Barry was briefly upgraded to a hurricane but weakened to a tropical storm again hours later.

Slow-moving tropical storm Barry has weakened after making landfall in Louisiana, but authorities in the southern US state have urged residents to remain “vigilant” as it is expected to produce days of heavy rain.

After briefly becoming the first hurricane of the Atlantic season, Barry was downgraded to a tropical storm following its landfall on Saturday.

Still, Louisiana Governor John Bel Edwards warned that Barry could still cause disastrous flooding across a wide stretch of the Gulf Coast of the United States overnight.

“This storm still has a long way to go before it leaves this state,” Edwards said on Saturday night during a press conference.

“Some people might think that the threat is over, that is not the case. Don’t let your guard down,” he added.

Although Edwards warned residents of potential flooding and more rain, he also said that the crest predictions for many had gone down due to a change in the rain forecast.

“However, even the revised crest heights qualify as major flooding and present major risks to life and property.”

Edwards also said he had talked to US President Donald Trump, who told him he was “pulling for Louisiana” and said he would make sure the state would get the assistance it needed.

Al Jazeera’s Jay Gray said that in some places evacuation orders were in place and that people in Louisiana state capital New Orleans were told to stay indoors.

“They were told to make sure they have plenty of food, medicine, water and other supplies they need for as much as three days,” Gray said from New Orleans.

“The issue [with Barry] is it is moving so slowly, and the rain just continues to fall in what is already a saturated area.”

Flash flood warnings have affected about 11.7m people in Louisiana, Mississippi, Alabama, Arkansas, Tennessee, Kentucky, Missouri and Illinois.

Residents and business owners in New Orleans laid down sandbags and boarded up windows while city officials set up shelters for residents.

Mississippi and Alabama have been hit the hardest so far, with the Alabama city of Mobile reporting a total of 136mm of rain in last 36 hours.

Dangerous storm surges

Shortly before reaching land on Saturday, Barry was briefly upgraded to a hurricane for several hours but as it approached mainland US, its power weakened.

Despite the downgrade to a tropical storm, the National Hurricane Center (NHC) warned people of dangerous storm surges, strong winds and rain.

“Life-threatening, significant flash flooding and river flooding will become increasingly likely later today and tonight as Barry moves inland, especially across portions of south-central and southeast Louisiana into Mississippi,” the NHC said.

With Barry threatening massive rainfall across several southern states, federal emergency declarations were issued to help free up resources to address the storm.

“We could be looking at widespread major flooding across several river basins,” the NHC said.

Some of the scenes of heavy rains, a potential storm surge, and flooding are reminiscent of 2005’s deadly Hurricane Katrina.

The costliest and deadliest hurricane in US history, Katrina submerged about 80 percent of New Orleans, causing some 1,800 deaths and more than $150bn in damage.

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Downpours also lashed coastal Alabama and Mississippi.

Parts of Dauphin Island, a barrier island in Alabama 322 kilometres from where Barry was headed, were flooded both by rain and surging water from the Gulf, said Mayor Jeff Collier, who was driving around in a Humvee to survey the damage.

He said the island still had power early on Saturday afternoon and wind damage was minimal.