Well, you experienced it 2 years ago. In May and June, weather conditions are such that the marine layer (a layer of low fog like clouds that lie over the Pacific) are able to move in over land. They do usually burn off (unless you are very close to the coast), but it can take the better part of the morning to do so. It can also move back in over land in the late afternoon and early evening. Hence, May grey and June gloom.
What this means for you at DL is mild weather. It keeps it from getting too hot during the day, but you may need a jacket or sweater in the morning and evening.
Michele