NEW ORLEANS (AP) — Hurricane Ida rapidly grew in strength early Sunday, becoming a dangerous Category 4 hurricane just hours before hitting the Louisiana coast as emergency officials in the region grappled with opening shelters for displaced evacuees despite the risk of spreading the coronavirus.

As Ida moved through some of the warmest ocean water in the world in the northern Gulf of Mexico, its top winds grew by 45 mph (72 kph) to 150 mph (230 kph) in five hours. The system was expected to make landfall early Sunday afternoon on the exact date Hurricane Katrina ravaged Louisiana and Mississippi 16 years earlier.

The hurricane centre said Ida is forecast to hit at 155 mph (250 kph), just one mph shy of a Category 5 hurricane. Only four Category 5 hurricanes have made landfall in the United States: Michael in 2018, Andrew in 1992, Camille in 1969 and the Labor Day Hurricane of 1935. Both Michael and Andrew were upgraded to category 5 long after the storm hit with further review of damage.

Ida threatened a region already reeling from a resurgence of COVID-19 infections, due to low vaccination rates and the highly contagious delta variant.

New Orleans hospitals planned to ride out the storm with their beds nearly full, as similarly stressed hospitals elsewhere had little room for evacuated patients. And shelters for those fleeing their homes carried an added risk of becoming flashpoints for new infections.

Hurricane force winds started to strike Grand Isle on Sunday morning. Before power was lost on the barrier island, a beachfront web camera showed the ocean steadily rising as growing waves churned and palm trees whipped.

Forecasters warned winds of 100 mph (161 kph) were expected soon in Houma, a city of 33,000 that supports oil platforms in the Gulf.

In New Orleans, where the worst weather is expected later, a light rain fell. Cars were parked on the median, which locals call neutral ground, because its a few feet higher and can protect against potential flooding

Ida intensified so swiftly that New Orleans officials said there was no time to organize a mandatory evacuation of its 390,000 residents. Mayor LaToya Cantrell urged residents to leave voluntarily. Those who stayed were warned to prepare for long power outages amid sweltering heat.

Nick Mosca was walking his dog, like most of those who were out.

“I’d like to be better prepared. There’s a few things I’m thinking we could have done. But this storm came pretty quick, so you only have the time you have,” Mosca said.

Governor John Bel Edwards vowed Saturday that Louisiana’s “resilient and tough people” would weather the storm. He also noted shelters would operate with reduced capacities “to reflect the realities of COVID.”

Edwards said Louisiana officials were already working to find hotel rooms for evacuees so that fewer had to stay in mass shelters. He noted that during last year’s hurricane season, Louisiana found rooms for 20,000 people.

Coastal Gulfport, Mississippi, was battered Sunday by intense rain. A Red Cross shelter posted signs displaying directions for evacuees, along with warnings about COVID-19. Shelter manager Barbara Casterlin said workers were required to wear face masks. Evacuees were encouraged to do the same. Anyone who refuses will be sent to an isolated area, she said, as will people who are sick.

“We’re not checking vaccinations,” Casterlin said, “but we are doing temperature checks two or three times a day.”

President Joe Biden approved emergency declarations for Louisiana and Mississippi ahead of Ida’s arrival.

Comparisons to the August 29, 2005, landfall of Katrina weighed heavily on residents bracing for Ida. A Category 3 storm, Katrina was blamed for 1,800 deaths as it caused levee breaches and catastrophic flooding in New Orleans and demolished oceanfront homes in Mississippi. Ida’s hurricane force winds stretched less than 40 miles (64 kilometers) from the storm’s eye or less than half the size of Katrina.

Officials stressed that the levee and drainage systems protecting the city had been much improved since Katrina. But they cautioned flooding was still possible, with up to 24 inches (61 centimeters) of rain forecast in some areas.

“Ida will most definitely be stronger than Katrina, and by a pretty big margin,” said University of Miami hurricane researcher Brian McNoldy. “And, the worst of the storm will pass over New Orleans and Baton Rouge, which got the weaker side of Katrina.”

About two million people live in and around New Orleans and Baton Rouge with many fewer people along the Mississippi River and wetlands to the south, which will get the brunt of the hurricane first.

Edwards said 5,000 National Guard troops were being deployed in 14 Louisiana parishes for search and rescue efforts. And 10,000 linemen were on standby to respond to electrical outages.

Hurricane Ida nearly doubled in strength, going from an 85 mph storm to a 150 mph storm in just 24 hours, which meteorologists called “explosive intensification.”

“Yikes! Ida leaves me stunned,” said Jeff Masters, a former NOAA hurricane hunter meteorologist and founder of Weather Underground.

He warned the region could face devastation to its infrastructure, which includes petrochemical sites and major ports. The state’s 17 oil refineries account for nearly one-fifth of the US refining capacity and its two liquefied natural gas export terminals ship about 55 per cent of the nation’s total exports, according to the US Energy Information Administration. Louisiana is also home to two nuclear power plants, one near New Orleans and another about 27 miles (about 43 kilometers) northwest of Baton Rouge.

The Interstate 10 corridor between New Orleans and Baton Rouge is a critical hub of the nation’s petrochemical industry, lined with oil refineries, natural gas terminals and chemical manufacturing plants. Entergy, Louisiana’s major electricity provider, operates two nuclear power plants along the Mississippi River.

A US Energy Department map of oil and gas infrastructure shows scores of low-lying sites in the storm’s projected path that are listed as potentially vulnerable to flooding.