The Portland area and the coast will probably see the most rain, but central oregon will get some too.
Steve Pierce, President of the Oregon American Meteorological Societt, says it's unusual to get this much rain in late fall.
"But for late September, this is a very active system and most locations, Bend, Redmond, Pendlton will pick up quite a bit of rain, but we don't have to worry about snow. It won't be a cold system."
Portland is poised to beat its previous wettest September on record in 1986, when more than four inches of rain fell during the month.