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

Power could be out for weeks in the areas hit hardest by Ida, Entergy said Monday.

"Where weather permitted, our crews were out at first light today assessing damage where it was safe to do so," the energy company said in a statement. "This will help us get a better idea of what we’re dealing with. It would be premature to speculate at this time when power will be restored given the extent of the damage."

The ability to reach some areas is hindered by road closures and flooding, the company said.

"In harder to reach areas, we use advanced technology, such as infrared cameras, drones and satellite imagery to assess damage by foot, vehicles, airboats, highwater vehicles and helicopters. Even so, lack of access in areas like waterways and marshes could delay our damage assessment," Entergy said.          

-ABC News' Josh Hoyos

Tropical Storm Ida, now about 40 miles southwest of Jackson, Mississippi, is still bringing flash flood warnings to Louisiana, Mississippi and Alabama on Monday.

Up to 18 inches of rain has pummeled Louisiana. Up to 9 inches fell in Mississippi.

A tornado watch remains in effect in Mississippi, Alabama and Florida.

By Tuesday, Ida will move northeast into the Tennessee River Valley.

By Wednesday night into Thursday, Ida will track into the Northeast, dropping up to 6 inches of rain. Major flooding is possible along the Interstate 95 corridor from New York City to Philadelphia.

-ABC News' Max Golembo

The Karnofsky Tailor Shop, a historic national landmark in New Orleans, is one of the multiple buildings that collapsed when Ida walloped the city.

The brick two-story shop, a former tailor business in the Central Business District of the city, dates back to 1913 and is where Louis Armstrong worked before embarking on his legendary jazz career.

The family that owned the shop provided a second home for Armstrong and loaned him money to purchase his first cornet, according to the National Park Service.

-ABC News' Ginger Zee

Louisiana Gov. John Bel Edwards told MSNBC Monday that search and rescue efforts are ongoing and he expects Ida's death toll to "go up considerably throughout the day."

Helicopters are surveying damage because it will take “many days” to reach Louisiana's southern coastal areas by ground, he said.

Nearly all of southeast Louisiana is without power, the governor said. All eight major lines that feed electricity to the New Orleans area have failed.

-ABC News' Josh Hoyos