|Wei Ji the Learner|
This is more for folks who live in the Seattle area, so I can pack as lightly but appropriately as possible for the weekend.
What has the weather been like the past few weeks, and are the numbers of mid-70s that NOAA is predicting reasonably accurate?
I know to bring some raingear, this is more 'heavy/medium/light jacket' and 'sweatshirts?' etc considerations.
Are you just going to the con, or are you planning on doing other activities while you are here?
Either way, I wouldn't bring a heavy jacket. Springtime in Seattle is unpredictable, it may be 80 degrees one day, 60 the next, and mornings/evenings can be chilly, but it's not going to get really cold. Bring a few layers, maybe a sweatshirt and a rain shell you can wear over it, and you should be good for the con. If you are also planning on doing other outdoor activities, I'd probably bring one more layer just to be safe (though you probably won't need it even then).
Be prepared for temperatures of 50-mid 70's. A single light jacket, sweatshirt, flannel or fleece should be plenty. (And since temperatures inside the convention can be changeable, it's worth layering for gaming.) It rains frequently in Seattle, but it's more of a drizzle and almost never pours. If you are planning on exploring the city pack a travel umbrella or a water-resistant jacket with a hat or hood, just in case.
Currently it's looking to be 60-80 with partly cloudy skies and a light breeze.
BUT since most of your day is spent inside there's something more important than the weather to consider - Air Quality. the Pollen is in HIGH GEAR right now, especially the cottonwood trees. If you have allergies to plant-based air particulates be sure to bring some extra meds to deal with it. I know I'm on 2/day during this season... :/