The hurricane center said there were no coastal watches or warnings in effect, but swells generated by Rosa were expected to cause risky surf and rip current conditions along portions of southwestern Mexico, the peninsula and southern California.
Hard driving conditions may begin Sunday afternoon as tropical moisture ahead of the storm washes into the state.
"Additional weakening is forecast", the Miami-based agency said, with Rosa expected to become a tropical storm by early Monday as it approaches central and northern Baja California, which adjoins the U.S. state of California.
"It could be a tropical depression or tropical storm at that point, but regardless it's going to be a significant increase", says Hopper.
The storm will be unlike Arizona's typical summer thunderstorms, said Randall Cerveny, an ASU professor at the School of Geographical Sciences and Urban Planning.
There's more rain on the way, but this time it's not from a monsoon storm.
Flash flooding will be a primary threat when rain rates of 1 inch to 3 inches an hour possible.
But O'Malley said there's still a good chance the rain could be much less than predicted.
In Arizona, remnants of tropical cyclones account for about 25% of the annual rainfall along the Colorado River, Norman said.
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