Hurricane Rosa was spinning toward the Baja Peninsula on Sunday as a Category 1 storm with sustained winds of 85 mph, the National Hurricane Center said.
Rosa is expected to make landfall Monday as a tropical storm, then continue moving northeast, soaking the southwestern United States.
High surf up to 10 feet and rip currents will begin along southern California coast Sunday, the hurricane center said. The potentially dangerous conditions are expected to last through Monday. Some 12- to 15-foot waves are possible on south-facing shores.
More than 10 million people are under flash flood watches at the US-Mexico border along California and Arizona, and north through Utah.
Rain is expected to begin Sunday afternoon in southeastern California and Arizona, where 2-4 inches is likely to fall, forecasters said. Some areas could get up to 6 inches.
All that rain can lead to life-threatening flash flooding in the deserts, forecasters said.
Firefighters in the Phoenix area are reminding residents they can fill up sandbags for free at some area fire stations, CNN affiliate KPHO/KTVK reports.
"What we're worried about is all the flooding that we potentially could have," Queen Creek Fire Department Capt. Kris Gale told the station. "It won't keep all of the water out. They're good for about 2 feet of water."
Southwest seeing increase in tropical rainfall
It's rare for a hurricane or tropical storm to impact the southwestern US, but CNN meteorologist Gene Norman says, "These events have begun to increase in recent years."
In Arizona, remnants of tropical cyclones account for about 25% of the annual rainfall along the Colorado River, Norman said.
The region bracing for Rosa's rain was already soaked by remnants of Hurricane Bud in June.
Arizona was also pummeled by a one-two punch of flooding from back-to-back remnants of Norbert and Odile, on September 8 and 18 respectively, in 2014.