The hurricane was lashing Mexico's Baja California peninsula on Wednesday, where authorities prepared by opening shelters and closing some roads.
As it heads north into cooler waters it is expected to bend to the west over the Pacific and weaken to a tropical storm or depression. It will start passing offshore Southern California Friday night, pushing heavy moisture in the area.
By Saturday, heavy rain can be expected in many inland parts of Southern California. Mountain communities and other areas with burn scars should be prepared for significant flash flooding.
Beaches will also see big waves for surfers, with 4-8 foot hurricane-pulse swells expected Saturday.
But surfers and beachgoers are advised to be cautious, as beaches in the Orange County area may also see some thunder showers.
The rain will bring relief to Southern California after a prolonged, record-setting heat wave that has lasted more than a week. The rain will drop temperatures in the region down to the 80s and 90s. It is likely to bring 1-2 inches of rain to mountain communities starting Friday night, leading to a good possibility of flash flooding.
Desert areas are also vulnerable, expecting about an inch of rain that could wash out some roads.
Kay's maximum sustained winds were 100 mph at midafternoon Wednesday, with forecasters saying it could brush the middle portion of the Baja peninsula Thursday or Friday.
The U.S. National Hurricane Center in Miami said Kay was centered about 205 miles west-southwest of the southern tip of the Baja peninsula. Kay was moving north-northwest at 13 mph.
Forecasters expected Kay to stay offshore as it moves more northward, roughly parallel to the coast, it was an expansive storm, with tropical storm-force winds extending up to 230 miles (370 kilometers) from the center.
The Associated Press contributed to this report.