Tropical Storm Debby formed Saturday, the National Hurricane Center said, making it the first named storm in the Gulf of Mexico this season.
A tropical storm warning was issued for part of Louisiana's coast from the Pearl River west to Morgan City, the center said in a late afternoon advisory. New Orleans was not in the warning area, however.
Debby had maximum sustained winds at 50 mph and was likely to strengthen further, the center said. At 8 p.m. EDT, Debby was nearly stationary, the center said.
Tropical-storm force winds extended 175 miles out from the center and should reach the U.S. coast by Sunday night.
"A slow northward motion is expected tonight, followed by a westward turn on Sunday" and then Debby's center should move over the northern Gulf over the next few days, the center added.
Areas from Louisiana to Florida's Panhandle were told to expect 3-6 inches of rain, and even 10 inches in some places. At 8 p.m. EDT, outer rainbands were lashing portions of west-central and south Florida, the Hurricane Center said.
As a precaution, many oil companies were evacuating rig workers and support personnel from offshore installations in the area.
About 8 percent of oil and natural gas production in the Gulf had been suspended, the U.S. government said Saturday.
Debby makes four named Atlantic storms before July 1 -- the first time that's happened since record-keeping began in 1851.
The Atlantic hurricane season officially runs from June 1 to Nov. 30, but got off to an early start this year. In May, Tropical Storm Alberto formed off South Carolina and then quickly fizzled, while Tropical Storm Beryl soaked the Southeast.
Tropical Storm Chris on Friday briefly became a hurricane before fizzling out. Still, it was notable for being unusually far north for an early hurricane.
Government forecasters predict nine to 15 named storms in the Atlantic, with four to eight becoming hurricanes.
Reuters contributed to this report.