 percent of normal capacity. Key network facilities were knocked off line by the storm overnight, the company said, “and while some have already been restored, some facilities remain down and are inaccessible.”

A spokeswoman for Verizon said on Monday that the company was “still actively assessing the situation on the ground as it is safe to do so.” She added, “While we are seeing sites out of service in the heaviest hit areas, overlapping sites are offering some coverage to residents and first responders who remain there.” Many cell sites were running on backup generators and batteries, she said.

Verizon said it was providing unlimited calling, texting and data to its customers most affected by Hurricane Ida. AT&T said it was waiving overage charges for customers in parts of Louisiana and Mississippi through Saturday. T-Mobile said on Sunday that most T-Mobile and Sprint customers in the affected area would be offered free talk, text and unlimited data through Friday.

Some utility customers who were in the direct path of the hurricane may not see electric service restored for as long as three weeks, according to Entergy. But 90 percent of customers will have power back sooner, it said.

Requests for comment from Entergy about the hardest hit areas and the next stages of restoration were not immediately answered early Monday.

As the storm swept across the city on Sunday, Entergy said that crews from at least 22 states and Washington, D.C., were joining the recovery effort.

The company said it was working to assess damage and identify a path forward to restore power to areas that could still receive it. It added that it had provided backup generation to the New Orleans Sewerage and Water Board.

Including other utilities as well as Entergy, about one million customers in Louisiana were without power early Monday morning, according to reports compiled by PowerOutage.us. Most were in the southeastern part of the state. In Mississippi, about 130,000 customers were reported to be without power, mainly in the southwest, the website said.

Entergy Louisiana warned customers that broken power lines can remain hazardous.

“Just because you can’t see any apparent danger, doesn’t mean there isn’t any,” the company said on Monday. “Downed power lines may still be energized. Keep your distance.”

As the remnants of Hurricane Ida move farther inland in the coming days, the storm system is expected to lose strength but will continue to pose a danger to many parts of the Southeast, the National Hurricane Center said.

Ida, which was downgraded to a tropical depression Monday afternoon, will continue to bring heavy rainfall, and possibly severe flooding, to Louisiana, the southern parts of Mississippi and coastal communities in Alabama through Monday evening. The rainfall totals could reach as much as 24 inches in some parts of southeast Louisiana.

“Heavy rain combined with storm surge has resulted in catastrophic impacts along the southeast coast of Louisiana, with considerable flash flooding and riverine flooding continuing farther inland,’’ the Weather Service said.

Coastal Alabama and the western parts of Florida could see six to 12 inches of rain through Tuesday morning, and parts of central Mississippi could see up to a foot of rain.

Tornadoes have been reported in Alabama — on the outskirts of Mobile and south of Troy — and more are possible on Monday night in Southern Mississippi, southwest Alabama and the western Florida Panhandle.

On Monday afternoon, the system was about 20 miles north-northwest of Jackson, Miss., moving toward the north-northeast at 9 miles an hour with maximum sustained winds of 35 m.p.h.

The storm is expected to weaken as it continues toward the northeast on Monday night, tracking toward the Middle Tennessee Valley, including Humphreys County, where 20 people were killed this month as flash floods tore through communities there. The area could see up to six inches of rain on Tuesday and Wednesday, the Hurricane Center said.

The National Weather Service in Nashville issued a flood watch for most of Middle Tennessee starting on Monday night.

By Wednesday, the storm is forecast to move through the Upper Ohio Valley, dropping as much as six inches of rain, and then continue into the Northeast later in the week.

All of these areas could experience flash flooding, the Hurricane Center said.

Johnny Diaz, Jacey Fortin and Derrick Bryson Taylor contributed reporting.

Hurricane Ida’s devastating winds, flooding and heavy rains swept through southeastern Louisiana on Sunday, and the brunt of its destructive path became apparent on Monday: Homes and businesses were damaged or destroyed, and hundreds of thousands of people were left without power.

Local and national volunteers and aid groups are prepared to rescue, feed, and give shelter to those who have been affected. Here is some guidance for those who wish to help.

Before you give, do your research.

Natural disasters create ripe opportunities for fraudsters who prey on vulnerable people in need and exploit the generous impulses of others who want to donate funds to help them. The Federal Communications Commission noted that scammers use phone calls, text messages, email and postal mail, and even go door-to-door. The Federal Trade Commission has tips on how to spot a fraudulent charity or fund-raiser.

Charity Navigator, GuideStar and other organizations provide information on nonprofit groups and aid agencies, and can direct you to the reputable ones.

If you suspect that an organization or individual is engaged in fraudulent activity after a natural disaster, report it to the National Center for Disaster Fraud, or to FEMA at 1-866-720-5721.

