By Jane Sutton
MIAMI (Reuters) - The first cyclone to threaten the U.S. coast this year formed over 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 Center said.
Some oil industry workers in the Gulf were evacuated 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.
The storm, the first of the 2013 Atlantic hurricane season to take aim at the United States, had top winds of 65 mph and was centered about 400 miles south of the mouth of the Mississippi River.
It was moving north-northwest and was expected to be at or near hurricane strength, packing sustained winds of about 74 miles per hour, by late Friday, the Miami-based Hurricane Center said.
Coastal residents could start feeling its bluster by Friday night and on its current track its center 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 also included metropolitan New Orleans, Lake Maurepas and Lake Pontchartrain. Tropical storms carry winds of 39 mph to 73 mph.
Heavy rains were forecast all along the Gulf Coast into northern Florida, the forecasters said.
The Hurricane Center 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 Tom Brown and Andrew Hay)