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Hurricane Marco forms off the Gulf Coast as Tropical Storm Laura is close behind

"It looks like the upper Gulf is going to get a one-two punch," a National Hurricane Center spokesman said.

Tropical storm Marco was upgraded to hurricane status on Sunday as it gained strength over the Gulf of Mexico. Marco could be part of an unprecedented twin strike to the U.S. Gulf Coast alongside Tropical Storm Laura, which could also strengthen to a hurricane this week.

Marco entered the Gulf of Mexico on Saturday evening and was headed toward landfall in Louisiana or Mississippi on Monday afternoon, according to National Hurricane Center projections. By Sunday afternoon, the Air Force Reserve Hurricane Hunters found that Marco had reached sustained winds of 75 mph.

To reach hurricane status, a storm needs to generate sustained winds of at least 74 mph.

The National Hurricane Center in Miami warned that Marco is expected to bring high gusts and life-threatening storm surge along the Gulf Coast.

Tropical storm Laura — about 40 miles northeast of Port au Prince, Haiti, on Sunday morning — was expected to strengthen to a hurricane by Tuesday afternoon, the center said. It could make landfall from Texas to Florida's Gulf Coast by Wednesday afternoon, forecasters said.

"It looks like the upper Gulf is going to get a one-two punch," hurricane center spokesman Dennis Feltgen said. "That’s pretty much unprecedented that close together."

The shortest time between U.S. landfalls for major storms is 23 hours between Sept. 4 and 5, 1933.

The Puerto Rico Electric Power Authority said Sunday that more than 30,000 of its customers, about 2 percent total, have already lost power as the storms approach.

Louisiana Gov. John Bel Edwards declared a state of emergency Friday ahead of the storms and on Saturday asked President Donald Trump to grant federal emergency status to the state.

"Tropical Storms Marco and Laura are forecast to impact Louisiana in quick sequence early next week," Edwards' office said in a statement.

Edwards warned residents they had until Sunday evening to finalize preparations and gather supplies, urging all residents to begin sheltering in place that night.

"You need to be prepared to ride out the storms, you and your family, wherever you are at dark tonight and that is because tropical storm force winds will be impacting coastal Louisiana before daylight tomorrow," Edwards said during a press conference Sunday.

Louisiana State University announced that it would close campus Monday, assuring students that the school would ensure access to meals as they sheltered in their residence halls. The school's administration said it would monitor both storms and provide updates to students and staff.

A mandatory evacuation of Plaquemines Parish in New Orleans will begin Sunday afternoon, parish officials announced on Saturday night. Plaquemines is the southernmost area of the city, surrounded by the Gulf of Mexico, and has been severely damaged by previous hurricanes, including Katrina in 2005.

Marco was in the Gulf of Mexico, about 360 miles southeast of the mouth of the Mississippi River, according to the hurricane center on Sunday morning. It was moving north-northwest at 13 mph.

A hurricane watch was in effect from Intracoastal City, Louisiana, to the Mississippi-Alabama border, including New Orleans, federal forecasters said.

Tropical Storm Laura had maximum sustained winds of 45 mph on Sunday, and was moving west at 18 mph.

A tropical storm warning was in effect for Puerto Rico, the U.S. Virgin Islands and parts of the northern coast of the Dominican Republic, and a tropical storm watch was issued for the central Bahamas and the Florida Keys from Ocean Reef to Key West and the Dry Tortugas, the hurricane center said.