Governors of Florida and Alabama followed Louisiana's Indian American chief executive Bobby Jindal in declaring emergency as hurricane Ida weakened to a tropical storm but still dangerously churned toward the US Gulf Coast.
Forecasters warned of heavy rainfall when Ida comes ashore near the Mississippi-Alabama state line.
Although the forecast track does not show Ida making a direct hit on Louisiana, Jindal said, residents should be alert and prepared for the possibility of flooding, particularly in low-lying areas.
Authorities were continuing to monitor evacuation routes, he said, in case they need to be utilised.
A tropical storm warning stretched across most of the Gulf Coast, from Louisiana to the Florida Panhandle, including the city of New Orleans and adjacent Lake Pontchartrain.
Authorities do not expect New Orleans to receive more water than the city can handle, Jindal said. Workers in some coastal parishes were told to stay home, and some schools were closed.
Voluntary evacuations were issued in some coastal parishes, and at least one parish opened a shelter. State officials do not plan to open any state-operated shelters, Jindal said.
Louisiana has offered assistance to other coastal states as they probably will bear the brunt of Ida, the governor said.
Declaring an emergency, Alabama Governor Bob Riley said: "Based on the latest information I have seen, Alabama lies directly in the path of Tropical Storm Ida."
Mississippi Governor Haley Barbour added: "While Ida appears to be weakening, we urge that residents, particularly in low-lying, flood-prone areas, continue to be vigilant, prudent, and alert to any changes."
On the forecast track, the centre of Ida should reach the northern Gulf Coast, forecasters said. After landfall, Ida is expected to turn eastward on Tuesday.
American Airlines cancelled 15 flights Monday and 25 Tuesday in cities along the Gulf Coast. Delta Air Lines posted a notice to travellers on its Web site that flights in Gulf Coast cities might be affected.
The US Coast Guard shut down all non-emergency traffic into and out of ports along the Gulf coast.
Ida is the Atlantic region's ninth named storm. The Atlantic hurricane season typically peaks in September and ends Nov 30.