Observation notes: Found by my grandson, who had picked it up and brought it to me, so no idea what plant it was on. Markings on the back looks like Forest Tent Caterpillar, but lacks blue flanks. Has bright yellow between segments instead. Submitted to CaterpillarIdentification.org and they're stumped.
I received the identification from Dr. P. J. Liesch of the University of Wisconsin Department of Entomology who describes it as a rare find. https://bugguide.net/node/view/36230