The storm, packing top sustained winds of 75 miles per hour (120 km/h), was whirling about 330 miles (535 kilometers) south-southwest of Mexico's Baja California peninsula, the US National Hurricane Center reported.
Expected to weaken gradually as of Monday with little change in strength forecast overnight, Hernan was passing to the west of Socorro Island, the Miami-based center said.
Hernan was advancing toward the northwest at nearly 15 miles per hour (24 km/h), with a northwestward to west-northwestward movement expected over the next 48 hours.