Seattle and Portland don't get the same kind of rain as lots of other places (such as Chicago or Boston). In the Northwest the most common rain is a light drizzle, downpours are very rare. Whereas in many parts of the country a normal rain storm would be considered a downpour in the Northwest. For that reason people in Seattle and Portland can get through the entire winter wearing a light jacket or even a cloth hoodie. Even though hoodies aren't waterproof, being exposed to the rain for a couple minutes will only make them slightly damp, rather than sodden through and through the way they might be if one attempted to use them in other rainy cities.

An umbrella is a bit of false protection from the rain as well. It's a nuisance to carry so often given that it will save you only from a mild inconvenience on most days. And as often as not it'll end up being a hindrance as you fight it from being destroyed or carried away by the wind.