Thanks to a savvy skywatcher in the UK, this unusual cloud formation is called altocumulus stratiformis perlucidus undulatus radiatus.
Which is to say, it is a mid-level altocumulus cloud that appears as cloudlets over a large area (stratiformis); have gaps between cloudlets (perlucidus), have cloudlets arranged in nearly parallel lines (undulatus) which apear to converge toward the horizon (radiatus). The official code in the UK for this type is CM5.
I took the photo on March 26, midday, in downtown Olympia, WA. The clouds seemed to be radiating from the direction of Mt. Rainier. I have a feeling this snow-covered mountain played a role in the cloud's unusual shape.
Daily, the world of clouds and cloudwatchers is expanding for me. The official website of the Cloud Appreciation Society www.cloudappreciationsociety.org is fabulous as is the UK's meteorological office www.metoffice.gov.uk
I hope to get the whats down this spring and, when the sky turns cloudless this summer and I can stop looking up all the time, turn to the books to start figuring out the whys.