A flash flood watch is in effect for Tucson and much of Southern Arizona from now until late Tuesday night.
The watch comes as excess moisture from Hurricane Polo feeds thunderstorms in northern Mexico. Up to two inches of rain are expected in most of Southeast Arizona, with two to four inches possible in the Sky Islands. Flash flood risk is highest in areas east of the Tucson metro area.
The rain is expected to be heaviest Monday night into early Tuesday morning. The National Weather Service has ranked the flood risk at four out of five, and residents are advised to stay out of washes and drainage channels, particularly when driving.
Meanwhile, across the border in the Mexican state of Sonora, schools are set to close Tuesday and Wednesday in anticipation of the hurricane.
The monsoon season officially comes to an end on Sept. 30.
