Astronomical fall begins Friday night, and autumn storms are already knocking on California’s door. A major September storm is forecast to bring heavy rains and strong winds to Washington, Oregon and Northern California beginning Sunday night. … The jet stream is forecast to strengthen across the Pacific Ocean this weekend, pushing an atmospheric river all the way from Japan to the western U.S. Atmospheric rivers are ribbons of moisture that can ferry large amounts of moisture thousands of miles through the sky.

Related articles: 

• Weather West: Fall-like weather pattern for CA as El Niño continues to strengthen; Odds of a second consecutive wet winter rise (though with caveats!)
• Western Farm Press: UC – Grow cover crops before wet winter
• Drought.gov: Drought Deteriorations Due to Late Summer Hot and Dry Conditions in Southern Portions of Intermountain West Region