Tropical Storm Marco began falling apart today, easing one threat to the United States Gulf Coast but setting the stage for the arrival of Laura as a potentially supercharged Category three hurricane with winds topping 177 km/h and a storm surge that could swamp entire towns.
The two-storm combination could bring a history-making onslaught of wind and coastal flooding from Texas to Alabama, all complicated by the coronavirus pandemic, forecasters said.
Still a tropical storm for now, Laura churned just south of Cuba after killing at least 11 people in the Dominican Republic and Haiti, where it knocked out power and caused flooding in the two nations that share the island of Hispaniola.
The deaths reportedly included a 10-year-old girl whose home was hit by a tree and a mother and young son who were crushed by a collapsing wall.
Laura was not expected to weaken over land before moving into warm, deep gulf waters that forecasters said could bring rapid intensification.
"We're only going to dodge the bullet so many times. And the current forecast for Laura has it focused intently on Louisiana," Governor John Bel Edwards told a news briefing.
Shrimp trawlers and fishing boats were tied up in a Louisiana harbour ahead of the storms. Red flags warned swimmers away from the pounding surf.
Both in-person classes and virtual school sessions required because of the coronavirus pandemic were cancelled in some districts.
A food bank that has been twice as busy as normal since March providing meals to people affected by the pandemic prepared to shut down for a few days because of the weather, but not before distributing a last round of provisions to the needy.
"We're very tired," said Lawrence DeHart, director of Terrebonne Churches United Foodbank in Houma.
State emergencies were declared in Louisiana and Mississippi, and shelters were being opened with cots set farther apart, among other measures designed to curb infections.
"The virus is not concerned that we have hurricanes coming, and so it's not going to take any time off and neither can we," Edwards said.
Louisiana was trying to avoid opening state-run shelters because of the Covid-19 threat.
The governor encouraged evacuees to stay with relatives or in hotels. But officials said they made virus-related preparations at state shelters in case they are needed.
As Marco collapsed, the National Hurricane Centre cancelled all tropical storm watches and warnings. Marco's winds died down to 65 km/h as it sloshed 65km southeast of the mouth of the Mississippi River.
Laura drenched much of Cuba as it headed northwest at 31 km/h with top sustained winds of 95 km/h.
Once Laura passes Cuba, the system could quickly strengthen over warm water, which acts as fuel to supercharge the storm.
Together, the two storms could bring a total of 0.6m of rain to parts of Louisiana, perhaps raising the storm surge to more than 3m along the Louisiana coast and pushing water 50km up the rivers in a worst-case scenario, said meteorologist Benjamin Schott, who runs the National Weather Service office in Slidell, Louisiana.
The double punch comes just days before the August 29 anniversary of Hurricane Katrina, which breached the levees in New Orleans, flattened much of the Mississippi coast and killed as many as 1800 people.