NEW ORLEANS — Preparations began Thursday along the central Gulf Coast as newly formed Tropical Storm Karen threatened to become the first named tropical system to menace the United States this year.

Hurricane and tropical storm watches were posted from southeast Louisiana to Florida and some oil and gas platforms in the storm’s projected path were being secured and evacuated.

The National Hurricane Center in Miami said Karen was about 485 miles (775 km) south of the mouth of the Mississippi River and had maximum sustained winds of 65 mph (100 kph).

The hurricane watch was in effect from Grand Isle, La., to Indian Pass in the Florida Panhandle. A tropical storm watch also was in effect for parts of the Louisiana coast west of Grand Isle, including the New Orleans area.

Karen was moving north-northwest at 12 mph (19 kph). It could be at or near hurricane strength by Friday, forecasters said.

While meteorologists said it was too soon to predict the storm’s ultimate intensity, they said it could weaken a bit as it approaches the coast over the weekend.

“Our forecast calls for it to be right around the border of a hurricane and a tropical storm,” said David Zelinsky, a hurricane center meteorologist.

Whether a weak hurricane or strong tropical storm, Karen’s effects are expected to be largely the same: heavy rain and the potential for similar storm surge.