The rest of the year, expect rain. If it doesn't come down, you'll be happy, but if it does, you'll be prepared. The local visitor's bureau will tell you that it rains about 36 inches a year in Portland, less than in Houston, Atlanta or Seattle, but don't let the statistics fool you. Pacific Northwest rain comes down a little at a time, spread over many months. Bring an umbrella, or do what the locals do and carry a cap or wear a jacket with a hood.
Despite the high probability of rainy weather, December is a good time to visit Portland, when downtown is decked out with holiday decorations, and you can do your gift shopping and pay no sales tax.
Unless you want to attend the annual Rose Festival, avoid Portland when it's going on in June. The crowds make it hard to get hotel reservations and almost impossible to get discount hotel rates. If you want to attend the festival, plan at least three months in advance. The Portland Marathon (early October) also draws lots of out of town visitors.