Summer
You can get lost in the fog for weeks at a time in summer. If you can navigate through the disorienting pea soup and tourist throngs, windy conditions offshore often produce waves that rival a marginal day in Sweden. In many areas during early summer, the water is actually colder than it is during winter, as upwelling coaxes the temperature to delve into the mid and upper 40s.
Fall
Always your best bet. Conditions can either be influenced by the much-desired "Indian Summer" (slack wind, oily glass groundswells, sunshine and 80 degrees on the beach) or hint at midwinter chaos, with 25-foot seas, heavy rain and rigid south wind. With fall, though, comes the highest chance for scoring relatively perfect, clean waves and days that are capable of fooling surfers to believe they're in a coldwater Indonesia.
Winter
Winter is the surly king of all seasons in this part of the world, funneling fierce storm tracks directly into the region and wreaking land havoc by way of flooding, landslides, power outages, downed trees, road washouts, and beach erosion. To put it bluntly, don't visit the Pacific Northwest in the winter unless your goal is to sit in the car. Not only do the storm systems seem to be on a three-month-long conveyor belt, the surf itself is usually far too giant and out-of-control to ride.
Spring
Throughout spring, rain begins to taper off somewhat, allowing therapeutic binges of sunshine to rouse the land. Wildflowers bloom profusely and residents start to cheer up a bit (but still not toward you), while swells continue to rumble down from the northwest, gradually decreasing in size and consistency around April. Icy north winds howl around the clock during spring, rendering windsurfing in bays and lagoons as a choice seasonal sport.