It was a mystery to me, until this morning. A Flickr member helped me find the correct identification! (Thanks PeteAndNoeWoods. <– Click that link to see some awesome nature photos!)
This is Cinnabar-red polypore, Pycnoporus cinnabarinus.
Pycnoporus means “with compact dense pores” and cinnabarinus “bright red”. It’s usually found on dead deciduous wood, though can sometimes be found on conifers. The top will fade to white-ish with time; the underside fades, too, but not as much. I guess it’s not strictly a winter fungus… it can be found year round.
Read more at Messiah.edu.