The problem this time of the year with high pressure is fog. In fact, you know when the foggest month of the year is? October. Not December or January. Why? Long nights and frequent clear or nearly cloud free skies that allow strong and extended infrared cooling to space. The fog is most persistent in low areas...such as the Puget Sound basin and the Willamette Valley.
On such nights we often have surface-based inversions in which the temperatures warm with height. You can watch this happening in real time by going to my department web page that shows the temperatures above Seattle every hour from the profiler at the NWS site at Sand Point:
http://www.atmos.washington.edu/cgi-bin/latest.cgi?profiler.spt
Monday morning update: you can see the inversion this morning in N Seattle...times are in GMT (red..14 GMT is 6 AM)
