Forecasters on Sunday upgraded Tropical Storm Florence to a hurricane, saying it was strengthening and taking aim at North and South Carolina, where it could make landfall by the end of the week.

“We expect a rapid intensification of Florence tomorrow,” Dennis Feltgen, a spokesman for the National Hurricane Center, said on Sunday. “We expect it to be a major hurricane when it makes landfall by Thursday night or early Friday.”

By Monday, Florence was expected to intensify to a Category 3 hurricane, with winds of 111 to 130 miles per hour, up from a Category 1 on Sunday. On Sunday evening, the center said Florence would “remain an extremely dangerous major hurricane through Thursday.”

[Evacuations were ordered in parts of the Carolinas on Monday.]

Mr. Feltgen cautioned that heavy rains could prove more damaging than the high winds. He said Florence could be a “major event,” affecting all of South Carolina in particular, not just its coast, with “historic freshwater rainfall.”