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

Category 3 Hurricane Ida is churning northwest and is now located about 25 miles west-southwest of New Orleans.

Sunday night, Ida will charge across southeastern Louisiana, bringing a threat of storm surge and flash flooding.

Areas of heavy rain will continue in Mississippi and Louisiana Monday morning, with 10 to 20 inches of rain in some areas. Strong, gusty winds are expected, bringing a threat of more power outages.

Even as Ida weakens, the heavy rain and flash flooding threat will persist well inland into Monday and Tuesday, eventually reaching the Tennessee Valley. Flash flood watches are in effect in Tennessee.

Hurricane Ida is Louisiana's second strongest hurricane ever when measuring by pressure. Ida's pressure fell to 930 hPa -- only 2005's catastrophic Hurricane Katrina was lower with 920 hPa.

-ABC News' Melissa Griffin

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