At least 17 million people are under a tropical storm warning from San Luis Pass, Texas, to the Alabama-Florida border, CNN meteorologist Michael Guy said.
The storm has already claimed at least one life. A 10-year-old boy died Wednesday in Fort Morgan, Alabama, when he was struck by a log that was being carried by a wave, the Baldwin County sheriff's office said.
The storm could bring up to 15 inches of rain to parts of southeastern Louisiana, southern Mississippi, southern Alabama and western portions of the Florida Panhandle through Thursday night.
"This rainfall could cause life-threatening flash flooding in these areas," the National Hurricane Center said in its latest advisory.
A few tornadoes and inundation of 1 -3 feet above the ground along the coast are also possible, the National Weather Service said.
At 11 p.m. ET Wednesday, Cindy was about 105 miles south of Lake Charles, Louisiana, and about 97 miles southeast of Galveston, Texas, with maximum sustained winds of 50 mph, according to the hurricane center advisory. It was moving northwest at 7 mph.
The center of the storm is projected to make landfall Thursday morning over southeastern Texas or southwestern Louisiana.
Cindy is the second tropical storm to form in the Atlantic Ocean since the hurricane season officially began June 1 and the third storm of the year. The season got an early start with Tropical Storm Arlene in April.
Heavy rain and some flash flooding already have been reported along the Gulf Coast, but more is on the way.
The most torrential rain would likely be to the east of the storm's center, because these areas will see a prolonged period of onshore flow, Guy said. Onshore flow happens when air moves from sea to land, usually with increased moisture.
As the storm comes ashore, the coast could also face winds of up to 50 mph.
'Prepare for the worst'
Flooding remains the greatest concern for state officials in Alabama, Louisiana, Mississippi and Texas.
Louisiana Gov. John Bel Edwards declared a state of emergency ahead of the storm.
"All arms of the state's emergency preparedness and response apparatus are taking Tropical Storm Cindy seriously, and we are calling on all Louisianans throughout the state to do so as well," Edwards said in a statement.
"Please do all you can to prepare for the worst while praying for the best."
New Orleans Mayor Mitch Landrieu urged residents Wednesday to remain alert, even though the National Weather Service had lifted a tropical storm warning in the city.
"We are not in the clear yet," he told reporters.
Up to 4 inches of rain fell Tuesday in New Orleans, and an additional 3 to 6 inches were expected over the next 48 hours, followed by weekend thunderstorms, Landrieu said.
Authorities in Louisiana announced they were closing courts and other government buildings across the state on Wednesday.
'A slow-motion disaster'
Alabama Gov. Kay Ivey declared a state of emergency to "guarantee state resources are on standby and are ready to assist impacted communities if necessary."
"There's been a lot of rain in the area over the last three to four weeks so the ground is pretty saturated," said Glen Brannan of Alabama's Mobile County Emergency Management Agency.
In Mississippi, the city of Biloxi declared a state of emergency because of risk of flooding. Residents in nearby counties were also offered sandbags.
"It's kind of like a slow-motion disaster for us now. It's not wind, it's rain we're concerned about," said Greg Flynn with the Mississippi Emergency Management Agency.
In Texas, elderly residents and families with medical issues were encouraged to evacuate their homes in Bolivar Peninsula near Galveston.
"The voluntary evacuation order is intended to alert residents of Bolivar Peninsula that emergency responders may be unable to reach them," Galveston County Judge Mark Henry said in a statement.
Texas Gov. Greg Abbott said emergency operations teams have been activated or are on standby to prepare for the storm's landfall.
"We stand ready to assist local communities in the event of an emergency, and I ask all Texans to keep those in the storm's path, and our brave first responders, in their prayers as they prepare for this storm," Abbott said in a statement.