(CNN) Southern California is getting drenched for the first time in six months.

Rainfall started developing in southern parts of the state Tuesday night -- the first substantial rain in Southern California since May.

More than 13 million people are under a flash flood advisory to include portions of California, southern Nevada and Utah, and most of Arizona, CNN Meteorologist Michael Guy said.

The storm is a combination of two weather systems -- the remnants of a tropical depression from the East Pacific and a weather system from Canada, Guy said.

Farther east, parts of Arizona could pick up 2-4 inches of rain, and Phoenix could receive 1.5 inches.

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