Can go any month -best for Olympic NP,Seattle,Oregon/Col. River Gorge.
 

I was looking at early June until I just read a post about the PNW and "June Gloom"....that June can be rather depressing weather wise. I certainly know you can't predict weather, but given we can travel anytime (would prefer not to travel during peak tourist season)....what are your recommendations?

Thanks for your input.

June is a mixed bag. This year it has been wetter than usual and cool.

Our summer starts shortly after July 4th and extends until Labor Day.

September usually has mild temperatures and little rain. October is nice too, but a little cooler and wetter than September.

HTTY

Another vote for September. Kids are back in school and so fewer visitors are out and about. We see some of best weather in September too.

Compromises, depends on what you want. In August/September/October you'll have the warmest and driest conditions, and maybe some autumn color at the end, so you won't get so wet in Olympic NP, particularly in the rainforests or on the coast. However, the waterfalls in the Columbia Gorge will be at very low flows and some might be just a trickle. The snow-capped mountains won't be, except for the big volcanoes.

In April/May/June, you'll stand a higher chance of getting wet in all of the above, and places like Mount Rainier or Hurricane Ridge will still be snowy. On the other hand, Seattle and Portland are glorious in spring color, the Gorge waterfalls will be roaring with melting mountain snow, and the orchard areas (e.g. Hood River Valley) will be in bloom. (Spring there: http://gardyloo.us/20100509_85a.jpg )

In the Olympic NP rain forests, you'll get dripped on, but the elk will still be around, and, having eaten all the undergrowth during the winter, will have left you with these amazing, eerie vistas of the forest floor with moss hanging from trees that disappear into the mist. Gorgeous and other-worldly.

You pick.

Good tradeoff blog by Gardy. Hey - just go when you can - and take a small rain poncho and you will be fine. Maybe a pair of 3/4 boots that shed water if it's really coming down.

Personally - I love walking around in the rain - the forest smells great after a fresh rain - pine trees and all that.

My vote would also be for September-probably the first 3 weeks.
Previous posters have not addressed the Columbia Gorge/Oregon part of your trip. Either SEA or PDX will meet your needs. Check the flight schedules with prices and the cost of the rental car from each airport.
The first thing to see would be the higher elevations such as Mt. Rainier and/or Mt. Hood. End the trip with the coastal locations including Olympic NP.
How far into Oregon were you planning on? Both states have a lot going for them as far as beautiful scenery goes.