The Gulf Coast braced for a history-making onslaught from the twin storms, with many evacuees recalling the damage caused by Hurricane Katrina in 2005, when catastrophic flooding breached the levees in New Orleans and as many as 1,800 people died.
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"What we know is there's going to be storm surge from Marco, we know that that water is not going to recede hardly at all before Laura hits, and so we've not seen this before and that's why people need to be paying particular attention," Gov. John Bel Edwards warned at a Sunday briefing.
Laura caused the deaths of at least 11 people in the Dominican Republic and Haiti, while knocking out power and causing flooding in the two nations that share the island of Hispaniola.
Marco had grown into a hurricane early Sunday, but the National Hurricane Center said its sustained winds decreased to 70 mph after nightfall. The center cautioned that Marco could still cause life-threatening storm surges and dangerous winds along the Gulf Coast.
In a video issued to ABC News, high flood water was seen taking over a residential street in the Dominican Republic early Sunday morning.
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Marco was centered about 115 miles south-southeast of the mouth of the Mississippi River heading northwest at 10 mph Monday morning.
Laura was centered about 175 miles east-southeast of Cayo Largo on Monday morning, and had maximum sustained winds of 65 mph. It was moving west-northwest at 21 mph and was predicted to strengthen into a hurricane by Tuesday morning as it followed a path likely to take it to the Louisiana coast by Wednesday night, forecasters said.
Haitian civil protection officials said they had received reports a 10-year-old girl was killed when a tree fell on a home in the southern coastal town of Anse-a-Pitres, on the border with the Dominican Republic. Haiti's prime minister said at least eight other people died and two were missing. In the Dominican Republic, relatives told reporters a collapsed wall killed a mother and her young son.
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Hundreds of thousands were without power in the Dominican Republic amid heavy flooding in both countries.
Despite Marco's weakening, a storm surge warning remained in place from Morgan City, Louisiana, to Ocean Springs, Mississippi. A tropical storm warning included Lake Pontchartrain in Louisiana, and metropolitan New Orleans.
A storm surge of up to 4 feet was forecast for parts of coastal Louisiana and Mississippi.
Stay updated on daily forecasts and severe weather updates on ABC13's weather page.
The Associated Press contributed to this report.