This National Oceanic and Atmospheric Administration satellite image taken Monday shows Ida when it was still a hurricane about 574 kilometres southeast of the mouth of the Mississippi River. (Associated Press/Weather Underground)

Forecasters downgraded Ida to a tropical storm on Monday morning as the former hurricane approached the U.S. Gulf Coast.

The U.S. National Hurricane Center said that as of 9 a.m. CT, Ida's maximum sustained winds topped out at 110 km/h, with higher gusts.

All hurricane warnings that were in place along the U.S. Gulf Coast were dropped. A tropical storm warning was put in effect from Grand Isle, La., east to Florida's Aucilla River.

Senior U.S. forecaster Richard Pasch said Ida was expected to reach the coast overnight.

"Anywhere from the Louisiana coast to the Florida panhandle will see the strong winds heavy rains and large waves and storm surge flooding, particularly in the eastern part of that area," he said.

The centre of Ida was about 300 kilometres south-southeast of the mouth of the Mississippi River and about 460 kms south-southwest of Pensacola, Fla. The storm was moving toward the north-northwest at 28 km/h, and was expected to turn northward.

Forecasters predicted Ida would make landfall Tuesday morning, then turn east.

While no mandatory evacuation orders were posted, Florida and Louisiana both declared states of emergency.

In Florida, the residents of Pensacola Beach and Perdido Key, as well as people who live farther inland in mobile homes, were encouraged to leave their homes. Schools were closed in the area Monday and Tuesday.

"Even though we're telling everybody to be prepared, my gut tells me it probably won't be that bad," said Steve Arndt, director of Bay Point Marina Co. in Panama City, Fla.

The first storm of the season to target the U.S., Ida was expected to dump between 75 and 150 millimetres of rain, with some areas possibly getting 200 millimetres through Tuesday, from central and eastern Gulf Coast regions northward to the Tennessee Valley and the southern Appalachians.

On Sunday, Ida lashed the Mexico resort of Cancun with heavy wind and rain.

In El Salvador, at least 124 people have died from flooding following three days of heavy rains that were indirectly tied to Ida.

With files from The Associated Press