Despite its long-standing reputation, the city of Seattle is not the wettest place on Earth. Proportionate to a lot of the rest of the country (especially the Southeast), it really does not get a lot of total precipitation. In fact, most of it comes down in a light sprinkling or drizzle, with only the occasional downpour. Where the reputation comes from are the sheer number of days that are overcast, around 200 at least, and the simple fact that people here tend to make it a much bigger deal than it really is. Heavy snow like what was experienced during the winters of 2007–2008 and 2008–2009 is actually pretty rare overall, though be warned about heading out onto the road if there has been a snowstorm. People here simply do not know how to behave on the roads when covered in even a little bit of ice and snow.
All this should not lead to the idea, however, that you never see the sun here. Much of the weather is quite beautiful, and warm, cloudless days are quite common in the spring and summer. The parks and local beaches are packed on those days, and it is rare that you would see the clouds rush in and rain out your picnic or softball game. It can actually get almost unbearably hot in the middle of the summer, but even then they are a wonderful contrast to the colder winter months.
In terms of what clothing to bring, it’s really best to prepare for every occasion. You probably won’t need much at all in the way of dedicated snow outfits, though it would be smart to bring some warm jackets and hoodies for when it does start to rain heavily. A few pairs of jeans are all you really need in terms of pants, though be sure to bring some shorts and sandals for when the sun does break through the clouds for more than two days at a time. As much of a hassle as it can be sometimes, the weather here really is quite nice.