CNN  — 

The Gulf Coast will get walloped by a tropical storm and a hurricane this week, bringing torrential rain, fierce winds and ferocious storm surges.

First up: Marco, which strengthened from a tropical storm to a Category 1 hurricane Sunday but weakened overnight to sustained winds of 70 mph – just under the hurricane threshold. Marco is expected to make landfall on the Louisiana coast Monday.

Then there’s Tropical Storm Laura, which is forecast to strengthen to a hurricane before it makes landfall on the US mainland late Wednesday or early Thursday. Laura is also expected to make landfall on or near the Louisiana coast.

National Weather Service meteorologist Benjamin Schott said Laura could possibly be a Category 3 storm as it approaches the state, bringing up to 10 feet of storm surge to the southeastern coastline. If the storm holds its strength, water could be pushed back up the state’s rivers as far as 30 miles inland, he said.

Parts of Louisiana will start to see the effects of Tropical Storm Laura by Wednesday morning, after Marco leaves the state Tuesday evening, Schott said.

“The unprecedented kind of thing here is that it’s the same state within 48 hours of each other,” Schott previously said.

“In modern meteorological history … there’s never been anything like this before where you could have possibly two hurricanes hitting within miles of each other over a 48 hour period.”

The imminent threat of Tropical Storm Marco prompted mandatory evacuations Sunday in Plaquemines Parish, Louisiana, and Grand Isle, Louisiana. As of late Sunday evening, the storm was about 185 miles south southeast of the mouth of the Mississippi River with maximum sustained winds of 70 mph, according to the National Hurricane Center.

And the one-two punch from Marco and Laura means “there may not be much of a window” for rescuers or power restoration crews to respond to victims between the two storms, Louisiana Gov. John Bel Edwards said.

‘Life-threatening storm surge’

“Ultimately, the big concern is going to be storm surge,” CNN meteorologist Allison Chinchar said.

Forecasters warned of “life-threatening storm surge and hurricane-force winds” Monday in parts of the Gulf Coast.

The hurricane center issued a storm surge warning for coastal cities from Morgan City, Louisiana, east to Ocean Springs, Mississippi.

“A storm surge warning means there is a danger of life-threatening inundation, from rising water moving inland from the coastline, during the next 36 hours,” the hurricane center said.

Marco is expected to bring a storm surge height of 4 to 6 feet for Biloxi, Mississippi, and Grand Isle, Louisiana, Chinchar said.

03B US weather Marco Laura

Bowl-shaped New Orleans is at risk

“Since New Orleans is not actually on the coast, it will be more indirectly impacted via Lake Pontchartrain, which is expected to have surge heights of 2 to 4 feet,” Chinchar said.

That, combined with 4 to 6 inches of rain, means New Orleans could flood Monday.

“New Orleans is especially at risk because it is essentially shaped like a bowl,” Chinchar said.

“Thanks to the drainage system they have set up, the city can act like a colander, allowing water to flow out,” she said. “But when it rains a lot in a short period of time the drainage system can become overwhelmed, and that colander acts more like a bowl and floods the city.”

President Donald Trump approved an emergency declaration on Sunday for Louisiana, ordering federal assistance to coordinate disaster relief efforts.

Texas Gov. Greg Abbott requested a federal emergency declaration for 23 counties in his state. Galveston County Judge Mark Henry said Sunday that Hurricane Marco would likely not be a factor for residents. “Unfortunately, we can’t say the same about Tropical Storm Laura,” Henry said.

Long lines formed outside Louisiana grocery stores as residents – long accustomed to hurricane prep – braced for both storms.

Stacie Osborn told CNN it took her 30 minutes to get through the checkout line at a market in New Orleans.

“I stocked up on enough food for the week, extra water and gassed up my car just in case – the usual,” she said.

Meantime, John Snow in Baton Rouge said the line at his local Costco was wrapped around the building just 10 minutes after opening.

“It ended up taking about 20 to 25 minutes to get into the store, which wasn’t ideal with the Louisiana heat and masks on,” he said, “but it wasn’t terrible.”

Snow said he was stocking up on the essentials and getting gas. “As always, we hope for the best but plan for the worst.”

Marco and Laura will dump torrential rain

Marco is expected to bring “a tremendous amount of rain, not just to Louisiana, but areas of Mississippi, Alabama and even Florida,” Chinchar said.

“Then you have Laura making its way into the Gulf, shortly after Marco makes landfall,” she said.

“Even on Monday and Tuesday, Key West, Miami – you’re going to get some of those outer bands from Laura producing very heavy rain and very gusty winds.”

Laura was thrashing the Dominican Republic on Sunday, “dumping a tremendous amount of rain there,” Chinchar said. It was over eastern Cuba Sunday evening, according to the hurricane center.

Tropical Storm Laura caused flooding Saturday in Salinas, Puerto Rico.

At least nine people have died in the Caribbean due to Tropical Storm Laura.

At least three people were killed in the capital of Santo Domino, according to the country’s Center of Emergency Operations. Among the victims was a 7-year-old boy who died with his mother after a wall collapsed in their home. The third person died after a tree fell on a house.

The Dominican Republic’s President Luis Abinader said Sunday that an army corporal was killed while helping with rescue efforts in Pedernales province.

Five people were killed in Haiti, according to country’s Civil Protection, including a 10-year-old girl.

By the time Laura reaches the US mainland, it’s expected to be a Category 2 hurricane with maximum sustained winds of at least 96 mph.

Laura is expected to impact many of the same areas that Marco will hit.

CNN’s Melissa Alonso, Abel Alvarado, Haley Brink, Ana Cucalon, Amanda Jackson and Slover Morrison contributed to this report.