I live in Salem, OR and have been doing the exact same thing. It sounds like we will be there the same days. We were there in Oct 2018 and had torrential downpours (like monsoon style rain) during the Halloween party. The best part of that night was my boyfriend was saying there is no rain in the forecast on the weather app as I am looking up and seeing storm clouds rolling in saying no, that is a thunderstorm coming. So the weather app was wrong in real time. It rained off and on the next day too and wasn't bad. This time we are prepared bringing our ponchos, rain gear, waterproof hiking shoes. I am hoping by bringing all of this with us, it will prevent the rain from happening.
But we are absolutely hoping for some sun that week!