  We report the first lattice QCD calculation of the form factors for the
standard model tree-level decay $B_s\to K \ell\nu$. In combination with future
measurement, this calculation will provide an alternative exclusive
semileptonic determination of $|V_{ub}|$. We compare our results with previous
model calculations, make predictions for differential decay rates and branching
fractions, and predict the ratio of differential branching fractions between
$B_s\to K\tau\nu$ and $B_s\to K\mu\nu$. We also present standard model
predictions for differential decay rate forward-backward asymmetries,
polarization fractions, and calculate potentially useful ratios of $B_s\to K$
form factors with those of the fictitious $B_s\to\eta_s$ decay. Our lattice
simulations utilize NRQCD $b$ and HISQ light quarks on a subset of the MILC
Collaboration's $2+1$ asqtad gauge configurations, including two lattice
spacings and a range of light quark masses.
