  We study regularity properties of the free boundary for solutions of the
porous medium equation with the presence of drift. We show the $C^{1,\alpha}$
regularity of the free boundary, when the solution is directionally monotone in
space variable in a local neighborhood. The main challenge lies in establishing
a local non-degeneracy estimate (Theorem 1.3 and Proposition 1.5), which
appears new even for the zero drift case.
