It was nearly a cloudless sky over Seattle Tuesday, except for the evening when a few clouds made a very visible appearance.
A line of what might be loosely described as "Jellyfish" clouds appeared over North Seattle Tuesday evening -- sort of like a Rockett's line of individual clouds neatly aligned east-west across the sky.
The white streaks are ice crystals that are precipitating from the cloud, then evaporating when they reach the dry air below. The fact that they are light ice crystals instead of raindrops gives them more of a floating wispy look that gives the "tentacle" look to the bottom of the clouds. (More details from this Atmospheric Optics site)
As to the source of the moisture for the clouds, it looks like there were three lines of clouds across Washington this evening -- one in Western Washington; two in Eastern Washington:
My thoughts are it might have been a line of deformation -- sometimes this occurs when you have two nearby low pressure centers tugging on the same area. In this case, we have a low in southeastern B.C. and another approaching into Northern California.
This can create an area where when air gets essentially pulled in opposite directions, it creates lower pressure aloft that can create a little bit of lift and some clouds. It sure was a pretty sight!