Flossie's winds increased to 75 mph (120 km/h) and a slight strengthening is expected during the next 24 hours, the Miami-based hurricane center said.
On Friday morning, the hurricane was located 1,300 miles (2,000 kilometers) southeast of Hawaii and was moving west at 14 mph (22 km/h), said Richard Knabb, a hurricane specialist with the center.
Flossie is expected to weaken by the time it approaches Hawaii in about four days and passes to the south, Knabb said. But Hawaii residents should remain on alert.
"Even a system passing to the south can bring significant rain" and surf swells, he said.