  The isovector axial form factor of the nucleon plays a key role in
interpreting data from long-baseline neutrino oscillation experiments. We
perform a lattice-QCD based calculation of this form factor, introducing a new
method to directly extract its $z$-expansion from lattice correlators. Our
final parametrization of the form factor, which extends up to spacelike
virtualities of $0.7\,{\rm GeV}^2$ with fully quantified uncertainties, agrees
with previous lattice calculations but is significantly less steep than
neutrino-deuterium scattering data suggests.
