By Jane Sutton
MIAMI (Reuters) - The first cyclone to threaten the U.S. coast this year moved across the Gulf of Mexico on Thursday and was forecast to sweep through offshore oil installations before hitting the mainland between Louisiana and the Florida Panhandle, the U.S. National Hurricane Centre said.
Some energy companies in the Gulf started shutting down production and evacuating workers from offshore platforms as Tropical Storm Karen approached a region that produces nearly a fifth of daily U.S. oil output.
Three days after much of the U.S. government was closed down over a budget standoff, the Federal Emergency Management Agency began recalling furloughed workers to help prepare for the storm.
Karen, the first storm of the 2013 Atlantic hurricane season to take aim at the United States, had top winds of 65 mph (105 kph) and was centred about 340 miles (545 km) south of the mouth of the Mississippi River.
It was moving north-northwest and was expected to strengthen to just below hurricane strength by late Friday, the Miami-based hurricane centre said.
Coastal residents could start feeling its bluster by Friday night. On its current track the storm's centre was expected to cross the coastline near the Mississippi-Alabama border by late on Saturday.
A hurricane watch was issued for the coast from Grand Isle, Louisiana, south of New Orleans, to Destin, Florida, alerting residents to expect hurricane conditions within 48 hours.
A tropical storm watch was in effect in Louisiana from west of Grand Isle to east of Morgan City. The watch area included metropolitan New Orleans, Lake Maurepas and Lake Pontchartrain. Tropical storms carry winds of 39 mph to 73 mph (63 kph to 118 kph).
Heavy rains were forecast all along the Gulf Coast into northern Florida, the forecasters said.
The National Hurricane Centre forecasters were exempt from the U.S. government shutdown because their work is vital to protecting life and property. 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.
(Reporting by Jane Sutton in Miami, Kristin Hays in Houston and Mark Felsenthal in Washington.; Editing by Andrew Hay, Christopher Wilson and Ken Wills)