These are mammatus clouds: basically, the result of updrafts and downdrafts churning the bottom of a cloud as it matures throughout the life of a storm. The distinct puffy pattern made by the churning winds almost always marks a severe weather system. Since I spent the past hour or two before taking these photos in our basement, sheltering from a tornado warning, this storm definitely qualifies as severe.
You can see them marking an obvious boundary for the storm as it slowly arcs out over Lake Michigan from my perch above Montrose Harbor. There were a few intrepid souls already back on the beach enjoying a nice summer evening.