Donations of money, rather than of goods, are usually the best way to help, because they are more flexible and can readily be redirected when needs change.

Here are some local organizations in the storm area.

All Hands and Hearts prepared for Ida by stationing its disaster assessment and response team in Beaumont, Texas. Its volunteers will enter areas affected by the storm when they can, meeting initial needs that will probably include chain-saw work to clear debris and trees, roof tarping, mucking and gutting flooded houses, and sanitizing homes with mold contamination.

The Second Harvest Food Bank, which serves South Louisiana, has prepared more than 3,500 disaster-readiness food boxes with items like rehydration drinks and nutrition bars, as well as bottled water. It also maintains cooking equipment that can be transported to heat prepared meals. Donations of bottled water and cleaning supplies are welcome. Volunteers can apply to help, but donations of money are the most efficient way to assist the aid effort, the organization said.

AirLink is a nonprofit humanitarian flight organization that ships aid, emergency workers and medical personnel to communities in crisis. It has joined Operation BBQ Relief to supply equipment, cooks and volunteers to prepare meals for people affected by the storm. Donations are welcome.

SBP, originally known as the St. Bernard Project, was founded in 2006 by a couple in St. Bernard Parish who were frustrated by the slow response after Hurricane Katrina. It focuses on restoring damaged homes and businesses and supporting recovery policies. Its Hurricane Ida plan needs donations, which will pay for supplies for home rebuilding and protective equipment for team members.

A number of volunteer rescue groups operate under some variation of the name Cajun Navy. One is Cajun Navy Relief, a volunteer disaster response team that became a formal nonprofit organization in 2017; it has provided relief and rescue services during more than a dozen of Louisiana’s floods, hurricanes and tropical storms. The team has identified supplies that are needed and is accepting donations.

Here are national organizations lending a helping hand.

AmeriCares, a health-focused relief and development organization, is responding to Ida in Louisiana and Mississippi and matching donations. Vito Castelgrande, the leader of its Hurricane Ida team, said the organization will begin assessing damage in the hardest-hit communities when it is safe to travel.

Mercy Chefs, a Virginia-based nonprofit group, was founded in 2006 after Hurricane Katrina devastated New Orleans, the hometown of its founder, Gary LeBlanc. The organization has served more than 15 million meals to people affected by natural disasters or who have other needs. The group has deployed two mobile kitchens to serve hot meals in Ida’s wake and is accepting donations.

GoFundMe has created a centralized hub with verified GoFundMe fund-raisers to help those affected by Ida. It will be updated on a regular basis with new fund-raisers as they are verified.

Project HOPE has sent an emergency response team with 11 medical volunteers and has distributed 8,000 hygiene kits, which include items like shampoo, soap, a toothbrush, deodorant and a first-aid kit. Donations can be made solely for Hurricane Ida emergency relief.

The Red Cross has mobilized hundreds of trained disaster workers and relief supplies to support people in evacuation shelters. About 600 volunteers were prepared to support Ida relief efforts, and shelters have been opened in Louisiana and Mississippi, with cots, blankets and comfort kits, and ready-to-eat meals. The organization has also positioned products needed for blood transfusions. Donations can be made through redcross.org, or 800-RED-CROSS, or by texting the word REDCROSS to 90999.

United Way of Southeast Louisiana is collecting donations for a relief fund to rebuild and provide long-term assistance, including community grants.

Hurricane Ida intensified overnight, becoming a Category 4 storm over the course of just a few hours. The rapid increase in strength raises questions about how much climate change is affecting hurricanes in the Atlantic Ocean. While researchers can’t say for sure whether human-caused climate change will mean longer or more active hurricane seasons in the future, there is broad agreement on one thing: Global warming is changing storms.

Scientists say that unusually warm Atlantic surface temperatures have helped to increase storm activity. “It’s very likely that human-caused climate change contributed to that anomalously warm ocean,” said James P. Kossin, a climate scientist with the National Oceanic and Atmospheric Administration. “Climate change is making it more likely for hurricanes to behave in certain ways.”

Here are some of those ways.

1. Higher winds

There’s a solid scientific consensus that hurricanes are becoming more powerful.

Hurricanes are complex, but one of the key factors that determines how strong a given storm ultimately becomes is ocean surface temperature, because warmer water provides more of the energy that fuels storms.

“Potential intensity is going up,” said Kerry Emanuel, a professor of atmospheric science at the Massachusetts Institute of Technology. “We predicted it would go up 30 years ago, and the observations show it going up.”

Stronger winds mean downed power lines, damaged roofs and, when paired with rising sea levels, worse coastal flooding.

