The first cyclone to threaten the US coast this year has formed over the Gulf of Mexico and was forecast to sweep through offshore oil installations before hitting the mainland between Louisiana and the Florida Panhandle, the US National Hurricane Centre said.
Some energy companies in the Gulf started shutting down production and evacuating workers from offshore platforms as Tropical Storm Karen on Thursday approached a region that produces nearly a fifth of daily US oil output.
Three days after much of the US 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 105 kph and was centred about 580 centre 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 119km per hour, 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 Centre, 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 63 kph to 118 kph.
Heavy rains were forecast all along the Gulf Coast into northern Florida, the forecasters said.
The National Hurricane Center forecasters were exempt from the US 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.