I had this exact problem yesterday with the heavy, sticky snow that was falling. I stopped and cleared it, and the helped for a whopping five minutes or so. I kind of wished the "smart" (Traffic Aware) cruise control would fall back to "dumb" (Traffic Oblivious) when it can't be smart.
And, boy, if you're using AutoPilot when this happens the car really goes crazy, beeping frantically and flashing red warnings.
A possibly related issue: later in the day after the snow stopped I was driving along on the interstate with no cars in front of me, using AP(1), and all of a sudden a truck showed up on the autopilot display (but not in reality) and the car braked fairly hard. After a moment it went away and the car resumed its cruise speed. A few minutes later this happened again. I'm wondering whether this might be some (really annoying) artifact caused by residual ice or snow in and around the front radar?