Ironically, what you'll have to worry about in Seattle is rain. As soon as it rains, everyone starts acting like they've never seen the stuff before. That, and even worse, once the sun comes out, especially if it's low in the sky. No one apparently was ever trained in how to use the visors.
Now, what you really need to worry about in Seattle is the black ice. As soon as the temperature starts getting to about 34 degrees, keep an eye out, because some areas will be a little colder. You won't see it, because it just looks like the roads are wet, and with all the hills and curves, it's easy to lose control. It's not something that other people don't have to deal with, but if you're not used to the cold, it might surprise you.