The US city of New Orleans prepared Wednesday to face the first tropical storm of the season, which could become a hurricane and led the governor of Louisiana to declare a state of emergency.

"It is still too soon to tell what the impact will be, but we believe there will be an impact," New Orleans Mayor LaToya Cantrell said at a press conference in the city which was devastated by Hurricane Katrina in 2005.

This satellite image obtained from NOAA/RAMMB, shows a tropical storm formed in the Gulf of Mexico. AFP

"Be prepared for the impacts," she warned, reminding residents that the city had already received up to eight inches of rain in three hours that morning. There was partial flooding.

New Orleans was placed under a storm-surge watch on Wednesday morning 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 in the city, known worldwide for its Mardi Gras and jazz.

Part of New Orleans is built below sea level.

"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.

If the storm becomes a hurricane as anticipated, it would be the first of the Atlantic season and would be named Barry.

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.

Louisiana governor John Bel Edwards declared the state of emergency, which allows the state to provide additional resources for storm preparation.

Agence France-Presse