Hilary, a small but powerful storm, strengthened to a Category 4 hurricane late on Thursday as its core continued to move parallel to the southwest Pacific coast of Mexico.
The storm was 85 miles southwest of the popular resort of Acapulco, packing maximum sustained winds of 135 miles per hour, the Miami-based National Hurricane Center said.
"Some additional strengthening is possible on Friday. There will be some fluctuations in intensity during the next day or so," forecasters added.
Hilary, now on the fourth rung of a five-step severity scale, is the seventh hurricane of the Pacific season. None has caused major damage so far.
A tropical storm warning was in place for the Mexican coast from Lagunas de Chacahua in the state of Oaxaca westward to Punta San Telmo in Michoacan.
The hurricane center said Hilary was expected to produce rainfall of 3-5 inches across parts of Guerrero and Oaxaca states in southern Mexico, with up to 10 inches possible in some areas. The storm was likely to bring life-threatening surf to Mexican coasts.