Title: A Free Boundary Problem for the Parabolic Poisson Kernel Abstract: We study a free boundary problem for caloric measure first introduced by Hofmann, Lewis and Nyström. In particular, we show that the oscillation of the Poisson kernel of a domain controls the oscillation of the unit space normal to that domain (this is the parabolic analogue of Kenig and Toro's work on harmonic measure). The key step is classifying "flat blowups." To do so, we adapt the work of Andersson and Weiss on solutions to a parabolic problem arising in combustion.