In reply to drolex:
When, and how long, are you in Seattle? Most of the year the Olys are very wet you know. The summertime N. Pacific high-pressure ridge usually arrives by mid-July and lasts until about mid-September. In that time the Cascades, further east are usually dry, but the Olys can still be cloud covered. I've been in the Olys twice. The most recent was last June and it was a lovely day on Mt Buckhorn - beautiful scenery. I think it's possible to obtain detailed maps for the National Park area. Try a search of the US National Parks for maps.
For a more reliable weather location than the Olys, the Cascades east of Seattle are very good during the summer. The possibilities are almost endless, from easy day-hikes to very hard multi-pitch trad climbs. They are also much easier to get to from Seattle.
I don't want to put you off the Olys, but if you are in Seattle for a short time it might be best to consider other locations too.
Let me know if you need more information.
