Wilmington, North Carolina (CNN) More than 1 million people on the southern East Coast of the United States faced mandatory evacuation orders as Hurricane Florence neared Category 5 strength Tuesday. Florence was upgraded twice Monday, from Category 3 to 4, prompting evacuation orders in coastal areas of North Carolina, South Carolina and Virginia. One year after historic storms that ravaged the American Gulf Coast and Puerto Rico last year, officials warned not to underestimate the threat the storm poses. Residents boarded up their homes, formed long lines at gas stations and emptied the shelves of hardware stores and supermarkets. The size of hurricane-force winds doubled over 12 hours from 30 to 60 miles across, the National Hurricane Center said Monday afternoon. The wind field is expected to keep growing, which will increase the storm surge and inland wind threats, the service said in a statement. "None of the guidance suggest that Florence has peaked in intensi