My wife and I rode through all of those countries toward the end of a 3-month ride starting in Lisbon. We experienced 35 days of rain toward the end of our trip. It rained all day on some of those days , and short showers on others, but it did get wet. We've ridden through Belgium, the Netherlands, and France three times and experienced significant rain on all three trips (total of 7 months and 6500 miles). Spain and Portugal are the only countries that we did not experience rain out of the 10 countries we rode through.
This site will give you a pretty good idea of what to expect:
Weatherspark
This slide from a presentation we made of our 2011 trip pretty much says it all.
Fitting fenders tends to take some time, and tools to do it right. I 'd suggest that you fit the fenders on your bike before you leave and then take them off the bike and pack them separately. I'm almost certain that fenders will make the trip more enjoyable. Living in Seattle, you probably have good rain gear already.