At least three people have died as a result of the storm.
Ida is barreling through Louisiana after making landfall in the state as a powerful Category 4 hurricane on Sunday afternoon.
It was one of the strongest hurricanes on record -- by both wind speed and pressure -- to roar ashore in Louisiana.
Ida, now a tropical storm, is hitting on the 16-year anniversary of Katrina, a Category 3 hurricane that ravaged the Gulf Coast. Hurricane Katrina unleashed a series of events, taking the lives of more than 1,800 people and leaving more than $100 billion worth of damage in its wake.
More than 789,000 customers in Louisiana are without power as of 9 p.m. ET. In Mississippi, more than 16,000 have lost power.
Flash flooding is ongoing in the New Orleans area where there's already been 4 to 7 inches of rain.
All of New Orleans has lost power as Hurricane Ida pounds Louisiana.
Power in the city is only coming from generators, according to New Orleans’ Office of Homeland Security and Emergency Preparedness.
Pumps are operating using self-generating sources of power.
In Lafourche Parish, south of New Orleans, the Thibodaux Regional Health System suffered a partial loss of power earlier Sunday, said Mindy Faciane, public information officer for the Louisiana Department of Health.
A boil water advisory has been issued for the entire East Bank of Jefferson Parish.
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
