Tropical storm Marco has become a hurricane over the Gulf of Mexico on a path toward the Louisiana coast.
Tropical storm Laura battered the Dominican Republic and Haiti and headed to the same part of the US coast, also as a potential hurricane.
It would be the first time two hurricanes form in the Gulf of Mexico simultaneously, according to records dating to at least 1900, said Colorado State University hurricane researcher Phil Klotzbach.
The National Hurricane Centre said Marco was about 300 miles south of the mouth of the Mississippi River and heading north-northwest at 14 miles per hour, packing winds of 75 miles per hour.
The centre warned of life-threatening storm surges and hurricane-force winds along the Gulf Coast.
Haitian civil protection officials said they had received reports that a 10-year-old girl had died when a tree fell on a home in the southern coastal town of Anse-a-Pitres, on the border with the Dominican Republic.
It was the first reported death from the storm.
Hundreds of thousands were without power in the Dominican Republic, as both countries on the island of Hispaniola suffered heavy flooding.
A hurricane watch was issued for the New Orleans metro area, which Hurricane Katrina pummelled in August 2005.
Laura was centred about 95 miles off the eastern tip of Cuba on Sunday morning, with maximum sustained winds of 50 mph. It was moving west-northwest at 21 mph.
Crews armed with megaphones in the Dominican capital of Santo Domingo had urged dozens of residents in flood-prone areas to evacuate before Laura’s heavy rains hit.
The storm left more than 100,000 people without water in the Dominican Republic on Saturday night, while earlier it snapped trees and knocked out power to more than 200,000 customers in neighbouring Puerto Rico.
It was forecast to move over Cuba on Sunday night or Monday.
Officials in the Florida Keys, which Laura might pass over on its route into the Gulf, declared a local state of emergency and issued a mandatory evacuation order for anyone living on boats, in mobile homes and in campers.
Tourists staying in hotels were warned to be aware of hazardous weather conditions and consider changing their plans starting on Sunday.
New warnings were added on Sunday morning — including a storm surge warning from Morgan City, Louisiana to Ocean Springs, Mississippi, and a hurricane warning from Morgan City to the mouth of the Pearl River.
A tropical storm warning included Lake Pontchartrain in Louisiana, and metropolitan New Orleans.
A storm surge of up to 6ft was forecast for parts of coastal Louisiana and Mississippi.
Louisiana governor John Bel Edwards, who declared a state of emergency on Friday, asked President Donald Trump for a federal emergency declaration.
“The cumulative impact of these storms will likely have much of Louisiana facing tropical storm/hurricane force impacts for a much longer period of time than it would with any one hurricane,” he wrote.
People in Louisiana headed to stores to stock up on food, water and other supplies.
Raymond Monday of Gretna, though, had only a generator on his cart at Sam’s Club. “We’ve got a freezer full of food” at home, along with large containers of water, he said.
Both storms were expected to bring three to six inches of rain to areas they were passing over or near, threatening flooding.
The hurricane centre said the storms were not expected to interact as the region faces an unusually active hurricane season.
“We are in unprecedented times,” Mississippi governor Tate Reeves said at a news conference on Saturday as he declared a state of emergency.
“We are dealing with not only two potential storms in the next few hours, we are also dealing with Covid-19.”