Hurricane Sally is making a slow-motion crawl towards the US Gulf Coast, threatening historic floods and prolonged rainfall as storm-force winds start to lash the shore.
The governors of four states are urging people to flee the coastline.
Sally could wallop the Alabama, Florida and Mississippi on Tuesday night or early Wednesday with massive flash flooding and storm surges up to two metres, the National Hurricane Center said on Tuesday.
Its languid pace recalls 2017’s Hurricane Harvey, which dumped a metre of rain over a period of days on the Houston area, causing major damage.
More than 60cm of rain was expected in some areas, with “extreme life-threatening flash flooding likely through Wednesday,” an NHC forecaster said.
While Sally’s winds decreased to 140 km/h at 4 pm local time, it was moving at a glacial pace of 3 km/h.
Sally was 135 km south of Mobile, Alabama and spreading tropical-storm-force winds onshore, the NHC said.
“We’re staying in good spirits. We’re going to ride it out,” said local motel owner Jennie Bonfiglio.
About half the 38 rooms had guests and the hotel was stocked with torches, batteries, bottled water and canned food, she said.
“The wind has definitely picked up and some of the streets are starting to flood. I definitely wouldn’t go outside unless you had to.”
Sally will slow more after landfall, causing Atlanta to see up to 15 cm of rain through Friday, said meteorologist Jim Foerster.
“It’s going to be a catastrophic flooding event” for much of the southeastern United States, with Mobile to the western part of the Florida Panhandle taking the brunt of the storm.
Damage from Sally could reach $US3 billion ($A4.1 billion), said Chuck Watson of Enki Research, which models and tracks tropical storms.
That could rise if the heaviest rainfall happens over land instead of the Gulf.
Governors from Louisiana to Florida warned people to leave low-lying communities and Mobile County Sheriff Sam Cochran told residents of flood-prone areas if they chose to ride out the storm, it would be “a couple of days or longer before you can get out”.
Nearly 11,000 homes are at risk in the larger coastal cities in Alabama and Mississippi, according to estimates from property data and analytics firm CoreLogic.