Tropical Storm Hilary strengthened into a major hurricane in the Pacific on Thursday and was expected to further intensify before approaching Mexico's Baja California peninsula over the weekend, forecasters said.

Heavy rainfall was also expected in the southwestern United States from Friday as the storm moves north, the US National Hurricane Center (NHC) said.

Hilary was packing maximum sustained winds of about 120 miles (195 kilometers) an hour, according to the NHC.

That made it a major Category Three hurricane -- the third most powerful on the Saffir-Simpson scale of one to five.

Hilary was located about 445 miles south of the tourist resort of Cabo San Lucas on the southern tip of Baja California, the NHC said.

"Additional rapid strengthening is forecast during the next day or so, and Hilary is likely to become a category four hurricane tomorrow (Friday)," it predicted.

"Flash flooding, locally significant, will be possible," it added.

The storm brought rain and rough seas to areas along Mexico's southwestern Pacific coast, including the tourist resort of Acapulco.

In the United States, parts of southern California and southern Nevada could see heavy rain through early next week, the NHC said.

Hurricanes hit Mexico every year on both its Pacific and Atlantic coasts, usually between May and November.