The first-ever Tropical Storm Watch was issued for Southern California around 8 a.m. Friday, according to the National Hurricane Center.
Hilary had sustained winds near 145 mph and was expected to continue intensifying through the day. It will still be a hurricane as it enters Mexico's Baja peninsula on Saturday night and will approach Southern California on Sunday as a tropical storm.
Baja California's western portion remains under a hurricane warning.
Forecasters say Hilary could cause extensive flooding and sustained winds in Southern California. Cities are preparing by building sand berms in low-lying areas. City leaders in coastal areas say beaches and piers will be shut down if need be.
The National Hurricane Center's Director Michael Brennan said in a Facebook Live update around 8:30 a.m. that Hilary has likely reached its peak strength.
"Our confidence has increased on the potential for a rare and very dangerous flash flooding event over portions of the southwest United States," Brennan said.
National Hurricane Center Deputy Director Jamie Rhome said the hurricane is currently traveling at 139 miles per hour and moving northwest at 12 miles per hour, faster than it was earlier Friday.
The hurricane is expected to make its impact on land Friday evening or night.
Speeds are expected to drop, and the hurricane will "weaken significantly" as it approaches Southern California; expected to sustain speeds around 50 miles per hour and becoming a tropical storm.
There's potential for widespread rainfall totals of 3 to 6 inches across portions of the Southern California deserts, and in the mountain areas as much as 10 inches. That's as much rain as these areas get in an entire year, Brennan added.
"For folks in Southern California the reminder is rainfall flooding is responsible for most of the fatalities during tropical storms and hurricanes in the United States about two-thirds of them," said Brennan. "So while the winds tend to get a lot of attention, the flash flooding and rain has killed the most people in the United States in the last 10 years."
The cities of and around Los Angeles, Palm Springs, San Diego and Las Vegas are under a tropical storm watch. Flash flooding is also expected in these areas with Palm Springs facing a significant amount of rainfall.
Rhome called the incoming rainfalls in these areas "potentially catastrophic" and said residents need to take action to protect themselves, loved ones and property.
Additional advice included enabling emergency alerts on cell phones, rescheduling plans, and not driving in areas where heavy rainfall is forecasted.
The storm will cause an unsettled weather pattern in the Bay Area. There will be chances for showers and isolated thunderstorms. Forecasters see the greatest rain chances from late Sunday into early Tuesday.
The last tropical storm to hit California was in 1939.