On September 1, Ida transitioned into a post-tropical cyclone as it accelerated through the Northeastern United States, breaking multiple rainfall records in various locations before moving out into the Atlantic on the next day.

Afterward, Ida's remnant moved into the Gulf of St. Lawrence and stalled there for a couple of days, before being absorbed into another developing low-pressure area early on September 5.

[4][10] There was also a remarkable number of hospitalizations and deaths in the Greater New Orleans Area as a result of carbon monoxide poisoning while using portable gas generators with inadequate ventilation,[11][12] including three in a family of four in Marrero, Louisiana on September 1, 2021.

As the wave moved over the Windward Islands, the convective coverage increased at which point the National Hurricane Center (NHC) began to the track the system as it continued westward into the Caribbean Sea.

On August 25, a vorticity maximum emerged from the eastern side of the larger low-pressure area and convection became concentrated and better organized around it,[4] prompting the NHC to assess this disturbance as a high likelihood of development as it turned north-northwestward.

[18] At 12:00 UTC on August 26, the system had attained sufficient organization to be classified as Tropical Depression Nine, about 115 mi (185 km) south-southwest of Negril, Jamaica.

Upon its development, the depression was moving north-northwestward before turning northwestward a few hours later, steered around the southwestern side of a subtropical ridge located over the western Atlantic Ocean.

[4] Favorable factors in the system's further development included warm waters of the northwestern Caribbean Sea, low wind shear, and a moist environment.

[23] Around the same time, the hurricane made its first landfall on Cuba's Isle of Youth with sustained winds of 80 mph (130 km/h) and a minimum central pressure of 987 mbar (29.15 inHg).

After crossing over, Ida made its second landfall at 23:25 UTC at Playa Dayaniguas in Pinar del Río, Cuba, with the same winds speeds and a slightly higher 988 mbar (29.18 inHg) pressure.

[4] During the day-time hours of August 28 microwave satellite imagery and Cuban radar data showed Ida's central core to be reorganizing with the formation of a convective ring around the center.

This, combined with the favorable conditions of light vertical wind shear and warm sea surface temperatures, led to a second, and more significant, period of rapid intensification that commenced around 12:00 UTC on August 28.

At around 12:00 UTC that day, Ida reached its peak intensity, with 1-minute sustained wind speeds of 150 mph (240 km/h) and a minimum central barometric pressure of 929 mbar (27.43 inHg), while located not far southwest the mouth of the Mississippi River.

[33][34] On the next day, Ida's remnants moved northeastward across Atlantic Canada, bringing heavy rain and gale-force winds to communities throughout the region.

[35] Ida's remnants reached the Gulf of St. Lawrence on September 3, where it made a slow, counter clockwise loop while maintaining maximum winds of 45–50 mph (75–85 km/h).

[36][37][38][needs update] On August 28, 800 individuals, including teachers and students monitoring turtles on the Guanahacabibes Peninsula, were evacuated due to Ida, according to the head of civil defense in the area.

[47] On August 27, the preseason NFL football game for the Arizona Cardinals and New Orleans Saints, originally scheduled for the next day at Caesars Superdome, was cancelled due to the storm's forecast of being a major hurricane at the time of its landfall.

Flash flood watches were issued at 2 p.m. EDT on September 1 for all five boroughs, including Long Island and Orange, Putnam, Rockland, Ulster, Dutchess, Sullivan, and Westchester counties.

[63] In Pennsylvania, Governor Tom Wolf signed a proclamation of disaster emergency on August 31 in anticipation of flooding, severe storms, and tornadoes from the remnants of Ida.

[74] Severe damage was recorded across the coastal areas of Louisiana, including in New Orleans, Golden Meadow, Houma, Galliano, LaPlace, Lockport and Grand Isle.

[104] Outside the post-Katrina flood protection system, eight feet (2.4 m) of water overtopped levees in lower Jefferson Parish [105] forcing residents in Lafitte to flee to their rooftops to escape floodwaters.

[4] Up to 12 breaks in its sand levee led to 100% of structures being damaged, with 40% being completely destroyed, or being blocked by piles of debris, and electrical lines were toppled and buried.

[109] This was thought to be in part due to Hurricane Zeta's impacts from the previous year; many weak and dead trees and substandard structures were removed in that storm, potentially reducing the amount of debris available for Ida to cause damage with.

Three people were injured in Saraland by an EF1 tornado that ripped portions of roofs from motels and an industrial buildings and knocked down trees, some of which fell on homes.

[136] On September 1, a planned MLB baseball game between the Philadelphia Phillies and Washington Nationals was postponed due to the rain caused by the remnants of the storm.

[143] Flooding from the Schuylkill River caused damage to SEPTA Regional Rail's Manayunk/Norristown Line between Miquon and Norristown, resulting in the suspension of service.

Ida was the third tropical system in as many weeks to soak the Northeastern United States, after Fred and Henri, which left the soil saturated and a greater risk of flooding.

The New York City Comptroller's office denied all claims, based on a 1907 legal decision that does not hold municipal governments responsible for damage due to “extraordinary or excessive” rainfall.

[179][180] The rains caused flooding, forced the evacuation of several houses and a retirement home, as well as road closures, in the Grande-Vallée to Rivière-au-Renard region of the Gaspé where the accumulations exceeded 3.9 in (100 mm).

Thousands of rescues had happened in Pennsylvania alone that day, and first responders helped bring communities to safety from halted subway trains the night of the flooding in New York City.