I spent 20+ years in Portland, OR before wandering led me to San Diego. In the Northwest, you either run in the rain or you don’t run. Definitely take care of your nipples. Otherwise, dress for the temperature, not the rain. If you want, you could wear a jacket, but in 18 miles it will be soaked through and uncomfortable. If the temps warrant it, I’d go shorts, short sleeve and gloves. If the temps are cooler, then long sleeve and tights.
Other than that, get out there and revel in the fact that you are out training while the competition sits inside on their collective ass, afraid of the a little weather.
Winter is trail running season! I wear this top by Sugoi when it’s pooring and down to low 40’s with a light pair of gloves and shorts. If its not cold at all I’d just wear a standard technical tee shirt. The guys at the running store around here swear by long sleeve wool shirts for really wet running. I haven’t found one that fits me right.
I had a hand in testing the Sugoi Mid Zero fabric years ago and it is outstanding,. The Zip-Neck Mid Zero top should be an essential part of any runner’s wardrobe who live in places that it does get cool to cold in the winter. It’s great on it’s own, used as a base layer or an insulating layer.
I lived in Vancouver for 10 years, so know all about running in the rain - a lot!