Hurricane Nora has formed in the eastern Pacific on a track that would bring it near the Puerto Vallarta area and then head toward a close encounter with resorts at the tip of Baja California Peninsula.
Nora had maximum sustained winds of 120km/h on Saturday morning, with tropical storm force winds extending more than 330 kilometres from the centre in some places.
The storm's large wind field and heavy rain mean much of Mexico's central and northern Pacific coast is likely to see floods, mudslides and perilous surf.
The weakened remnants may even bring rain next week to the US southwest, the Great Basin and central Rockies.
The US National Hurricane Center forecast showed Nora skirting close to the bay sheltering Puerto Vallarta by Sunday and then shooting straight up the Gulf of California a day later, passing very close to the Los Cabos resorts.
It was centred about 370km south of Cabo Corrientes and was heading to north-northwest at 17km/h.
The Hurricane Center said the Mexican states of Guerrero, Michoacan, Colima and Jalisco could see up to 30 centimetres of rain, with isolated maximums still higher.