“Even if storms themselves weren’t changing, the storm surge is riding on an elevated sea level,” Dr. Emanuel said. He used New York City as an example, where sea levels have risen about a foot in the past century. “If Sandy’s storm surge had occurred in 1912 rather than 2012,” he said, “it probably wouldn’t have flooded Lower Manhattan.”

2. More rain

Warming also increases the amount of water vapor that the atmosphere can hold. In fact, every degree Celsius of warming allows the air to hold about 7 percent more water.

That means we can expect future storms to unleash higher amounts of rainfall.

3. Slower storms

Researchers do not yet know why storms are moving more slowly, but they are. Some say a slowdown in global atmospheric circulation, or global winds, could be partly to blame.

In a 2018 paper, Dr. Kossin found that hurricanes over the United States had slowed 17 percent since 1947. Combined with the increase in rain rates, storms are causing a 25 percent increase in local rainfall in the United States, he said.

Slower, wetter storms also worsen flooding. Dr. Kossin likened the problem to walking around your back yard while using a hose to spray water on the ground. If you walk fast, the water won’t have a chance to start pooling. But if you walk slowly, he said, “you’ll get a lot of rain below you.”

4. Wider-ranging storms

Because warmer water helps fuel hurricanes, climate change is enlarging the zone where hurricanes can form.

There’s a “migration of tropical cyclones out of the tropics and toward subtropics and middle latitudes,” Dr. Kossin said. That could mean more storms making landfall in higher latitudes, like in the United States or Japan.

5. More volatility

As the climate warms, researchers also say they expect storms to intensify more rapidly. Researchers are still unsure why it’s happening, but the trend appears to be clear.

In a 2017 paper based on climate and hurricane models, Dr. Emanuel found that storms that intensify rapidly — the ones that increase their wind speed by 70 miles per hour or more in the 24 hours before landfall — were rare in the period from 1976 through 2005. On average, he estimated, their likelihood in those years was equal to about once per century.

By the end of the 21st century, he found, those storms might form once every five or 10 years.

“It’s a forecaster’s nightmare,” Dr. Emanuel said. If a tropical storm or Category 1 hurricane develops into a Category 4 hurricane overnight, he said, “there’s no time to evacuate people.”

LaPLACE, La. — The power was out. The wind was uprooting trees around her house and peeling the shingles from her roof. The driving rain started pouring into the house, even seeping out through the electrical sockets.

Lea Joseph took her children out to the car, where they tried to sleep. Though the car was shaking as the storm passed, it still felt safer than the house. Her mind was racing with fear and, as she described it, all of the what-ifs, imagining the worst outcomes and what might have happened if she had tried to flee before Hurricane Ida swept in.

“I felt bad, because I should have left with my kids,” she said. “I’m scared. My son is crying. He kept asking, ‘When is the eye passing, when is the eye passing?’ They know what’s going on.”

With the big blow past on Monday, her 13-year-old son, Cesar, showed videos he had shared with his friends on Snapchat, recording the wind and the water as the storm descended on their home.

“I wasn’t scared,” he said. “My brothers were.”

He recalled that Cesar’s 11-year-old brother, Juan, kept calling out, “Hold the door, hold the door.”

“I was crying,” Juan said on Monday as he stood on a flooded street, the water lapping over his rubber boots. He was scared, he said, but also relieved to be on the other side of the storm.

His mother’s regrets had not ebbed. “My car ain’t the best to be driving,” she said, “but I should have drove it like that.”

When the next storm comes to southeastern Louisiana, will she try to ride it out? “Never again, never again,” Ms. Joseph said. “Not as long as I’ve got little ones. Not a Category 1. Not anything.”

In many ways, she knew, the storm was not over. Her home had been severely damaged, and it could be weeks before electricity returns. Even so, she said, “We’re trying to keep as calm as possible for the children.”

In LaPlace, a city of just under 30,000 people on the eastern bank of a crook in the Mississippi River, many houses were left mangled and streets remained flooded on Monday.

Water covered the pavement on Whitlow Court, a strip of mobile homes that had been rattled and battered by Ida. Every truck that tried to drive down the street created a wake. Neighborhood residents were hungry and tired. The water supply was out. So was the electricity. No one had any cellphone service.

David Sanford considered himself something of a hurricane veteran: He moved to Louisiana eight years ago from Pensacola on the equally storm-prone Florida coast. Even so, Ida terrified him, he said. The storm set his mobile home vibrating, and a skylight over the bathroom popped, dumping rainwater inside.

“It was just rough,” Mr. Sanford, 64, recounted, sitting back on a dry patch at the end of the street on Monday. “This one right here was the worst one I’ve been in.” The howling wind “didn’t slack up at all,” he said. “That was a huge storm.”