Tropical Storm Ida has intensified as it swirls towards Cuba, showing hallmarks of a rare, rapidly intensifying storm that could speed across Gulf waters and slam into Louisiana as a major hurricane, the National Hurricane Centre warns.
"The forecast track has it headed straight towards New Orleans. Not good," said NOAA's Jim Kossin, a climate and hurricane scientist.
Ida posed a relatively low threat to tobacco-rich western Cuba, where forecasters predicted a glancing blow on Friday.
The real danger begins over the Gulf, where forecasts were aligned in predicting Ida will strengthen very quickly into a major hurricane before landfall in the area of the Mississippi River delta late on Sunday or early Monday, experts said.
"Ida certainly has the potential to be very bad," said Brian McNoldy, a hurricane researcher at the University of Miami. "It will be moving quickly, so the trek across the Gulf from Cuba to Louisiana will only take 1.5 days."
Friday morning, Ida's maximum sustained winds swiftly rose from 75km/h to 95km/h as it moved away from Grand Cayman toward Cuba's Isle of Youth at about 24km/h. Tropical storm-force winds extended as far as 130km from the centre.
A hurricane watch for New Orleans and an emergency declaration for the state of Louisiana were declared.
"Unfortunately, all of Louisiana's coastline is currently in the forecast cone for Tropical Storm Ida, which is strengthening and could come ashore in Louisiana as a major hurricane as Gulf conditions are conducive for rapid intensification," said Governor John Bel Edwards.
"By Saturday evening, everyone should be in the location where they intend to ride out the storm," the governor added.
A hurricane watch was in effect from Cameron, Louisiana, to the Mississippi-Alabama border - including Lake Pontchartrain, Lake Maurepas and metropolitan New Orleans.
Dangerous storm surge was also possible for the Gulf Coast. Depending on the tide as Ida approached the coast, 2.1 to 3.4 metres of storm surge was forecast from Morgan City, Louisiana, to Ocean Springs, Mississippi.
"There is an increasing risk of life-threatening storm surge, damaging hurricane-force winds, and heavy rainfall Sunday and Monday, especially along the coast of Louisiana," the hurricane centre said.
The mayor of Grand Isle, a Louisiana town on a narrow barrier island in the Gulf, said a voluntary evacuation late Thursday would become mandatory on Friday.
The storm was forecast to drop anywhere from 15 to 30cm of rain over parts of Jamaica, Cuba and the Cayman Islands, with the potential for more in some isolated areas.
Forecasters warned of possible flash floods and mudslides and tidal storm surge of up to a metre or more above normal, along with "large and destructive waves."