It's not actually overly humid in the SeaTac area. Go south to Olympia and then further into that area is where the rainforest is, but it has to do with a sharp elevation climb that is sort of unique for the peninsula. Seattle is a running joke and absolutely false billed as a rainy city.If they can do outdoor games in California, Nevada (though that one was obviously a tad problematic), and Texas, they can certainly do one in the Pacific Northwest. I think the humidity presents a bigger obstacle than the temperature, but that could be probably be mitigated.
Simple fact, clouds need to dump moisture to get over the Cascades/Rainer. Which happens past Seattle, and usually just contributes to the very long ski season mostly.
Add in consistent air movement, the ocean keeping things consistently temperate, and you have one of the best cities weather wise. If it gets over 85 people start getting hotels, if it gets below freezing it's the coming of the ice age, yet in like April/May some days you could actually enjoy a pretty nice day on the beach, and then be skiing in the next hour or two.
That area is pretty unique, and special. I miss it.