Traffic on the Kenai falls off drastically after August 10. Everything starts to shut down after labor Day, as in no water at the camp sites. As noted, others have reported a broad area of snow including Ft St John and Grand Prairie on September 13. We have been snowed on on May 24 from Tok to the Kenai in a previous year.
For fishing, 15 July onward. That will get you a shot at Kings, peak Red season, and Silvers August into October. Halibut will still be good through August.
Fire season is May into June, then declining. Rain usually starts Fathers Day for the balance of the summer, but is a light misty thing most times. Fall color was pretty sparse this year when we left September 1st.
Have a good time.