Hurricane Sally is drawing closer to the US Gulf Coast, threatening historic floods, the National Hurricane Center says, with more than 60cm of rain expected in some areas.
The second strong storm in less than a month to threaten the region, Sally's winds decreased to 140km/h, and early on Tuesday was 100km east of the mouth of the Mississippi River, the NHC says, moving at a glacial pace of 3km/h.
It could wallop the Mississippi, Alabama and Florida coasts on Tuesday with massive flash flooding and storm surges of up to 2.7 metres in some spots.
Mobile, Alabama Mayor Sandy Stimpson warned residents he expected a "tremendous amount of flooding" and said the city was barricading intersections likely to see high water.
Mississippi and Louisiana called for evacuations of low-lying areas and President Donald Trump issued an emergency disaster declaration for both states. Alabama closed the state's beaches and recommended evacuations of residents in low-lying areas.
Ports, schools and businesses closed along the coast. Energy companies buttoned up or halted oil refineries and pulled workers from offshore oil and gas production platforms.
The hurricane is expected to dump between 25cm and 50cm of rain on the coast.
Mississippi appears most likely for landfall but Jim Foerster, the chief meteorologist at DTN, an energy, agriculture and weather data provider, said Sally's biggest threat was it would be a "rainmaker" across a wide stretch of the Gulf Coast.
Sally is the 18th named storm in the Atlantic this year and will be the eighth of tropical storm or hurricane strength to hit the United States - something "very rare if not a record", according to Dan Kottlowski, senior meteorologist at AccuWeather.
Australian Associated Press