I fucking love Seattle!! IMO - One of the most pot friendly cities in the US.
As for Hempfest... Yeah, its a freakin blast! As long as youre not flaunting the fact that your smoking, then no one will bother you. BE CAREFUL about taking anything for free. Ive seen plenty of people eat brownies or cookies that were laced with extras. Unless you're Ok with the suprise, it can really ruin the day.
For those of you with dogs, dont bring them in without a little pack on their back. Its easier to sell to the guards that they're for "medical reasons" when they're assisting you with something.
Hope you make it up, and have a great time!!