The storm was heading towards oil and gas facilities in the central Gulf of Mexico at speeds of more than 100mph.
US oil companies were shutting production and evacuating workers from the Gulf in the face of Ida, a Category 2 storm. Oil rose more than $1 to above $78 a barrel.
In El Salvador, rivers burst their banks and hillsides collapsed under relentless rains triggered by Ida's passage, cutting off parts of the mountainous interior from the rest of the country.
El Salvador's government said most of those who died were killed as mudslides and floods swept away rudimentary houses.
The bulk of the Central American country's coffee is grown in areas far from the worst affects of the flooding but the national coffee association had no estimate of potential damage to the harvest.
The National Hurricane Center said Ida was expected to weaken but would likely remain a hurricane as it approached the northern Gulf Coast. It was forecast to hit somewhere between Louisiana and Florida.
Several large producers shut down some oil and gas production as a precautionary measure.
The Louisiana Offshore Oil Port, the only terminal in the United States capable of handling the largest tankers, said it would stop unloading ships due to stormy seas.
A quarter of US oil and 15% of its natural gas are produced from fields in the Gulf and the coast is home to 40% of the nation's refining capacity.
A tropical storm warning was in effect for parts of Louisiana and Mississippi, including New Orleans, which is still recovering from the devastation of Hurricane Katrina in 2005.
Louisiana Governor Bobby Jindal declared a state of emergency on Sunday, allowing the government to mobilize troops and rescue workers.
If Ida makes landfall in Louisiana, it would be the first storm to strike the state since Hurricane Gustav came ashore in September 2008.
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