The clouds were moving consistently from south to north (left to right in the photos). I believe that the consistent shape break up is due to the Olympic Mountains, which arise pretty much straight out from the edge of the bulk head.
I set my camera up on a tripod as the first bank of clouds arrived, put on neutral density filters (CPL plus a six stop), set the aperture set at F/16, and ISO at 64. Then I took the above sequence of photos between 4:00pm and 6:25pm. The length of the exposures varied depending on the ambient light but generally went from about 5 seconds for the first shot, at about 4:00pm to 10 for the last one, which was during twilight.
XingfuMama 