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(CNN) — Hurricane Laura may reach extremely dangerous Category 4 strength before slamming into Louisiana and eastern Texas overnight and is expected to hit parts of the coast with “unsurvivable storm surge,” the National Hurricane Center says.

Laura was moving over the Gulf of Mexico late Wednesday morning with maximum sustained winds of 125 mph — a Category 3 storm.

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It is expected to strengthen to a Category 4 sometime before making landfall near the Texas-Louisiana state line late Wednesday or early Thursday, forecasters say.

The storm’s power and low-lying geography could combine for devastating results: “Unsurvivable” storm surges of 15 feet or higher could overwhelm coasts from south of Texas’ Port Arthur to Louisiana’s Intracoastal City, the hurricane center said.

And surges several feet high could reach nearly to Interstate 10 in Louisiana, endangering cities like Lake Charles, some 35 miles inland, forecasters say.

“Catastrophic, life-threatening,” National Hurricane Director Ken Graham told CNN on Wednesday morning about the storm’s potential.

“Every little bayou, every little river that normally drains your rain, is going to flow in the opposite direction with storm surge,” he said. “And it (will get) out of those banks and (go) over the land.”

About 1.5 million people are under some type of evacuation order across parts of Texas and Louisiana ahead of Laura, which killed at least nine people when it raked parts of the Caribbean as a tropical storm earlier this month.

“Only a few hours remain to protect life and property,” the National Hurricane Center said online around 10 a.m. CT.

The Houston area — especially eastern Harris County — also should expect damaging winds. Road tolls there have been waived to help people leaving the area, Gov. Greg Abbott said.

A hurricane warning extends from the Galveston area — as well as parts of Harris County just east of Houston — to Intracoastal City, Louisiana, south of Lafayette.

Houston itself is under a tropical storm warning, meaning strong winds — up to 73 mph — are expected.

“Trees will come down. Power lines will come down … especially in the eastern part of Houston proper,” CNN meteorologist Chad Myers said.

Shelters in Texas have been preparing for two threats — stocking personal protective equipment, preparing for social distancing measures and making testing available for coronavirus while the state prepares for Hurricane Laura, Abbott said.

Storm path: Track Laura’s location

Storm surges, damaging winds and tornadoes are among the threats

Laura is expected to make landfall late Wednesday or early Thursday near the Texas-Louisiana state line, possibly as a Category 4, the National Hurricane Center said.

Destructive winds will be one of the big problems. A Category 4 landfall would mean winds at least 130 mph near the storm’s center — strong enough to destroy homes and knock out power for weeks.

But the storm’s eyewall isn’t the only place to watch. “Hurricane-force winds … are also expected to spread well inland into portions of eastern Texas and western Louisiana,” the National Hurricane Center said.

Even before landfall, winds of tropical-storm force — 39 mph and greater — should reach the area of the Texas-Louisiana state line, perhaps by 2 p.m. CT, Myers said.

Threats include storm surges of:

• 15 to 20 feet along a 55-mile stretch of coastal Louisiana from Johnson Bayou to Rockefeller Wildlife Refuge

• 10 to 15 feet from Texas’ Sea Rim State Park to Louisiana’s Johnson Bayou, and from the Rockefeller Wildlife Refuge to Louisiana’s Intracoastal City

• 8 to 12 feet from Intracoastal City, Louisiana, to Morgan City, Louisiana

• 6 to 9 feet from Port Bolivar, Texas, to Sea Rim State Park, Texas

• 4 to 7 feet from Morgan City, Louisiana, to the mouth of the Mississippi River.

From Wednesday through Friday, Laura is expected to produce rainfall of 5 to 10 inches, with isolated maximum amounts of 15 inches, across parts of northwestern Gulf Coast.

Over the lower to middle Mississippi Valley, 2 to 4 inches of rainfall with isolated totals of 6 inches are expected, the National Hurricane Center said.

A few tornadoes also are expected Wednesday night from far southeast Texas to Louisiana and Mississippi, the hurricane center said.

Laura’s impending arrival comes as the Gulf Coast avoided a powerful storm in Marco, which significantly weakened before reaching the US.

3 years after Harvey

Texas evacuations Tuesday fell on the third anniversary of Hurricane Harvey hitting Texas.

Abbott noted that there were more evacuations announced than before Hurricane Harvey struck.

Houston is the largest city in the region. It is particularly vulnerable to flooding and could see big impacts from Laura. The concrete-filled city has notoriously poor drainage systems and a propensity to flood, such as during the overwhelming rainfall from Hurricane Harvey in 2017.

Mandatory evacuations in Jasper and Hardin Counties fall just north of Beaumont, Texas, which was hard-hit during Harvey.

Deaths in the Caribbean

At least nine people have died in the Caribbean, including several in the Dominican Republic and Haiti, due to Laura.

The victims include a 7-year-old boy who died along with his mother after a wall collapsed in their home in the Dominican Republic. Another person died after a tree fell on a house.

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

Five people were killed in Haiti, including a 10-year-old girl, the country’s civil protection agency said.

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