Almost 800 rescued in St. John the Baptist Parish after Hurricane Ida

First responders will likely not be able to respond to calls in Louisiana until daybreak Monday, Gov. John Bel Edwards warned.

"I have no doubt we're going to see extreme devastation in Grand Isle and elsewhere," he said.

Over 15,000 people are in shelters across the state, the governor said.

Sixty-one intermediate care facilities, 22 nursing homes and 18 assisted living facilities have been evacuated, but no "tier one" hospitals have had to be evacuated, he said.

At least 442,925 customers in Louisiana are without power as of 5 p.m. ET.

Edwards said his biggest concern is outages because of how many people are in hospitals. "I understand that generators are great, but over time they tend to fail," he said.

-ABC News' Will McDuffie

Category 4 Hurricane Ida, moving northwest at 11 mph, is slamming Louisiana with powerful winds, torrential rain and dangerous storm surge.

When measuring wind speed, Ida is one of the three most powerful hurricanes to strike Louisiana. Along with Ida, Laura in 2020 and the Last Island hurricane in 1856 also all clocked in at 150 mph.

A tornado watch is in effect in parts of Louisiana, Mississippi, Alabama and Florida, and storm surge could top 8 feet along the Louisiana and Mississippi coast.

Ida will weaken through the night but will remain a dangerous hurricane, bringing rounds of very heavy rain and possibly flash flooding to the New Orleans area.

Louisiana and Mississippi could see 10 to 20 inches of rain in some areas.

The heavy rain and possible flash flooding will move inland Monday and Tuesday. Waverly, Tennessee, which was hit with deadly, catastrophic flooding last weekend, could see another 2 to 5 inches of rain late Monday into Tuesday.

-ABC News' Dan Peck

Louisiana Lt. Gov. Billy Nunguesser spoke with ABC News about the damage to the state and said it looked like it was going to be rough for the southern end.

“I believe it’s gonna be every bit as bad as Katrina," he told ABC News.

Nunguesser said flooding is already cataclysmic, and the levees near Myrtle Grove and other places were either breached or topped by water.

The lieutenant governor said he was especially concerned about Plaquemines Parish, which has oil and gas related infrastructure.

Nunguesser said he has heard reports of a giant floating platform on which oil workers live coming unmoored and floating somewhere in Barataria Bay.

-ABC News' Matt Gutman

President Biden spoke about the latest updates from FEMA headquarters in Washington, D.C., and promised he was going to "put the country's full might behind the rescue and recovery."

Biden said the agency has already prepositioned resources to the states most impacted by Ida including 2.5 million meals, 3 million liters of water and generators.

He urged residents in Mississippi and Louisiana to heed the warnings by their local leaders.

"Take it seriously, really...seriously," the president said.