SEATTLE - A "pineapple express" is a term for those really warm, wet storms in the winter where we get a train of moisture that stretches from near Hawaii (thus, the name "pineapple") toward the Northwest. It's usually responsible for our main flooding events.
But why not in summer? Our jet stream (which carries the moisture from Hawaii) isn't as strong in the summer time, and is also usually much farther north. Also, the differences in temperature in the mid-latitude areas aren't as large over the Pacific Ocean in the summer, so that hinders storm development.
Thus, we just don't have the ingredients together to get a strong jet stream tapping the moisture from the Hawaiian tropics and bringing it to our doorstep.