The Mexican government issued a hurricane watch and a tropical storm warning for Mexico's coast from Punta Maldonado to Bahias de Huatulco.
A watch means tropical storm conditions with sustained winds of 63 to 117 kph (39 to 73 mph) are possible in that area within 48 hours.
The National Hurricane Center in Miami said John was nearly stationary but was likely to move closer to the southern Mexico coast Tuesday and Wednesday. It's also forecast to get stronger before landfall.
Through Thursday, John is expected to produce 15 to 30 centimeters (6 to 12 inches) of rain across coastal areas of Chiapas state with more in isolated areas. In areas along and near the Oaxaca coast to southeast Guerrero, between 25 and 50 centimeters (10 and 20 inches) of rain with isolated higher totals can be expected through Thursday.