Hurricane Bud formed Sunday and was to gain strength rapidly in the north Pacific, west of Mexico, US forecasters said.
At 2100 GMT, Bud, the second hurricane of the season after Aletta, was packing top sustained winds of 75 miles an hour (120 kilometers per hour) and moving northwest at nine miles per hour (15 kph) the Miami-based National Hurricane Center said.
"Bud could strengthen rapidly during the next 24 hours or so," the NHC warned.
Its churning center was about 255 miles (410 KM) south of Manzanillo Mexico, the center said.
Mexico issued a Tropical Storm Watch for the southwestern coast of Mexico from Manzanillo to Cabo Corrientes, popular resort areas.
A Tropical Storm Watch "means that tropical storm conditions are possible within the watch area, in this case within the next 24 through 48 hours," the NHC said.