The storm Hernan strengthened into a Category 1 hurricane far off Mexico's western coast Sunday as it headed out into the Pacific Ocean and posed little threat to land.

The U.S. National Hurricane Center in Miami said Hernan was about 535 kilometres south-southwest of the southern tip of the Baja California Peninsula at mid-afternoon and was moving to the northwest at 24 km/h. It had maximum sustained winds of 120 km/h, just over the threshold for being a hurricane.

Forecasters said the storm centre was expected to pass on Sunday to the southwest of Socorro Island, a part of a volcanic archipelago that is home to a Mexican naval station.

The hurricane centre said Hernan likely would begin weakening by Monday and dissipate by Thursday.