Went this past April. Weather was cool and overcast in Seattle, rainy in the rainforest.
I wore a pull-over every day and was fine.
Take a rain jacket. Wore it every day. My wife forgot hers and it is surprisingly hard to find one in Seattle. Ended up getting one at an Army surplus store to avoid paying REI prices.
For Olympic, cdub06 put together a nice day trip for us.
Left Seattle, first stop, Quinault Rainforest trail. Swung by Ruby Beach. Then our big hike for the day Hoh River to Mineral Creek Falls. Brief hike for Madison Falls and a few stops for Cresent Lake. Burgers in Gig Harbor. Full day, but cdub drove all day and I didn't, so I was good.
I'll share some photos of the weather.

Hoh River

Lake Crescent

Ruby Beach

Kerry Park, Seattle