The tropical storm had maximum sustained winds of 50 mph (85 kph), the Miami-based center said. It was moving west-northwest at 9 mph (15 kph) and forecast to approach the coast by late Wednesday.
A hurricane warning was in effect from Puerto Ángel to Punta Maldonado in coastal southern Mexico. The hurricane watch stretches west of Punta Maldonado to Acapulco and east of Puerto Ángel to Bahías de Huatulco. A tropical storm watch was posted east of Puerto Ángel to Salina Cruz.
A hurricane warning means hurricane conditions are expected in the area, and preparations to protect life and property should be rushed to completion, according to the NHC advisory.
Heavy rainfall up to 20 inches (51 centimeters) was forecast for parts of Oaxaca and Guerrero with lighter amounts in Chiapas, Michoacan, Colima and Jalisco states.
The rainfall may produce flooding and mudslides, the center said, and storm surge could produce coastal flooding.