In today's situation, where yesterday was sunny and warm and today is expected to be some 20 degrees cooler, a marine push blew in after a thermal low moved inland last night. (Thermal lows are formed by hot air rising, and then air rushing in to fill that area of lower air pressure near the surface The thermal low was the reason it was so warm here.)
That replacement air comes in from the higher pressure over the ocean. Thus we got a push of air from the ocean through the Strait of Juan de Fuca and around the southern Olympics and into the Puget Sound area.
The next day is then typically cloudy and cool, with high temperatures as much as 15-30 degrees cooler than the day before.
For More Information:
KOMO Weather FAQ -- www.komotv.com