Tropical Storm Karen formed in the southeastern Gulf of Mexico on Thursday and could become a hurricane before hitting the U.S. coast between Louisiana and the Florida Panhandle, forecasters at the U.S. National Hurricane Center said.
Energy companies began evacuating some workers from oil and natural gas platforms in the Gulf of Mexico on Wednesday.
(Read more: Oil mixed as shutdown impasse lingers)
Anadarko Petroleum Corp said Thursday it had shut production at its Neptune platform, which has capacity to produce 14,000 barrels per day of oil and 23 million cubic feet per day of natural gas.
The storm, the first to threaten the U.S. coast during the 2013 Atlantic hurricane season, had top winds of 65 mph (105 kph) and was centered about 485 miles (775 km) south of the mouth of the Mississippi River.
A hurricane watch was issued for the coast from Grand Isle, Louisiana, eastward to Indian Pass, Florida, alerting residents to expect hurricane conditions within the next 48 hours.
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Karen would become a hurricane if its sustained winds reach 74 mph (119 kph). That was expected to happen late Friday.
"Some weakening is anticipated as Karen approaches the Gulf Coast but the storm could still be near hurricane strength at landfall," the forecasters said.
A tropical storm watch was in effect in Louisiana from Grand Isle west to Morgan City, and for New Orleans, Lake Maurepas and Lake Pontchartrain. Tropical storms carry winds of 39 mph to 73 mph (63 kph to 118 kph).
The Hurricane Center forecasters were exempt from the U.S. government shutdown because their work is vital to protecting life and property. But their parent agency, the National Oceanic and Atmospheric Administration, advised that some weather satellite images available to the public on its website "may not be up to date" because of the shutdown.
(Read more: Government workers feel sting of shutdown)