Tropical Storm Marco collapses after making landfall, but threat looms from Laura

Tropical Storm Marco weakened to a tropical depression Monday night after making landfall near the Mississippi River — but officials along the Gulf of Mexico were bracing for another storm, Laura, that is expected to become a hurricane.

Marco made landfall around 6 p.m. local time as a tropical storm, and just before 10 p.m. it was a tropical depression, the National Hurricane Center said. Heavy rain was still possible.

But the larger threat to parts of the U.S. coast looms from Laura, which prompted hurricane watches from Port Bolivar, Texas, near Galveston, to west of Morgan City, Louisiana, west of New Orleans. Storm surge watches stretched from San Luis Pass, Texas, to Ocean Springs, Mississippi.

"We're only going to dodge the bullet so many times. And the current forecast for Laura has it focused intently on Louisiana," Louisiana Gov. John Bel Edwards told reporters earlier Monday.

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Back-to-back storms head for Gulf Coast

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And just 260 miles west of New Orleans, the mayor of Port Arthur, Texas, on Monday night issued a mandatory evacuation order effective at 6 a.m. Tuesday, which was called "a direct result of the imminent dangers from Tropical Storm Marco and Tropical Storm Laura."

Jefferson County, which includes Port Arthur, also issued an evacuation order for 6:30 a.m. Tuesday, and Orange County, to the northeast, issued a 6 a.m. Tuesday evacuation order.

Marco continued to weaken Monday. At one point Sunday, it was a hurricane before it weakened to a tropical storm.

Marco was forecast to dissipate by early Wednesday.

With Marco exiting stage left, Tropical Storm Laura is entering stage right. And Laura is forecast to be bigger and stronger than Marco.

At 11 p.m. ET, Laura — which had maximum sustained winds of 65 mph — was around 80 miles northeast of the western tip of Cuba, where it was causing heavy rain and flash flooding, the hurricane center said. The storm was moving west-northwest at 20 mph.

It is forecast to become a hurricane Tuesday, and it is forecast to approach the U.S. Gulf Coast on Wednesday night.

Whenever Laura reaches the Gulf Coast, its impacts will be felt "well ahead of the center," with winds reaching parts of southeast Louisiana by Wednesday, Hurricane Center Director Ken Graham said in a video briefing late Monday afternoon.

"By the time the tropical storm-force winds get there, well, you have to have things wrapped up," Graham said. "It's too dangerous to be driving. It's too dangerous to be on a ladder or carrying plywood."

At least 11 people were killed in the Dominican Republic and Haiti as the storm knocked out power to the two countries and caused flooding, The Associated Press reported.

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A tropical storm warning for the middle Florida Keys was discontinued, but one remained in place from the Seven Mile Bridge to Key West, the hurricane center said late Monday.

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When Laura enters the Gulf of Mexico on Tuesday, the forecast gets interesting.

Landfall was projected to be late Wednesday or early Thursday as a strong 105 mph Category 2 storm, nearing Category 3, near the Texas and Louisiana border.

But no one knows just how strong Laura will get.

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One possibility is rapid intensification, in which case maximum sustained winds would have to increase by 35 mph in 24 hours. It's far from impossible — the elements are there to support the potential for explosive strengthening: warm Gulf of Mexico water in the mid- to upper 80s, low wind shear and plenty of moisture.

Should that happen, the rapid intensification would occur from Tuesday afternoon to Wednesday afternoon.

That will be important to watch, as a stronger Laura would head more west toward Houston, whereas a weaker storm would be steered more toward the Texas-Louisiana border.

Laura is expected to soak parts of the Gulf Coast with 4 to 8 inches of rain, with isolated totals of up to a foot, from Wednesday to Saturday, forecasters said.

Storm surge was also expected to cause flooding, and Graham warned that the water produced by tropical systems is the most dangerous and deadly part of this type of storm.