Hurricane Rosa strengthened Thursday to a Category 4 storm off northern Mexico's Pacific coast, though it was expected to weaken again before making landfall, the US National Hurricane Center said.
The NHC said swells generated by Rosa will affect portions of southwestern Mexico's coast and the southern Baja California Peninsula late this week and over the weekend.
Rosa, with maximum sustained winds decreasing to nearly 120 miles per hour (195 kph), was now a Category 3 storm, coming down a notch on the five-step Saffir-Simpson Hurricane Wind Scale.
Rain chances will rise in the Valley tonight and rain looks to consume the Valley Monday morning through Tuesday night.
It warned of strong rip currents in San Diego and Orange counties. Some areas could get up to six inches.
Some of the flooding could be significant, according to the U.S. National Weather Service.
"This storm still has a punch, it's still unsafe", said David Roth, a meteorologist with the National Weather Service's Weather Prediction Center in College Park, Maryland.
The National Weather Service announced flash flood watches through Tuesday for areas including southern Nevada, southeast California, southwest and central Utah and the western two-thirds of Arizona.
However, no coastal watches or warnings are in effect, it added.
Rosa, which is now a Category 3 storm, weakened slightly off Mexico's Pacific Coast on Friday.
Heavy rainfall is expected across the Baja peninsula as well as the southwestern United States, with the potential for life-threatening desert flash flooding and mountain landslides, the hurricane center said.