Hilary, now a powerful Category 4 hurricane, is expected to make landfall in Mexico's Baja California on Sunday and then weaken to a tropical storm before crossing into California Sunday night.
The first tropical storm watch in California's history has now been issued. The watch is in effect for portions of Southern California including San Diego, Palm Springs, Riverside and the city of Los Angeles.
How to help Maui fire victims from afar: Organizations and efforts underwaySouthern California is bracing for heavy rain, potentially historic flash flooding, mudslides and gusty winds on Sunday through Monday morning.
The worst of the rain will hit Southern California on Sunday from Palm Springs to the Mexican border. Some desert areas could see more than one year's worth of rain in just one or two days.
A flash flood watch has been issued for Joshua Tree National Park, according to the National Park Service.
“We’re starting a phased closure process, and will be fully closed within the next 36 hours,” Donovan Smith, a NPS park ranger, told ABC News Friday.
The park's management will be assessing the park and conditions as the storm progresses, he said.
"We’ve closed the park for weather conditions in the past, but this level of storm is quite new for this area,” Smith said.
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.
LA County Sheriff Robert Luna told reporters Friday afternoon that the county's various first responder and emergency management teams have already begun preparing for the storm.
He noted that water rescue personnel and rescue aircraft are on alert and ready for immediate response and responders have already made outreach to the homeless population in the county.
"I don't think any of us...[ever] thought [we'd] be standing here talking about a hurricane or a tropical storms," Luna said.
Southern California beachgoers should also be mindful of high surf.
The Los Angeles Dodgers, San Diego Padres and Los Angeles Angels have rescheduled their Sunday games due to the storm, Major League Baseball announced Friday.
President Joe Biden said Friday that his "team is closely monitoring" the storm.
"FEMA has prepositioned personnel and supplies in the region and they're ready," Biden said. "I urge everyone, everyone in the path of this storm to take precautions and listen to the guidance from state and local officials."
In Mexico, a hurricane watch was issued in Baja California where rough seas, scattered showers and thunderstorms are in the forecast for Saturday.
A hurricane prevention zone has been established from north of Punta Abreojos to Punta Eugenia in Baja California Sur, the Mexican secretary of the environment said Friday. Ships are urged to take "extreme precaution" due to the strong winds and high waves forecast for the area.
A tropical storm warning is in effect for Cabo San Lucas. Hilary's outer bands could graze the popular resort town but it is not forecast to directly hit Cabo.
ABC News' Anne Laurent, Samira Said, Tristan Maglunog and Ellie Kaufman contributed to this report.