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

Ida has weakened from a Category 4 to a Category 3 hurricane Sunday evening as it pummels Louisiana with dangerous wind, rain and storm surge.

In Shell Beach, Louisiana, the storm surge reached over 8 feet.

A flash flood warning remains in effect in New Orleans, while a flash flood emergency has been issued in the area of Braithwaite, Louisiana.

An extreme wind warning is in effect in parts of southeastern Louisiana, and tornadoes are possible in Louisiana, Mississippi, Alabama and Florida.

-ABC News' Melissa Griffin

As of 6 p.m. ET, Category 4 Hurricane Ida was moving northwest near Houma, Louisiana.

Sunday night's forecast calls for heavy rain, high wind gusts, dangerous storm surge and possibly tornadoes and flash flooding.

A flash flood warning has been issued in New Orleans where 1 to 3 inches of rain is already on the ground with another 4 to 8 inches expected.

An extreme wind warning is in effect in parts of southeastern Louisiana extending to areas just southwest of New Orleans. Wind gusts up to 125 mph are possible Sunday night in St. John the Baptist, St. James, St. Charles, Terrebonne, Lafourche and Jefferson parishes.

-ABC News' Melissa Griffin

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