(Reuters) – Hurricane Laura was expected to rapidly strengthen to a Category 4 hurricane on Wednesday on a steady track to hit the Texas-Louisiana Gulf Coast after dark, causing catastrophic damage, the National Hurricane Center said.

The storm, already a Category 3 on the five-step Saffir-Simpson scale for measuring hurricane intensity early Wednesday, was about 290 miles (465 km) southeast of Galveston, Texas, with maximum sustained winds of 115 miles (185 km) per hour, the Miami-based forecaster said. Category 4 hurricanes can pack winds of up to 156 miles (250 km) per hour.

More than 420,000 Texas residents and another 200,000 people in neighboring Louisiana were under mandatory evacuation orders as state and federal emergency management teams rushed to provide shelter accommodations and get first responders in place for possible rescues.

“You only have a few hours to prepare and evacuate for #HurricaneLaura. Wherever you are by noon is where you’ll have to ride out the storm. Be smart and be safe,” Louisiana Governor John Edwards tweeted Wednesday morning.

The storm was moving at about 15 miles (24 km) per hour. Storm surge along the Gulf Coast could raise water levels to as high as 12 feet to 15 feet (3.7 meters to 4.6 meters) in Intracoastal City and Morgan City, Louisiana, and Laura was expected to drop 5 to 10 inches (13 to 25 cm) of rain over the region, the National Hurricane Center (NHC) said.

“This storm surge could penetrate up to 30 miles inland from the immediate coastline in southwestern Louisiana and far southeastern Texas,” the NHC said in a Wednesday morning advisory. “All preparations to protect life and property should be rushed to completion in the next few hours.”

The storm was also expected to spawn tornadoes Wednesday night over Louisiana, far southeastern Texas, and southwestern Mississippi, the NHC said. It added there would likely be widespread flooding in urban areas from far eastern Texas across Louisiana and Arkansas from Wednesday to Thursday.

‘PREPARE FOR THE WORST’

In Texas, many residents spent Tuesday packing up and bracing their homes and businesses for the anticipated pummeling by Laura starting on Wednesday.

“We’ve been through this before; we’re prepared. Everybody downtown has boarded up their businesses and people are leaving the city,” said Gina Spagnola, president of the Galveston Chamber of Commerce, as she boarded up windows on her own waterfront home on Tuesday.

Hidalgo County, Texas, urged voluntary evacuation in the coastal region surrounding Houston, and shelters were set up in San Antonio, Dallas and Austin.

Houston, about 45 miles (70 km) north of the open water of the Gulf of Mexico, was pummeled in 2017 by Hurricane Harvey, which killed at least 68 people and caused $125 billion in property damage. Hurricane Laura is forecast to dissipate quickly after landfall, with the greatest danger more from wind and storm surges than rain.

Lina Hidalgo, the top executive for Harris County, which encompasses Houston, warned of deadly winds and a destructive storm surge after Laura makes landfall late Wednesday or early Thursday. She said that put millions of lives at risk as the storm moved westward and took aim at the fourth-biggest city in the United States.

“This storm certainly can cause unprecedented devastation,” Hidalgo said at a news conference on Tuesday. “We truly have to say: Prepare for the worst.”

(Reporting by Gabriella Borter; editing by Jonathan Oatis)