Heavy rain and pounding surf driven by Hurricane Sally hit the Florida and Alabama coasts on Tuesday as forecasters expected the slow-moving storm to dump continuous deluges before and after landfall, possibly triggering dangerous, historic flooding along the northern Gulf Coast.

"It's going to be a huge rainmaker," said Phil Klotzbach, a research scientist and meteorologist at Colorado State University. "It's not going to be pretty."

The National Hurricane Centre expected Sally to remain a Category 1 hurricane, with top sustained winds of 130km/h at landfall late Tuesday or early Wednesday. The storm's sluggish pace made it harder to predict exactly where its centre will strike.

The hurricane's slow movement exacerbated the threat of heavy rain and storm surge. Sally remained dangerous even after losing power, its fiercest winds having dropped considerably from a peak of 161km/h on Monday.

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Tuesday evening, hurricane warnings stretched from east of Bay St Louis, Mississippi, to Navarre, Florida. Rainfall of up to 50cm was forecast near the coast. There was a chance the storm could also spawn tornadoes and dump isolated rain accumulations of 76cm.

A man walks near the Gulf as Hurricane Sally moves in, in Gulf Shores. Photo / AP
A man walks near the Gulf as Hurricane Sally moves in, in Gulf Shores. Photo / AP

Heavy rain and surf pounded the barrier island of Navarre Beach, Florida, on Tuesday afternoon and road signs wobbled in the gusty wind. Rebecca Studstill was among those watching. Studstill, who lives inland, was wary of getting stuck on the island, saying police close bridges once the wind and water get too high. "Just hunkering down would probably be the best thing for folks out here," she said.

Two large casino boats broke loose on Tuesday from a dock where they were undergoing construction work in Bayou La Batre, Alabama. MJ Bosarge, who lives near the shipyard, said at least one of the riverboats had done considerable damage to the dock.

"You really want to get them secured because with wind and rain like this, the water is constantly rising," Bosarge said. "They could end up anywhere. There's no telling where they could end up."

A man takes a photo on the beach at Gulf State Park. Photo / AP
A man takes a photo on the beach at Gulf State Park. Photo / AP

In Orange Beach, Alabama, towering waves crashed onshore Tuesday as Crystal Smith and her young daughter, Taylor, watched. They drove more than an hour through sheets of rain and whipping wind to take in the sight.

"It's beautiful, I love it," Smith said. "But they are high. Hardly any of the beach isn't covered."

Captain Michael Thomas, an Orange Beach fishing guide, secured boats and made other last-minute preparations. He estimated up to 13cm of rain had fallen in as many hours.

"I'm as prepared as I can be," Thomas said.

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A few kilometres away in Gulf Shores, Alabama, waves crashed over the end of the long fishing pier at Gulf State Park. Some roads in the town already were covered with water.

Stacy Stewart, a senior specialist with the National Hurricane Centre, warned that floods in the affected areas could be deadly.

"This is going to be historic flooding along with the historic rainfall," Stewart said. "If people live near rivers, small streams and creeks, they need to evacuate and go somewhere else."

Storm surge from Hurricane Sally overtakes the outside parking lot and the first floor of the Palace casino parking garage in Biloxi. Photo / AP
Storm surge from Hurricane Sally overtakes the outside parking lot and the first floor of the Palace casino parking garage in Biloxi. Photo / AP

Donald Jones, a National Weather Service meteorologist in Louisiana, said Sally could unleash flooding similar to what Hurricane Harvey inflicted in 2017 when it swamped the Houston metropolitan area.

Mississippi Governor Tate Reeves said on Tuesday that people in the southern part of the state should prepare for heavy rain and the potential for flash flooding, even if the hurricane comes ashore to the east. He said about 120 people were in shelters in Mississippi.

As rain grew heavier on Tuesday, many businesses appeared to be closed at exits along the I-10 highway that runs parallel to the Gulf Coast from Louisiana to Florida.

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An empty vehicle sits in floodwaters in a driveway in Pascagoula. Photo / AP
An empty vehicle sits in floodwaters in a driveway in Pascagoula. Photo / AP

In Gulfport, Mississippi, white plastic bags hung over some gas station pumps to signal they were out of fuel. Along a bayou that extended inland from the Gulf, three shrimp boats were tied up as shrimpers and others tried to protect their boats from waves and storm surge. Most boat slips at Gulfport's marina were empty. Metal storm shutters or plywood covered the windows of many businesses.

David Espinosa walked the streets of Pascagoula, Mississippi, on Tuesday afternoon, drenched by the rain. He wasn't worried much about Sally, having found his pickup truck in a tree after Hurricane Katrina wrecked much of Mississippi's coast in 2005.

Espinosa had just moved back to the area days earlier, after a long stint in Oklahoma City.

"We just didn't know there would be another hurricane when we got back," Espinosa said. "Here we go again."

In Alabama, officials closed the causeway to Dauphin Island and the commuter tunnel that runs beneath the Mobile River.

Storm surge from Hurricane Sally overtakes the outside parking lot and the first floor of the Palace casino parking garage in Biloxi. Photo / AP
Storm surge from Hurricane Sally overtakes the outside parking lot and the first floor of the Palace casino parking garage in Biloxi. Photo / AP

Alabama Governor Kay Ivey urged residents near Mobile Bay and low-lying areas near rivers to evacuate if conditions still permitted a safe escape. The National Hurricane Centre predicted storm surge along Alabama's coast, including Mobile Bay, could reach 1.8m above ground.

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"This is not worth risking your life," Ivey said during a news conference Tuesday.

The storm was moving at only 4km/h Tuesday afternoon, centered about 135km south of Mobile, Alabama, and 145km southwest of Pensacola, Florida. Hurricane-force winds stretched 65km from its centre.

After making landfall, Sally was forecast to cause flash floods and minor to moderate river flooding across inland portions of Mississippi, Alabama, northern Georgia and the western Carolinas through the rest of the week.

President Donald Trump issued emergency declarations for parts of Louisiana, Mississippi and Alabama on Monday, and tweeted that residents should listen to state and local leaders.

Waves crash near a pier, at Gulf State Park. Photo / AP
Waves crash near a pier, at Gulf State Park. Photo / AP

The threat to Louisiana eased as officials in some areas reversed evacuation orders that had been issued for areas that had been feared to be a risk of flooding from Sally.

Florida Governor Ron DeSantis declared an emergency in 13 counties.

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On the barrier island of Pensacola Beach, Florida, the Sandshaker Lounge was open on Tuesday afternoon, filled with about 30 locals and tourists staying at nearby hotels.

"I think I'm the only business open," said bartender Kyra Smith. "It's pretty windy, but nobody's being knocked down. We want everybody to be safe."

Waters from the Gulf of Mexico pour on to a local road, in Waveland. Photo / AP
Waters from the Gulf of Mexico pour on to a local road, in Waveland. Photo / AP

Smith said most locals have lived in the area for decades and have weathered many storms bigger than Sally. "We're just going to ride it out," she said.