Even if it rains this weekend, it's gonna be in the low-mid 70s down here, way too warm for a jacket, IMO. I'm gonna roll with fenders and call it good. Shorts and short-sleeve shirt will be worn.

I'd leave Seattle pre-dawn Saturday or Sunday, or take a bus down to Tacoma or even OLY (if leaving mid-day) because I hate cycling in most parts of Seattle when most people are awake. This is assuming you're going straight south and not taking in the Olympic Peninsula.