Moisture from Hurricane Rosa could cause flash flooding to Baja California, eastern California, Las Vegas and Arizona as it approaches the coast late Sunday.
Rosa still had maximum sustained winds of 120km/h this morning and it was centred about 515km southwest of Punta Eugenia in Mexico.
Remnants of the Rosa are expected to hit Arizona on Sunday, bringing heavy rain, high winds and the threat of localized flooding, the National Weather Service said.
Exactly how much rain and where it will fall is uncertain, but Riverside County's Emergency Management Department has warned residents near the Holy and Cranston wildfire burn areas that intense rain could cause unsafe debris flows.
The forecast for the Prescott area includes more than an inch of rain both Monday and Tuesday, with thunderstorms predicted for both days. It was heading north-northeast at 12 miles per hour (19 kph).
Swells generated by the hurricane were also expected to produce life-threatening surf and rip currents off Southern California.
Hurricane Rosa weakened slightly but was still a major hurricane Friday as as it churned toward the Mexican coast, the National Hurricane Center said.
The National Weather Service announced flash flood watches through Wednesday for areas including southern Nevada, southeastern California, southwestern and central Utah and the western two-thirds of Arizona.
Rosa will begin weakening overnight into Saturday. It was moving west at 6 miles per hour.