surak,
Even if your bike frame had no provision for mounting fenders, there are ways to "jury rig" something so you can enjoy riding in Seattle year around.
My winter/rain bike has no eyelets on the front fork or the rear dropouts for rigidly mounting fenders, but a local bike shop (sadly, no longer in business) made some fenders work for me, in spite of the "less-than-zero" clearance offered by my brakes (I cannot run 25cm width tires, only 23 or less.) They split the fender on either side of both brake calipers and made it work.
If one rides solo in the rain, the stock-length fenders are enough to keep the water off your back. If you do group rides, you need to add "buddy flaps" to the rear, to keep water off the rider behind you. That flap should be long enough to almost touch the cement. A front "buddy flap" will minimize the direct road spray to your own feet/body.
Just remember, there is no way that you will stay dry. You will get wet, but the goal is to stay warm and wet, not cold/hypothermic and wet. Road water constantly spraying your back due to no-fenders will chill you severely. Fenders plus WOOL base layer and WOOL socks (e.g. Woolie Boolie) will make your winter cycling so much more bearable!
I speak from experience. I do close pacelining, even in the most horrific of weather.
Best of luck to you!