Summer is the busiest time for tourists, but also the best as far as weather goes. Portland can be hot in summer, with temperatures reaching up to 90°F. Expect temperatures to vary widely from night to day, sometimes by as much as 40 degrees. Most attractions are open daily year-round, but summer is the busiest tourist season, when you can expect the most congestion and the highest prices.
Sources:
www.travelportland.com
www.pdxhistory.com
www.portlandworldweb.com
www.bootsnall.com
www.fodors.com