I love winter fly fishing in the PNW, especially on the dry side of the Cascades. The worst weather is usually December through January, but it's much milder than back east by a long shot, although freezing fog is pretty bad. I like to fish either tailwaters or spring creeks if I can because the water will be warmer and bugs are usually more plentiful, but when it's a bit warmer freestones can fish well too. I generally fish days when it's above freezing, which is pretty often, so I don't have to deal with ice in the guides except when I fish in shaded parts of some canyons.
I like winter because of less people, small bugs, and bankers hours. I generally fish about 11 to 3 in winter. That's when the water has warmed a little, bugs start to hatch, and fish will feed. Bugs hatch pretty much every day, but a cloudy day that's not too cold can bring out strong midge and baetis hatches that will bring an amazing number of fish to the surface. There's nothing like a deserted canyon in February with fish rising everwhere to midges, and the harder it snows the more the fish rise.
Like others said, layers, and wool are your friends. Cotton is your enemy, and bring extra clothes in case you fall in. Snow will sometimes build up on the bottom of felt soles, it hasn't happened to me often in Oregon, but some guys use rubber soles in winter. I try and not wade too deep and move often to keep my blood circulating. Standing in one spot till your feet go numb in a good way to get really sick, and makes wading very difficult and dangerous. I learned the hard way, and now I seldom wade deeper than my knees in winter and walk often.
