Hurricane Laura has officially been upgraded to a Category 4 storm as it bears down on the U.S. Gulf Coast.

Laura has been gaining in strength as it moves over the warm waters of the Gulf of Mexico. About half a million people have been evacuated ahead of the approaching storm. It’s being called the largest evacuation since the start of the COVID-19 pandemic.

“It’s not just going to be a coastal event,” said senior hurricane specialist Stacy Stewart. “Laura’s going to be moving well inland, bringing hurricane force winds possibly to the Arkansas-Louisiana border and then tropical storm force winds into Arkansas.”

Forecasters say Laura grew in size and strength in just a matter or hours to become, what they call, a “formidable storm.”