At 0600 GMT Cindy was located about 200 miles (325 kilometers) south of Morgan City, Louisiana, and 270 miles (435 kilometers) southeast of Galveston, Texas, the National Hurricane Center reported.
The storm packed winds of 60 miles (95 kilometers) per hour, and was moving towards the northwest at seven miles (11 kilometers) per hour.
Cindy is forecast to hit the coast of southwest Louisiana and southeast Texas late today, and move inland on Thursday.
"This rainfall could cause life-threatening flash flooding," the NHC warned.
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Cindy is not expected to reach hurricane status, but is nevertheless large: tropical storm force winds "extend outward up to 275 miles, mainly north through northeast of the center," the NHC said.
The weather service also warned of possible isolated tornadoes today from southern Louisiana to northern Florida.
The NHC issued a tropical storm warning for the coastal region between the central coast of Texas to the western Florida panhandle.