A tropical storm brewing in the Gulf of Mexico threatens to dump heavy rains and dangerous flash floods across a large swath of the southern United States, the National Weather Service warned Friday.

Oil companies evacuated workers from offshore rigs ahead of the arrival of Lee while Louisiana's governor Bobby Jindal declared a state of emergency, urging residents to "prepare for the worst and hope for the best."

The slow-moving storm could bring the same kind of flooding that residents in the northeast are still grappling with after Hurricane Irene tore up the east coast last weekend, officials warned.

Irene affected more than 40 million people, was blamed for nearly 50 deaths, triggered historic flooding and caused what one risk assessment firm estimated to be more than $10 billion in damage before blowing itself out over Canada.

The biggest danger from Tropical Storm Lee -- the 12th named storm of the Atlantic season, which is already dumping rain across coastal Louisiana -- could be in the Appalachians.

"If we get the five to 10 inches that come out into a tropical storm in that kind of terrain, the flash flooding is fast and it's violent," Bill Read, director of the National Hurricane Center in Miami, told reporters.

With some areas forecast to receive up to 20 inches of rain over the Labor Day holiday weekend, residents in low-lying areas from Louisiana and Mississippi all the way up to Kentucky and Tennessee should prepare themselves for extensive flooding, he cautioned.

It could also bring isolated tornados.

Mississippi Governor Haley Barbour declared a state of emergency in several counties, urging residents to prepare well in advance.

"Do not underestimate the impact of this system of tropical weather," he said.

The weather service is also monitoring the strengthening of Hurricane Katia, which had been downgraded to a tropical storm earlier in the week but regained hurricane status Friday after passing over warmer water.

Forecast models vary, and Katia is still well out to sea, but several tracks show the hurricane taking aim at the US eastern seaboard sometime next week.

The current hurricane could clip islands ringing the eastern Caribbean this weekend, with the NHC warning that Katia would send "life-threatening surf" barreling into the Lesser Antilles later Friday.

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