Tropical Storm Karen forms in the Gulf of Mexico; hurricane watch from La. to Fla.

Updated at 3:45 p.m.

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.

Louisiana Gov. Bobby Jindal declared a state of emergency after Karen threatened to come ashore over the weekend along the Gulf coast. The declaration authorizes the state Office and Homeland Security and other state agencies to activate emergency plans for the tropical storm's possible impact on Louisiana parishes.

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.

CBS News weather consultant David Bernard reports that Karen is expected to slow as it approaches the coast, increasing the likelihood of major rains along the Gulf Coast and across the Southeast through the weekend.

The National Hurricane Center in Miami said Karen was about 430 miles south of the mouth of the Mississippi River on Thursday afternoon and had maximum sustained winds of 65 mph.

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.

NOAA Storm Tracker: Atlantic

Karen was moving north-northwest at 12 mph. It could be at or near hurricane strength by Friday before approaching the northern Gulf Coast a day later, 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.

Grand Isle Mayor David Camardelle, whose barrier island community about 60 miles south of New Orleans is often the first to order an evacuation in the face of a tropical weather system, said the town is making sure its 10 pump stations are ready. He is encouraging residents and clean out drainage culverts and ditches in anticipation of possible heavy rain and high tides.

Otherwise, residents were monitoring the storm and hoping to dodge the foul weather.

"Hopefully, this one is just a little rain event," said Camardelle "We don't need a big storm coming at us this late in the season."

Zelinsky said residents in the warning areas should listen to their local emergency managers for advisories. "Now is the time to begin making preparations," Zelinsky said.

Forecasters said a cold front approaching from the northwest was expected to turn Karen to the northeast, away from the Louisiana coast and more toward the Florida Panhandle or coastal Alabama. But the timing of the front's arrival over the weekend was uncertain.

Grand Isle suffered damage from Hurricane Isaac in August 2012. Isaac clipped the mouth of the Mississippi River for its official first landfall before meandering northwest over Grand Isle and stalling inland. Though a weak hurricane, Isaac's stall built a surge along the southeast Louisiana coast that flooded communities in neighboring Plaquemines Parish.

Mike Steele, a spokesman for the Louisiana Governor's Office of Homeland Security and Emergency Preparedness, said a conference call was planned Thursday morning for state and parish officials to discuss the storm.

"Just so we can be in contact and see if there are any needs we can help meet," Steele said.

Karen was expected to pass over Gulf oil and gas fields from Louisiana to Alabama, but early forecasts suggested the storm would miss the massive oil import facility at Port Fourchon, La., just west of Grand Isle, and the oil refineries that line the Mississippi River south of Baton Rouge.

Oil giant BP said it has begun securing offshore rigs and evacuating non-essential workers from its four company-operated production platforms in Karen's projected path.

Other oil companies were expected to take similar action.