Clouds have been absent for a few days around Seattle so it makes sense when they returned they'd want to put on a show.
Seattle and other parts of the Puget Sound region were treated to a display of "mammatus clouds" as rain approached Thursday afternoon.
The clouds and are usually a signal of turbulent, unstable air, and are usually seen after a strong storm has passed, although in Thursday's case, it was before any rain arrived.
Their formation is somewhat unique in that they are caused by sinking motion -- typically associated with drying weather as sinking air warms and dries as it does so.
Check the University of Illinois atmospheric sciences site for a more detailed explanation of how mammatus clouds form.
Here is some time lapse video of the clouds as they approached South Everett, courtesy Jonathan Cooper of Seattle Image Photography:
And here is one from Daron Johnson, also of South Everett
And finally one from Greg Johnson -- who has 2 cameras focused on theskies over Skunk Bay