Hilary, now a powerful Category 4 hurricane, is expected to make landfall in Mexico's Baja California by Sunday morning and then weaken to a tropical storm before crossing into California Sunday night.
Southern California -- including San Diego and Palm Springs -- is bracing for heavy rain, flash flooding, mudslides and gusty winds.
The greater Southwest area, including Las Vegas, could also see flash flooding from this very rare event.
Most areas will see 2 to 6 inches of rain, but some areas could see up to 10 inches of rain.
A tropical storm watch is in effect for Cabo San Lucas. Hilary's outer bands could graze the popular Mexican resort town but it is not forecast to directly hit Cabo.
A hurricane watch was issued further north in Baja California.