The US city of New Orleans was under a storm-surge watch Wednesday along with a stretch of Louisiana coast as a tropical storm formed in the Gulf of Mexico, threatening the region with potentially life-threatening rains.
The storm-surge watches warn residents of possible flooding from rising waters and coastal inundations.
"Conditions appear favorable for this system to strengthen to a hurricane as it approaches the central Gulf Coast by the weekend," the Miami-based National Hurricane Center said.
A flash flood emergency was declared earlier Wednesday in New Orleans, part of which is built below sea level and was devastated by Hurricane Katrina in 2005.
Parts of the southeastern Louisiana coast were already experiencing heavy rains and flooding, the NHC said.
Jefferson Parish, which includes parts of New Orleans, was drenched in four to six inches of rain (10-15 centimeters) and two to three more inches were expected.
The threat of a hurricane by the weekend -- the first of the Atlantic season if it materializes -- prompted a flurry of bad weather alerts.
At 1800 GMT, the storm system was 155 miles (250 kilometers) southeast of the mouth of the Mississippi River.
The storm was packing sustained winds of 30 miles per hour (48 kilometers), still way below hurricane force but on track to become a tropical storm by Thursday and a hurricane by Friday.
It was expected to unload as much as one foot (30 centimeters) of rain over the Gulf coast through early next week, the forecasters said.
Local authorities also issued warnings to residents as the Mississippi River neared flood levels, rising to 16 feet (4.9 meters) in New Orleans.
The levees protecting the city are built to hold back the river to a depth of 20 feet, a level that forecasters warned could be reached by Saturday morning.
The US Army Corps of Engineers, which operates the flood control system, said it was "closely monitoring" the situation.
New Orleans Mayor LaToya Cantrell closed the city hall and urged non-essential employees to stay home, while the city's airport reported numerous flight delays.