Portland's climate is mild and famously wet. The rainy season runs from October through May; winters are grey and drizzly rather than cold, with snow rare in the city (though Mount Hood 80 km east gets heavy snow from November). Summer (late June through September) is genuinely excellent: warm, largely dry, and with long evenings. July and August are the driest months.
The best time to visit is June through September. The Oregon Brewers Festival is in late July; the Portland Rose Festival is in June; the Alberta Arts District's Last Thursday runs year-round but peaks in summer. Spring brings spectacular rhododendron and cherry blooms in the parks and gardens — the Cherry Blossom Festival in the South Park Blocks is typically in late March.
Visitors who don't mind the rain will find the city quieter and cheaper in autumn and winter. November through February are the wettest months. The ski season on Mount Hood runs from late November through late spring; Timberline Lodge offers summer skiing on the Palmer Glacier.