DELCAMBRE, La. • Laura strengthened Wednesday into a menacing Category 4 hurricane, raising fears of a 20-foot storm surge that forecasters said would be “unsurvivable” and capable of sinking entire communities. Ocean water topped by white-capped waves began rising ominously as the monster neared land.

Authorities implored coastal residents of Texas and Louisiana to evacuate and worried that not enough had fled by the time winds began picking up.

The storm grew nearly 87% in power in just 24 hours to a size the National Hurricane Center called “extremely dangerous.” Drawing energy from the warm Gulf of Mexico waters, the system was on track to arrive late Wednesday or early Thursday as the most powerful hurricane to strike the U.S. so far this year. “It looks like it’s in full beast mode, which is not what you want to see if you’re in its way,” University of Miami hurricane researcher Brian McNoldy said.

Maximum sustained winds increased to 150 mph before nightfall, and forecasters said up to 15 inches of rain could fall in some places.

One major Louisiana highway already had standing water as Laura’s outer bands moved ashore with tropical storm-force winds. Thousands of sandbags lined roadways in tiny Lafitte, and winds picked up as shoppers rushed into a grocery store in low-lying Delcambre. Trent Savoie, 31, said he was staying put. “With four kids and 100 farm animals, it’s just hard to move out,” he said.

Texas Gov. Greg Abbott and Louisiana Gov. John Bel Edwards fretted that the dire predictions were not resonating despite authorities putting more than 500,000 coastal residents under mandatory evacuation orders.

Edwards activated the state’s entire National Guard. In Lake Charles, Guard members drove school buses around neighborhoods, offering to pick up families. Across the state line in Port Arthur, Texas, few stragglers boarded evacuation buses, and city officials announced that two C-130 transport planes offered the last chance to leave.

Abbott warned that people who fail to get out of harm’s way could be cut off from help . A Category 4 hurricane can render wide areas uninhabitable for weeks or months. The threat of such devastation posed a new disaster-relief challenge for a government already straining under the coronavirus pandemic.

This GOES-16 GeoColor satellite image taken Wednesday shows Hurricane Laura over the Gulf of Mexico. Hurricane Laura strengthened Wednesday into “an extremely dangerous Category 4 hurricane,” The National Hurricane Center said. Laura is expected to strike Wednesday night into Thursday morning along the Louisiana-Texas border.

NOAA viA the Associated press