You may recall the Lake Michigan ice balls that were seen at the Sleeping Bear Dunes lakeshore last March. Well, they’re baaaaack. You can see a great video of the ball ice at Accuweather. Here’s the explanation of how balls ice form that I put together from AIR PHOTO INTERPRETATION OF GREAT LAKES ICE FEATURES by Ernest W. Marshal & Frazil ice at Wikipedia:
Ball ice consists of roughly spherical masses of slush and frazil ice that accrete in turbulent water. Frazil ice is a collection of loose, randomly oriented needle-shaped ice crystals that form in open, turbulent, supercooled water. Lumps that form in the less turbulent zones are typically flattened discs, while those formed in the extremely turbulent zone near the shoreline ice where wave action is strongest form into spheres.
The author explains that ball ice is a feature common to all of the Great Lakes and can occur at any time during the winter where water turbulence breaks up a slush layer. You can read more about this in Great Lakes Ice Features.