Louisiana has declared a state of emergency as it prepares for a tropical depression expected to bring up to 15in (38cm) of rain over the weekend.

Tropical Depression 13 - to be named Lee if upgraded, as expected, to a tropical storm - is creeping north through the Gulf of Mexico.

New Orleans Mayor Mitch Landrieu warned the city could face major flooding, with 8-10in of rain forecast.

Meanwhile, Katia has regained hurricane status with winds of 75mph (120km/h).

The Atlantic hurricane season usually brings about a dozen named storms, but Katia is already the 11th with half the season still ahead.

Busy storm season

The US is still recovering from Hurricane Irene, which hit the east coast last weekend, killing more than 40 people. Nearly 900,000 homes and businesses still have no electricity.

On Friday morning the tropical depression was slowly drifting northward, packing winds of 35mph, and located about 210 miles south-west of the mouth of the Mississippi river.

It could become a tropical storm later on Friday, according to the National Hurricane Center.

Louisiana Governor Bobby Jindal declared the state of emergency on Thursday evening, saying coastal and inland areas could be hit by flash floods within 48 hours.

The Army Corps of Engineers, which operates the flood control structures in New Orleans, says it is not planning to close any of the structures yet.

Some offshore platforms for BP, Exxon Mobil and Shell off the Louisiana coast have already shut down and workers have been evacuated.

Tropical storm warnings are now in effect from the Texas state line to the eastern Mississippi coast, including New Orleans.

Flash flood warnings have been extended east along the Alabama coast and Florida Panhandle.

The developing weather system could bring some much-needed rain to Texas, which is in the grip of a severe drought.

Louisiana, which has also been affected by the drought, is already seeing sporadic rainfall that could help extinguish nearby marsh fires.

"Sometimes you get what you ask for," New Orleans Mayor Landrieu said. "Unfortunately it looks like we're going to get more than we needed."

He said the city, which was devastated by Hurricane Katrina in 2005, was prepared for the storm, with emergency crews on standby.

National Hurricane Center director Bill Read said: "We've got a huge area of moisture. We've got a developing wind field.

"We're probably going to see some tremendous rain amounts and the corresponding flooding that goes with that."

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