Louisiana Gov. Bobby Jindal declared a state of emergency Thursday after Tropical Storm Karen formed in the Gulf of Mexico and 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.

Forecasters said the storm is expected to approach the mouth of the Mississippi River and then shuttle off to the east toward the Florida Panhandle. But they warned the forecast track could change and advised residents and business in Louisiana, Mississippi, Alabama and Florida to prepare for possible tropical storm conditions.

Meanwhile, officials in coastal parishes advised residents to make preparations and took early steps to prepare for the storm.

Oil companies were preparing as well. Shell and BP said they were securing offshore rigs and evacuating non-essential workers. Others were expected to do so as well.

In New Orleans, the Army Corps of Engineers planned to close a storm-surge gate protecting the New Orleans area.

The gate designed to keep possible storm surge from Lake Borgne, east of the city, from entering the Inner Harbor Navigation Canal — known locally as the Industrial Canal — through the Gulf Intracoastal Waterway. In 2005, flood wall breaches along the canal contributed to catastrophic flooding during Hurricane Katrina. Karen was not expected to approach Katrina's strength or take a similar path.

Authorities in Plaquemines Parish and on the barrier island of Grand Isle were advising residents and business to make preparations, though no evacuations had been ordered.

Plaquemines President Billy Nungesser said all pump stations were operating and parish crews were on standby. Plaquemines' east bank took heavy damage from tidal surge that piled up after Hurricane Isaac stalled in August 2012.