By Chris Baltimore

HOUSTON (Reuters) - Ida weakened to a tropical storm on Monday as it churned toward oil and gas facilities in the Gulf of Mexico and was forecast to hit the U.S. Gulf Coast early on Tuesday between Louisiana and Florida.

Ida posed the first real storm threat of the 2009 hurricane season to Gulf of Mexico oil and natural gas production, and forced some companies to shut down off-shore platforms and evacuate personnel.

Alabama Governor Bob Riley declared a state of emergency to allow its government to mobilize troops and rescue workers. Louisiana issued a similar declaration on Sunday.

At its earlier hurricane strength, Ida triggered floods and mudslides that killed 124 people in El Salvador.

Ida's top sustained winds dropped to 70 miles per hour, and the storm is unlikely to regain hurricane strength before it makes landfall, the National Hurricane Center said.

Oil rose more than $2 to near $80 a barrel on Monday on fears that Ida would cut the U.S. oil and gas supply.

The Louisiana Offshore Oil Port, the only U.S. terminal capable of handling the largest tankers, stopped unloading ships due to stormy seas. And the Independence Hub, a major offshore natural gas processing facility, also was closed.

A quarter of U.S. oil and 15 percent of its natural gas are produced from fields in the Gulf, and the coast is home to 40 percent of the nation's refining capacity.

The Miami-based hurricane center discontinued hurricane warnings along the U.S. Gulf Coast, and said a tropical storm warning was in effect from Grand Isle, Louisiana, eastward to Aucilla River, Florida, meaning that tropical storm conditions are expected within 24 hours.

NEW ORLEANS IN WARNING AREA

The warning area included New Orleans, which is still recovering from the devastation of Hurricane Katrina in 2005.

At 1 p.m. EST (1800 GMT), the storm was about 115 miles south-southeast of the mouth of the Mississippi River, the hurricane center said.

River traffic between New Orleans and the Gulf of Mexico was halted due to rough seas, the U.S. Coast Guard said.

In Mobile, Alabama, on the U.S. Gulf Coast, Governor Bob Riley warned the state's residents to be on their guard, and declared a state of emergency for the state.

"At this point, we don't know how substantial this damage could be," Riley said. "We hope it continues to dissipate."

Local authorities reported flooding from waves and storm surge at the developed west end of Dauphin Island, the barrier island off Mobile, which was heavy damaged by Katrina in 2005.

The hurricane center earlier on Monday had downgraded Ida from a Category 2 hurricane to Category 1, the lowest rank on the five-step Saffir-Simpson intensity scale, before further downgrading it to a tropical storm.

In El Salvador, rivers burst their banks and hillsides collapsed under rains triggered by Ida's passage, cutting off parts of the mountainous interior from the rest of the nation.

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.

Ida first became a hurricane on Thursday off the Caribbean coast of Nicaragua.

(Additional reporting by Jose Cortazar and Michael O'Boyle in Cancun, Nelson Renteria in San Salvador, Ivan Castro in Managua, Erwin Seba in Houston and Kelly Dugan in Mobile, Alabama; Editing by Will Dunham)