Well, not really. In fact, Seattle actually receives less rain than Atlanta or Houston. According to our Wikipedia article:
"Despite being on the margin of the rain shadow of the Olympic Mountains, the city has a reputation for frequent rain. This reputation derives from this frequency of precipitation as well as the fact that it is cloudy an average of 226 days per year (cf. 132 in New York City). Nonetheless, the so-called "rainy city" receives a smaller quantity of actual precipitation annually, at 37.1 inches (94 cm), than New York City, Atlanta, Houston, and most cities of the Eastern Seaboard of the United States. Seattle was also not listed in a study that revealed the 10 Rainiest Cities in the continental United States.  Most of the precipitation falls as drizzle or light rain, with only occasional downpours."
In short, it is gray more often than it is not. If something happens to break up the monotony of the gray, drivers in Seattle freak out and turn I-5 and I-405 into parking lots. The sun comes out? We end up with a 55-car pile-up a week before a 12-car collision followed by 16 separate accidents involving at least 42 vehicles. A little snow? Nobody leaves their houses (and those that do end up in a 60-car pile-up).
And what was the horror that turned traffic black today (as you can see in the screen-shot of Live Maps Traffic to the left)? What was it that caused 6 separate accidents?
It rained. Good grief.